Facebook hit git performance issue on large repository

http://thread.gmane.org/gmane.comp.version-control.git/189776

by kaeso • 14 years ago

378 205 14 years ago

thread.gmane.org

courtewing• 14 years ago

This was actually pretty fascinating to me. On one hand, I am astonished at how long it takes to perform seemingly trivial git operations on repositories at this scale. On the other hand, I'm utterly mystified that a company like Facebook has such monolithic repositories. Even back when I was using SVN a lot, I relied on externals and such to break up large projects into their smaller service-level components.

I'd be very interested to see some benchmarks on their current VCS solution for repositories of this scale.

guan• 14 years ago

From a followup post: “We already have some of the easily separable projects in separate repositories, like HPHP. If we could split our largest repos into multiple ones, that would help the scaling issue. However, the code in those repos is rather interdependent and we believe it’d hurt more than help to split it up, at least for the medium-term future. We derive a fair amount of benefit from the code sharing and keeping things together in a single repo, so it's not clear when it’d make sense to get more aggressive splitting things up.”

ntkachov• 14 years ago

Remember that this is a synthetic repository.

vessenes• 14 years ago

He notes that these repositories are somewhat broken up already, and wants to keep them together.

There are good reasons to keep code in one repository; particularly, git's submodule support has a number of nasty interface tradeoffs; I wouldn't say it breaks git, but you have to keep a clear understanding of all your submodules in your head when you have a lot of them.

OK, it pretty much breaks git to have submodules that are interdependent. I know this because I am currently moving one of my organizations off this exact plan -- it's the opposite of useful and speedy to have to worry about versions across a large number of backend / frontend repositories.

It is MUCH easier and therefore better for developers to put them together, and release together.

chucknthem• 14 years ago

Not sure if it's still the case, but Google hosts all their internal source code on a modified version of perforce, so they essentially have everything in one repo.

wmf2• 14 years ago

Given that Facebook is compiled into a single 1 GB executable, a git repo with 1.3 M files doesn't really surprise me.

jlarocco• 14 years ago

What? Do you have a reference for that?

xxpor• 14 years ago

Oh God, it's like Amazon was 10 years ago.

functionform• 14 years ago

What do Facebook and the National Institutes of Health have in common? I'm pretty sure this will end with Facebook building their own versioning system from scratch and give it some kitchsy name like "Retro".

djtriptych2• 14 years ago

I hope these guys do take the route of developing a large-scale performant patch.

Git as so many interesting uses at scale as just a tool that navigates and tracks DAGs over time.

djb_hackernews• 14 years ago

That's projected growth for two of their projects. Sounds like they have something brewing...

Still amazed that breaking it up would do more harm than good when the code isn't even written yet...

Judson• 14 years ago

I read it as them being unable to break up a project, and the repo being a projection of future commits to a project that can't be split up.

yuvadam2• 14 years ago

While I'd be interested in seeing this issue further unfold, just the prospect of a 1.3M-file repo gives me the creeps.

I'm not sure what the exact situation at Facebook is with this repository, but I'm positive that if they had to start with a clean slate, this repo would easily find itself broken up into at least a dozen different repos.

Not to mention the fact that if _git_ has issues dealing with 1.3M files, I wonder what other (D)VCS they're thinking of as an alternative that would be more performant.

wbkang• 14 years ago

Pretty sure Perforce performs fine with that.

yuvadam2• 14 years ago

Well, of course that at some specific scale, you're gonna start to have trouble with any DVCS maintaining a complete local copy of such a huge repository.

It's even worse that just disk space and performance issues.

I can totally imagine a huge, busy repository where by the time you've pulled and rebased/merged your stuff, the repo has already been committed to again, invalidating your fast-forward commit and forcing you to pull again and again before you have any chance of pushing back your changes.

This is an inherent problem with DVCS that just can't be solved (trivially) when working on huge repositories that span millions of files and involve thousands of developers.

AndrewDucker• 14 years ago

If you read further down the thread they say that that's already had the non-interlinked files split out. What they've got left isn't easily broken up.

clord• 14 years ago

Best argument I've ever seen for not wanting to work at Facebook... wow that's a lot intertwined spagetti code.

Our source repo at work (a C++ compiler with full commit history going back to the early 90s...) is smaller and more componentized!

InclinedPlane• 14 years ago

A lot of big companies have repos 10 or 100 times that size. With tens of millions of files, sometimes up to 100 gigs or more of data under source control.

brown9-22• 14 years ago

True, but don't most places organize one git repo per project, rather than one for the entirety of the company's source code?

dpcx• 14 years ago

I don't want to imagine the actual kind of code that requires 1.3M files to run.

nbm9• 14 years ago

Keep in mind that your average repository doesn't only contain code that is compiled and executed (or interpreted), there is also documentation, static assets such as images (that may be processed), configuration, computed files (that may make sense to pre-compute once rather than compute on a hundred people's environments every build), and so forth.

Also, it doesn't only include the current file set - they include files that have been deleted, been split into modular files, been merged, been wholesale rewritten, put into a new hierarchy (some VCS systems handle this better than others).

(I work at Facebook, but not on the team looking into this stuff. I'm a happy user of their systems though. Keep in mind that the 1.3 million file repo is a synthetic test, not reality.)

0x0• 14 years ago

The follow-up email still mentions a working directory of 9.5gb. I cannot fathom working on a code repository consisting of 9.5gb of text. There must be something else going on here, even considering any peripheral projects like the iOS and android apps, etc.

(edit: if there are huge generated files intermingled with code, shouldn't those be hosted on a "pre-generated cache" web server instead of git, for example?)

sek7• 14 years ago

http://thread.gmane.org/gmane.comp.version-control.git/18977...

They keep every project in a single repo, mystery solved.

Edit:

> We already have some of the easily separable projects in separate repositories, like HPHP.

Yeah, because it makes no sense, it's C++. They probably use for everything PHP i assume then. Is there no good build management tool for it?

masklinn• 14 years ago

> They keep every project in a single repo, mystery solved.

That's not true:

> It is based on a growth model of two of our current repositories (I.e., it's not a perforce import). We already have some of the easily separable projects in separate repositories, like HPHP. If we could split our largest repos into multiple ones, that would help the scaling issue. However, the code in those repos is rather interdependent and we believe it'd hurt more than help to split it up, at least for the medium-term future.

They already have multiple repositories, the stats they're doing there is based on "two of [their] current repositories" implying more than two.

sek7• 14 years ago

Why would he take HPHP as an example then? It should be obvious that there is not much interdependence with the other code.

Sounds to me like this: http://thedailywtf.com/Articles/Enterprise-Dependency-Big-Ba...

fragsworth• 14 years ago

> They keep every project in a single repo, mystery solved.

This kind of "Duh, look what you're doing" response isn't really justified.

Sure, splitting up your repository would make things faster, but having to maintain multiple repositories is a major headache for the end-users of git. If it's possible, why not fix its scalability so that you don't have to worry about it?

metachris• 14 years ago

Git submodules make it really easy to split up large projects into smaller components.

Also, as Ævar writes in the first response, "there's only so much you can do about stat-ing 1.3 million files".

lnguyen5• 14 years ago

I'm pretty sure that cloning a repository of that size can't be all that fun.

You tend to split repositories based on team responsibilities. I doubt that every developer needs access to update all million+ files.

What this comes down to is that they've made certain architecture decisions that ideally would be changed but it's not possible to do so at this time.

echaozh• 14 years ago

I agree. Users are always right, even for open source software. Requiring the users to change their way is an easy way to lose them. A major player like Facebook willing to use Git is a good thing, as it may help spread the software in companies like Facebook. Git should try its best to please its user, rather than the other way round.

dblock• 14 years ago

Others have tried and keep throwing more and more smart people at the problem they just shouldn't have.

MSFT with Windows codebase that runs out of several labs. Crazy branching and merging infrastructure. They use source-depot, originally a clone of perforce.

Google with all their source code in one Perforce repo.

Facebook will be on perforce before we know it.

The solution is an internal Github, not one giant project.

sek7• 14 years ago

Google has everything in one Perforce repo? You mean the search engine, do you?

I agree btw, the Github mindset is the best one. Create for every project a new repo and connect them with build tools. But why not hire 100 SOA-Consultants, they have enough money now.

mikeocool• 14 years ago

No, literally the entire codebase for all of their products is in one Perforce repo. Ashish Kumar, manager of the Engineering Tools team, mentions it in this presentation: http://www.infoq.com/presentations/Development-at-Google

EricBurnett3• 14 years ago

Pointing at a big company and saying "they're doing it wrong" is easy enough to do, but you have to remember that every decision comes with tradeoffs. Take Google's codebase, since it's the one I know the best. A couple of the key decisions:

* Single rooted tree. Separated repositories would make it harder to share code, leading to more dupication.

* Build from head. We build everything from source, statically linked. No need to worry about multiple versions of dependencies, no lag between a bug fix and it being available to any and all binaries that need it, whenever they're next updated.

I don't think that an "internal Github" is going be a magic bullet here. It's more likely it would be a matter of trading one set of hard problems for another, as we all of a sudden need to figure out how to do cross-dependencies sanely, deal with multiple versions of libraries, etc, at scale. You are correct that one monolithic Perforce repo is a bit of a pain point, but that doesn't necessarily mean that the right decision is to shatter our codebase into different pieces - we'd rather make our repo scale better. For reference, we've already got hundreds of millions of lines of code, 20+ changes/minute 6 months ago (so what, 30+ now?), and plans for scaling the next 10x are in motion.

If you're interested, I recommend http://google-engtools.blogspot.com/. It details a number of the problems we've run into, and our solutions for dealing with them at scale.

mindcrime2• 14 years ago

Single rooted tree. Separated repositories would make it harder to share code, leading to more dupication.

I'm not convinced that the difference between a singly rooted tree and a multiple-rooted tree is going to make that much difference. I mean, think about it... if you 100k's or even millions of files, is anybody going to parse through all of that, looking for a reusable function, even if it is on their workstation?

And sure a compiled language would catch naming collisions on functions or whatever, but nothing stops somebody from creating a method

doQuickSort( ... )

and somebody else creating

quickSortFoo(...)

where they are semantically equivalent (or very nearly so).

It seems to me that the problem of duplicating code, because you don't know that a method already exists to do what you're trying to do, is the same problem regardless of how your tree is laid out; and is ultimately more of a documentation / process / discipline issue. But I'd be curious to hear the counter-argument to that...

akg• 14 years ago

I don't think Git was designed to perform well with such a large repo. In this case, the best-practice is probably compartmentalizing the code and using Git submodules. The Git submodule interface is a little un-friendly, but I think it does work well for such large repos. I've been using submodules successfully for our development that tracks source files as well as binary assets.

losvedir• 14 years ago

Huh, fascinating. git was initially created for the Linux kernel development, and I haven't heard of any issues there. Offhand I would have said, as a codebase, the Linux kernel would be larger and more complex than facebook, but I don't have a great sense of everything involved in both cases.

So what's the story here: kernel developers put up with longer git times, the kernel is better organized, the scope of facebook is more massive even than the linux kernel, or there's some inherent design in git that works better for kernel work than web work?

cbs• 14 years ago

From the sounds of it facebook has a really, really big ball of highly coupled code.

joelhaasnoot2• 14 years ago

Sounds like PHP.

Oh wait, it is.

haberman• 14 years ago

Not necessarily.

In the open-source world, when you want to change an API, you have to either add the change as a new API (leaving the existing API intact) or break backward compatibility and maintain parallel versions, gradually migrating users off of the old version.

Both of these options are a huge pain, and have a direct cost (larger API surface or parallel maintenance/migration efforts). When your entire repo and all callers are in the same code-base, you have a much more attractive option: change the API and all callers in a single changelist. You've now cleaned up your API without incurring any of the costs of the two open-source options.

This is why it can be nice, even if you have a bunch of nicely structured components, to have all code in a single repository.

marginalboy• 14 years ago

It isn't surprising if Facebook has a large, highly coupled code base. Given their reputation for tight timelines and maverick-advocacy, I'm continually surprised the thing works at all.

nbm9• 14 years ago

I wouldn't say that a large repository implies that the code is highly-coupled. There are advantages for keeping certain code together in a single repo. Being able to easily discover users of functions of a library, being able to "transactionally" commit an update to a library (or set of libraries) and the code that uses it, being able to do code review over changes of code in various places, being able to discover if someone else has solved this problem before, and so forth. If you only have your project and its libraries checked out, you don't serendipitously discover things in other projects.

As mentioned in this talk on how Facebook worked on visualizing interdependence between modules to drive down coupling at https://www.facebook.com/note.php?note_id=10150187460703920 , there are at least 10k modules with clear dependency information in a front-end repo, and the situation probably is a lot better now that they have that information-dense representation to work from (I don't work on the PHP/www side of things, I spend most of my time in back-end and operations repos).

mariusmg• 14 years ago

The FB mobile auth workflow breaks at least once a month.

zargon• 14 years ago

The linux kernel has an order of magnitude fewer files than Facebook's test repository (25,000 in version 2.6.27, according to http://www.schoenitzer.de/lks/lks_en.html) and only 9 million lines of text.

This is on the largish side for a single project, but if Facebook likes to keep all their work in single repo then it isn't too difficult to go way beyond those stats. Think of keeping all GNU projects in a single repo.

aidenn0• 14 years ago

The linux kernel is several orders of magnitude smaller. They are talking about 1.3 million files totalling nearly 10GB for the working tree. My kernel checkout has 39 thousand files totaling 489MB.

dustingetz• 14 years ago

the obvious answer, repeatedly mentioned in comments:

> factor into modules, one project per repo

where i work we have a project with clear module boundaries, but all in the same repo. we have an "app" and some dependencies including our platform/web framework. none of these are stable, they're all growing together. Commits on the app require changes in the platform, and in code review it is helpful to see things all together. Porting commits across different branches requires porting both the application change and the dependent platform changes. Often a client-specific branch will require severe one-off changes so the platform may diverge -- it is not practical for us (right now) to continually rebase client branches onto the latest platform.

this is just our experience, not facebook's, but lets face it: real life software isn't black and white, and discussion that doesn't acknowledge this isn't particularly helpful.

sek7• 14 years ago

This is what git submodules are for, but when they can't use them they don't have clear module boundaries.

snprbob86• 14 years ago

We've experienced this.

We've got a superproject with our server configs, and sub projects for our background processing, API, and web-frontend respectively.

Often, each project can evolve and be versioned 100% independently. However, often you need to modify multiple projects and (especially with server config changes) coordinate changes via the super project.

It's a little hairy sometimes and often feels like unnecessary overhead, but the mental boundary is extremely valuable on it's own. Being able to add a field to the API and check that commit into the superproject for deployment before the front end features are done is nice. The social impact on implementation modularity is valuable. We write better factored code by letting Git establish a more concrete boundary for us.

iamleppert• 14 years ago

I can believe this working with a former facebook employee. They do not believe in separating or distilling anything into separate repos. Why the fuck would you want to have a 15GB repo?

Ideally they should have many small, manageable repositories that are well tested and owned by a specific group/person/whatever. At least something small enough a single dev or team can get their head around.

Sheesh.

SoftwareMaven• 14 years ago

And then each of those dev teams can spend 1/2 their time writing code other people in the company have already written or every team can spend 1/2 their time publishing and reading documentation about what has been written.

There is no simple answer. There is only optimization for a particular problem-set you are trying to minimize.

brown9-22• 14 years ago

> And then each of those dev teams can spend 1/2 their time writing code other people in the company have already written or every team can spend 1/2 their time publishing and reading documentation about what has been written.

I don't see what this has to do with a discussion of one repo vs multiple repos.

You think that in a multi repo world, the engineers aren't as aware of what code exists and where as they are in a single repo world? You think that code duplication and needing to read docs magically doesn't exist in a single repo world?

The number of repositories is just an organizational construct. Communication still must take place no matter what.

earino• 14 years ago

If this was the crazy size of your git repo, why wouldn't you make a tool that took your git repo and versioned it? Keep it in repos that can all be performant, since most of the time you are working with "time local" information?

pwpwp• 14 years ago

Git was designed for the Linux kernel, and it's simply not big: a couple thousand files, broken up into directories of dozens or hundreds of files.

http://www.schoenitzer.de/lks/lks_en.html#new_files

ctz• 14 years ago

I'm surprised Facebook and all its peripheral development has that much source. I would expect something like 5-10 million lines of code, not ~100 million lines implied by the example.

nbm9• 14 years ago

The example is synthetic, so don't worry too much about the implications.

It is useful to keep in mind that Facebook isn't just the front-end (and isn't just code, also images, configuration, and so forth).

Just talking about open source stuff, Facebook also generates code like Cassandra, Hive (data warehousing application), Phabricator (a code review and lifecycle tool), HipHop for PHP (the translator/compiler, the interpreter, and the virtual machine), FlashCache (a kernel driver), Thrift, Scribe, and so forth.

We also have had to build applications to support our operations, so think about what sort of effort goes into building scalable monitoring, configuration management, automatic remediation, logging infrastructure, and so forth.

I don't know the actual lines of code across it all, and wouldn't mention it if I did, but people often underestimate the scale here.

moe• 14 years ago

And all of that must live in a single repository... because?

lbrandy• 14 years ago

Wow. I was expecting an interesting discussion. I was disappointed. Apparently the consensus on hacker news is that there exists a repository size N above which the benefits of splitting the repo _always_ outweigh the negatives. And, if that wasn't absurd enough, we've decided that git can already handle N and the repository in question is clearly above N. And I guess all along we'll ignore the many massive organizations who cannot and will not use git for precisely the same issue.

So instead of (potentially very enlightening conversation) identifying and talking about limitations and possible solutions in git, we've decided that anyone who can't use git because of its perf issues is "doing it wrong".

jamespo• 14 years ago

I would have liked this comment better if it came up with some solutions itself, although maybe it's not easy to solve?

djtriptych2• 14 years ago

That's the problem. It's NOT an easy problem to solve.

A lot of posts on hn describing some problem elicit "Why, that's no problem at all!" responses or "That's the wrong problem to think about" responses.

Honestly that mindset is often really useful in programming, but when we get a problem that doesn't have a shortcut and is relevent, conversation goes to shit. Because I guess that's when programmers normally go into a hole and brute-force brain it out.

How to use mass comms to talk about a difficult open problem is, I suppose, itself an open problem.

lincolnq• 14 years ago

I had the same reaction as you. </meta>

Stat'ing a million files is going to take a long time. Perforce doesn't have this problem because you explicitly check out files (p4 edit). (Perforce marks the whole tree read-only, as a reminder to edit the file before you save.)

It seems like large-repo git could implement the same feature. You would just disable (or warn) for operations which require stat'ing the whole tree.

Then the question is how to make the rest of the operations perform well -- git add taking 5-10 seconds seems indicative of an interesting problem, doesn't it?

ori_b3• 14 years ago

It seems to me that you could have a daemon that uses inotify to make operations O(changed) vs O(size).

joeyh2• 14 years ago

The git add problem is because .git/index is rewritten from scratch each time a new change is staged. With a 100 mb index file, that takes as long as it takes to write that much data to disk (cache). Much room for improvement here.

sek7• 14 years ago

You have a point.

It is just surprising when git was designed for the Linux kernel and we all here have a Github mindset.

kenrik• 14 years ago

With that in mind it seems like there is a market for a git replacement for these huge repos.

to3m• 14 years ago

I fear the market would be small.

It is my guess (though I have no proof) that most places with particularly large repositories have lots of binary files in them. It's hard to get a 15GB repository if you just have text.

This sort of thing suggests a centralized check-in/check-out model, because binary files are difficult to merge sensibly, and nobody wants to spend terabytes of hard drive space storing the repository locally. And your centralized check-in/check-out needs, whatever scale they might be, are probably tolerably well served by one of the existing solutions.

forrestthewoods3• 14 years ago

It's called Perforce and anyone dealing with binary files has been using it for years.

akkartik3• 14 years ago

You're right.

I found the original email equally disappointing, though. It boils down to "We pushed the envelope on size, it's too slow, we'd like to speed it up." Well, duh.

He uses the word 'scalability' early in the email, but shows no indication that he knows what it means. I'd love to hear if different operations slow down at different rates as the repo accumulates commits. Do they scale linearly, sublinearly, or superlinearly as the repo grows? Are there step functions at which there's a sudden dramatic slowdown (ran out of RAM, etc.)?

lnguyen5• 14 years ago

It's intentionally vague but with enough details that if you're actually in a position to help, you'll recognize what's going on and actually directly contact to get more information.

You don't spill internal processes and configurations without some kind of disclosure agreements and certainly not in a public forum.

kinofcain• 14 years ago

Your comment was at the top so I continued to read expecting to find a bunch of ignorant group think about how git is awesome and Facebook is dumb, but that's not really what's going on down below.

I don't know what facebook's use case is, so I have no idea if their repositories are optimally structured. However, I've used git on a very large repository and ran into some of the same performance issues that they did (30+ seconds to run git status), so I don't think it's terribly hard to imagine they're in a similar situation.

What we did to solve it is exactly what you're excoriating the people below for suggesting: we split the repos and used other tools to manage multiple git repos, 'Repo' in some situations, git submodules in others.

However, we moved to that workflow mainly because it had a number of other advantages, not just because it made day-to-day git operations faster.

I hope git gets faster, some of the performance problems described are things we saw too, but things are always more complicated and I see nothing below that looks like the knee-jerk ignorant consensus you're describing.

Sometimes the answer to "it hurts when I do this" is "don't do that... because there's other ways to solve the same issue that work better for a number of other reasons and we haven't bothered fixing that particular one because most of the time the other way works better anyway."

wisty• 14 years ago

On a simliar note, I've heard there of people who would hit the limit on fortran files, so they put every variable into a function call to the next file, which itself contained one function and a function call to the next file after that (if necessary).

Making stuff modular is often a good idea.

ay• 14 years ago

It is intuitively obvious that it is better to be rich and healthy than poor and ill. Sadly, the reality choices are neither. And you can not just split a repo. Hearing some ideas for that case would have been interesting.

Solving a scaling problem by splitting it is, well, obvious.

And, yes, I also ran github on a couple of projects at $work and the issues are real, seen them.

So, if it hurts when I try to use git - the answer will be don't use git... But the conveniences are so tempting...

robfig2• 14 years ago

In violent agreement here.

Git and HG: 1. Require you to be sync'ed to tip before pushing. 2. Cannot selectively check out files.

The former means that in any reasonably sized team, you will be forced to sync 30 times a day, even if you are the only one editing your section of the source tree. The latter means that Joe who is checking in the libraries for (huge open source project) for some testing increases everyones repo by that much, forever, even if it's deleted later.

Needless to say, the universal response is that I'm doing it wrong. Perforce 4 life!

But seriously, it says that Google adopted Git for their repo --- does anyone know how they use it? I would expect them to want a linear history, but their teams are way too big to be able to have everyone sync'ed to tip to push...

caf2• 14 years ago

Require you to be sync'ed to tip before pushing.

That's not the case. In fact, in the context of Linux kernel development, there's many emails on LKML where Linus is telling someone that they shouldn't be merging random-kernel-of-the-day into their development branch.

ori_b3• 14 years ago

Git is not used for their main repo. Git is used as a local cache for perforce where a branch roughly corresponds to a CL. Only subtrees of interest are checked out.

loeg• 14 years ago

I'm curious what their performance numbers look like if they host the .git repo on tmpfs -- 15GB isn't unreasonable on a beefy (24-32GB of ram) machine.

wmf2• 14 years ago

Probably the same as the warm cache results, since that's basically what tmpfs is. I wonder if git does all that stat()ing serially or in parallel, though...

durin42• 14 years ago

I don't have the link handy, but IIRC we did some experiments with that for Mercurial and found that stat() in parallel didn't really help much until you were using NFS or similarly slow network-type latency filesystems.

patangay2• 14 years ago

For what it's worth, Josh has been doing all his tests on machine with SSDs and 72GB of RAM.

jrockway3• 14 years ago

Yes, it's well known that big companies with big continuously integrated codebases don't manage the entire codebase with Git. It's slow, and splitting repositories means you can't have company-wide atomic commits. It's convenient to have a bunch of separate projects that share no state or code, but also wasteful.

So often, the tool used to manage the central repository, which needs to cleanly handle a large codebase, is different from the tool developers use for day-to-day work, which only needs to handle a small subset. At Google, everything is in Perforce, but since I personally need only four or five projects from Perforce for my work, I mirror that to git and interact with git on a day-to-day basis. This model seems to scale fairly well; Google has a big codebase with a lot of reuse, but all my git operations execute instantaneously.

Many projects can "shard" their code across repositories, but this is usually an unhappy compromise.

People always use the Linux kernel as an example of a big project, but even as open source projects go, it's pretty tiny. Compare the entire CPAN to Linux, for example. It's nice that I can update CPAN modules one at a time, but it would be nicer if I could fix a bug in my module and all modules that depend on it in one commit. But I can't, because CPAN is sharded across many different developers and repositories. This makes working on one module fast but working on a subset of modules impossible.

So really, Facebook is not being ridiculous here. Many companies have the same problem and decide not to handle it at all. Facebook realizes they want great developer tools and continuous integration across all their projects. And Git just doesn't work for that.

Splines• 14 years ago

At Google, everything is in Perforce, but since I personally need only four or five projects from Perforce for my work, I mirror that to git and interact with git on a day-to-day basis.

At MS we also use Perforce (aka Source Depot), and I've toyed with the idea of doing something similar. Have you found any guides for "gotchas" or care to share what you've learned going this route?

jrockway3• 14 years ago

I used git-p4 at my last job, and the only thing that ever got weird was p4 branches. At Google we have an internal tool that's similar to git-p4, and it always works perfectly for me. Enough developers are using it such that most of the internal tools understand that a working copy could be a git repository instead of a p4 client.

So if you're planning on doing this at your own company, my advice is to write your own scripts that make whatever conventions you have automatic, and to move everyone over at the same time. That way, you won't be the weird one whose stuff is always broken.

I think most people got burned by cvs2svn and git-svn and think that using two version control systems at once is intrinsically broken. It's not. svn was just too weird to translate to or from. (People that skipped svn and went right from cvs to git had almost no problems, I'm told.)

pnathan• 14 years ago

> Yes, it's well known that big companies with big continuously integrated codebases don't manage the entire codebase with Git. It's slow, and splitting repositories means you can't have company-wide atomic commits. It's convenient to have a bunch of separate projects that share no state or code,

Can you expand on this? I would love to talk more about the "well known" part, I've never run across it before. I am a maintainer (tools guy actually) of a hg repo with about 120 subrepos, and the whole approach with subrepos is something that we're not thrilled about. Oh, and if you want to communicate via email, I'd be up for that too.

thurn• 14 years ago

Facebook uses Subversion for its trunk, actually, and just gets developers to use git-svn. This issue is primarily a problem because git-svn is a lot more serious about replicating the true git experience (keep everything local) than Google's p4-git wrapper is. They really just need to be a little less religious about keeping everything local.

nbm9• 14 years ago

Just a minor clarification - some of the repositories use Subversion (as mentioned in the "Push" tech talk), and some are pure git.

jpdoctor2• 14 years ago

$100B company, maybe they can afford to put some people onto solving this for the open software community (and put the solution into the open), especially since nobody else in the community seems to have this problem.

rogerbinns• 14 years ago

If you proposed a good solution I'm sure they'd be happy to provide time and money and open source the result. But most of the responses aren't even that there is a solution - they say to split the repository into smaller pieces and spend time and money internally having their internal developers deal with that.

A good solution will benefit everyone who uses git. Codebases get larger over time. There is more forking and experimentation. More spoken languages can be supported. More computer languages can be interfaced. The O(n) operations becoming less than that will benefit you in the future as your code grows.

jpdoctor2• 14 years ago

> If you proposed a good solution I'm sure they'd be happy to provide time and money and open source the result.

If they provided money, I'd provide the time in order to produce a good result. See the problem?

More to the point: FB is all take and no give, as near as I can tell.

groby_b6• 14 years ago

They are putting people on this. Thankfully, those people are smart enough to ask for help before blindly going off and doing their own thing.

Now, if it's going to end up OSS, that's a different question. (I'm not implying it's not - I'm saying that's a decision that could go either way)

gokhan• 14 years ago

Large repos bring their own problems, and results in some design decisions accordingly. For example, Visual Studio itself is 5M+ files and this affected some of the the initial design decisions (Server side workspaces, for this example) when developing TFS 2005 (the first version) [1]. That decision suits MS but not the small to medium clients well. So they're now alternating that design with client side workspaces.

It's not wise to offer Facebook to split the repository. Looks like it's time to improve the tool.

[1] http://blogs.msdn.com/b/bharry/archive/2011/08/02/version-co...

julian372• 14 years ago

Somewhat off-topic, could somebody explain why

  echo 3 | tee /proc/sys/vm/drop_caches

rather than just

  echo 3 > /proc/sys/vm/drop_caches

Is it because the output to stdout lets you be extra sure that the right data was sent to the kernel?

I'm just wondering if this is an idiom with a deeper meaning that I'm not aware of.

EDIT: I'm guessing that when you run it in a script (without set -x), rather than on the command line, you can see in the log what it is you sent?

pdw• 14 years ago

Because you can

    echo 3 | sudo tee /proc/sys/vm/drop_caches

but

    sudo echo 3 > /proc/sys/vm/drop_caches

won't work.

julian372• 14 years ago

Right, that's what I was missing. That makes sense, and come to think about it, is a very useful addition to my toolbox. Thanks pdw and jochu!

tmhedberg• 14 years ago

You can also accomplish the same thing with:

    sudo sysctl -w vm.drop_caches=3

jochu• 14 years ago

Aside from reasons you mentioned, I can imagine it being because it easily allows one to add a sudo or being habit because of it. For example:

  echo 3 | sudo tee /proc/sys/vm/drop_caches

Will allow you to write as root and

  sudo echo 3 > /proc/sys/vm/drop_caches

Will be a permission error. It executes the echo as root and the write as the user

slashclee• 14 years ago

These times are for spinning-platter hard drives. I wonder what the numbers look like on a modern SSD?

xxiao• 14 years ago

git is not memory efficient by design, i used to push about 1G commit to the server and it hangs forever, i had to abort it and push in as small chunks instead

lnguyen5• 14 years ago

There's two issues: the width of the repository (number of files) and the depth (the number of commits).

Since "status" and "commit" perform fairly well after the OS file cache has been warmed up, that probably can be resolved by having background processes that keep it warm. (Also, how long would it take to just simply stat that number of files? )

The issue of "blame" still taking over 10 minutes: We need to know far back in the repository they're searching. What happen if there's one line that hasn't been changed since the initial commit? Are you being forced to go back to through the whole commit history?

How old is the repository? Years? Months? I'm probably guessing in the at least years range based on the number of commits (unless the developers are extremely commit-happy).

At a certain point, you're going to be better off taking the tip revisions off a branch and starting a fresh repository. It doesn't matter what SCM/VCS tool you're using (I've been the architect and admin on the implementation of a number of commercial tools). Keep the old repository live for a while and then archive it.

You'll find that while everyone wants to say that they absolutely need the full revision history of every project, you rarely go back very far (aka the last major release or two). And if you do need that history, you can pull it from the archives.

bos• 14 years ago

Facebook engineer here, working on this problem with Joshua.

What this comes down to is that git uses a lot of essentially O(n) data structures, and when n gets big, that can be painful.

A few examples:

* There's no secondary index from file or path name to commit hash. This is what slows down operations like "git blame": they have to search every commit to see if it touched a file.

* Since git uses lstat to see if files have been changed, the sheer number of system calls on a large filesystem becomes an issue. If the dentry and inode caches aren't warm, you spend a ton of time waiting on disk I/O.

An inotify daemon could help, but it's not perfect: it needs a long time to warm up in the case of a reboot or crash. Also, inotify is an incredibly tricky interface to use efficiently and reliably. (I wrote the inotify support in Mercurial, FWIW.)

* The index is also a performance problem. On a big repo, it's 100MB+ in size (hence expensive to read), and the whole thing is rewritten from scratch any time it needs to be touched (e.g. a single file's stat entry goes stale).

None of these problems is insurmountable, but neither is any of them amenable to an easy solution. (And no, "split up the tree" is not an easy solution.)

tonfa• 14 years ago

Nice to see you're still in the DVCS business ;)

Terretta• 14 years ago

> waiting on disk I/O

Out of curiosity, why are these benchmarks using regular disk and flash disk? At only 15 GB, what happens using ram disk? Sure SSD is fast, but for these things it's still really slow.

groby_b6• 14 years ago

An inotify daemon could help, but it's not perfect: it needs a long time to warm up in the case of a reboot or crash

So does, presumably, the cache when you use lstat. (Let's scratch presumably. It does. Bonus points if you can't use Linux and use an OS that seems to chill its caches down as soon as possible. )

I hope I'm wrong, but the proper solution to this seems to be a custom file system - not only will it allow you to more easily obtain a "modified since" list of files, it also allows you to only get local files "on demand". (E.g. http://google-engtools.blogspot.com/2011/06/build-in-cloud-a...)

That still doesn't solve the data structure issues in git, but at least it takes some of the insane amount of I/O off the table.

I'm looking forward to see what you guys cook up :)

caf2• 14 years ago

You might be able to do the "custom file system" as a pass-through FUSE filesystem.

dochtman• 14 years ago

Heh, a familiar name working on this!

So for your first item, it seems like it should be possible to add a (mostly immutable) cache file doing the job of Mercurial's files field in changesets, right? I.e. for each commit, list the files changed. Should be more efficient than searching through trees/manifests for changed files, at least.

For large (in files) trees, it seems like there's no easy solution, except for developing some kind of subtree support. However, that's similar to just splitting up the repository (along the lines of hg subrepo support), in the sense that now you have no real verification that non-checked-out parts of the tree will work with the changes in the part you do have checked out.

Still, the inotify daemon seems like it could alleviate things a bunch; particularly if the repository is on a server anyway, i.e. it's not rebooted that often.

DrCatbox• 14 years ago

You seem to know enough about this problem to solve it given enough time.

Why doesnt facebook solve this git on huge repos problem and put out a patch for others to see? Oh, right, you want somebody else to solve the problem for you, for free!

joelthelion2• 14 years ago

I see you've given the issue a lot of thought.

Sorry for asking the obvious, but do you really need huge amount of data to keep development productive? How often do you use history that is several years old? Could you not archive it?

Or is the sheer number of files the problem, even ignoring history?

This is not a "git is perfect, fix your workflow" post, but I'm genuinely interested in what you have to say. Also, it seems like making git faster is a increasingly difficult task, given the amount of effort that has already been put into it.

willvarfar• 14 years ago

Do you think adapting git to use, say, LevelDB and letting that do its job with incremental updates and maintaining a secondary index by path could help?

And that might dovetail nicely with an inotify daemon?

ramanujan• 14 years ago

This looks like it could be of assistance:

http://source.android.com/source/version-control.html

  Repo is a repository management tool that we built on top 
  of Git. Repo unifies the many Git repositories when 
  necessary, does the uploads to our revision control 
  system, and automates parts of the Android development 
  workflow. Repo is not meant to replace Git, only to make 
  it easier to work with Git in the context of Android. The 
  repo command is an executable Python script that you can 
  put anywhere in your path. In working with the Android 
  source files, you will use Repo for across-network 
  operations. For example, with a single Repo command you 
  can download files from multiple repositories into your 
  local working directory.

http://google-opensource.blogspot.com/2008/11/gerrit-and-rep...

  With approximately 8.5 million lines of code (not 
  including things like the Linux Kernel!), keeping this all 
  in one git tree would've been problematic for a few reasons:

  * We want to delineate access control based on location in the tree.
  * We want to be able to make some components replaceable at a later date.
  * We needed trivial overlays for OEMs and other projects who either aren't ready or aren't able to embrace open source.
  * We don't want our most technical people to spend their time as patch monkeys.

  The repo tool uses an XML-based manifest file describing 
  where the upstream repositories are, and how to merge them 
  into a single working checkout. repo will recurse across 
  all the git subtrees and handle uploads, pulls, and other 
  needed items. repo has built-in knowledge of topic 
  branches and makes working with them an essential part of 
  the workflow.

Looks like it's worth taking a serious look at this repo script, as it's been used in production for Android. Might allow splitting into multiple git repositories for performance while still retaining some of the benefits of a single repository.

jmccaffrey• 14 years ago

Having worked with repo professionally, I'm not a fan. You lose simple ability to track dependencies across repositories or even revert to a previous consistent point in time without diligent tagging. Even with good tags, restructuring your project setup and changing your repo manifest can still break your ability to go back in time.

cdibona• 14 years ago

Basically, if you want to manage a large collection of git source repositories, you'll probably end up using Repo and Gerrit and piggybacking on the work of the android ecosystem (and beyond, gerrit is used all over the place now)

There really isn't another solution out there right now (at least not anything open source) for very large single repositories.

exDM693• 14 years ago

> There really isn't another solution out there right now

What about Git submodules? They do fundamentally the same thing as Repo, but it's a built-in Git feature and not a bunch of scripts.

Repo can make your life very hard and you have to be a black belt Git ninja to understand what's going on when things don't go as you intended. Git submodules don't depend on having arbitrary GUID strings in your commit messages either (like Repo's Change-Id).

GitHub's reviews can handle Git submodules (but it's not free or open source). If someone knows any open source code review tools that can handle, please tell us.

Sorry for beating a dead horse, but I really want to save someone from fucking up (or at least re-centralizing) their workflow with repo scripts, when native git is better.

exDM693• 14 years ago

> Looks like it's worth taking a serious look at this repo script, as it's been used in production for Android. Might allow splitting into multiple git repositories for performance while still retaining some of the benefits of a single repository.

Stay away from Repo and Gerrit. I use them at work, and they make my life miserable.

Repo was written years ago when Git did not have submodules, a feature where you can put repositories inside repositories. Git submodules is far superior to Repo, and allows you to e.g. bisect the history of many repositories.

I'm hoping that Google comes to it's senses and starts phasing out Repo in favor of Git submodules in Android development.

joelthelion2• 14 years ago

What about gerrit?

DannoHung2• 14 years ago

Multiple people in this conversation section have asserted that code sharing is way easier when all the code is in a single repo, but from my understanding of sub-modules, it would be a fairly simple matter of setting up your pre/post-commit hooks to update submodules to a branch automatically and get useful company wide change atomicity (after all, changes should only propagate between teams/projects once they have some stability).

Putting aside the question of whether or not an enormous singular repo can be broken up intelligently into modular projects, is there something about the submodule approach that makes it a uniquely unsuitable way for sharing changes amongst projects?

groby_b6• 14 years ago

If you limit change propagation, your changes won't propagate as fast. That goes for bugs and bug fixes.

I can certainly see why you would have the latter propagated instantaneously, or close to it.

There's also the point that if you don't propagate change to everybody at the same time, you'll have dozens of slightly different versions of those projects across your company. The question of submodules vs. large repo is not as easily decided as you think - there are large upsides (and downsides) to both approaches.

DannoHung2• 14 years ago

No, you're misunderstanding what I'm saying with regards to publishing stable changes.

Say you have two branches: master and next. Stable work goes in master, unstable work goes in next. When the code is ready for consumption, you merge it into master.

Anyone who is using a project has it setup as a submodule. They add post/pre-commit hooks to update all project submodules. These submodules pull from master.

This way, everyone will get all stable changes on all submodule projects at the time of the next change to their own project.

alok-g• 14 years ago

Is anything known for scalability to such sizes for Subversion, Mercurial, Bazaar, and others?

charlieok• 14 years ago

I think it's a bad practice to keep a giant code base in one repo. Split the code base into purpose-specific modules, just as you would split any project into purpose-specific modules. In fact, those two things might well line up 1:1.

If a project depends on other projects, have it reference the other projects. Where appropriate, include exact version numbers and/or commit hashes. Gemfiles are good examples of this good practice at work.

Yes, git has submodules for this sort of thing, but after investigating that route, I decided against using git submodules. Use something independent of the VCS instead. Then git won't do weird or unexpected things when you switch branches. Also, you might want to mix in projects that use other version control systems. And really, why unnecessarily couple a project to its version control system?

If (when?), even after splitting a megaproject into manageable subprojects, these performance issues creep in, I'd certainly be interested in whatever improvements people are coming up with...

teyc• 14 years ago

This is an interesting social AND a technical problem. The problem for FB is that it is all too easy for them to just fork git, create the necessary interfaces and then hope the git maintainers would accept it (they mightn't) or release it into the wild (and incur bad karma and wrath of OS developers who'd see this has schism or even heresy).

They've reached out to the developers on git, and I guess that's a first step.

r15habh2• 14 years ago

Does Facebook really believe that because they have the most users, they should also have the biggest git repo?

Amazon and Google have already solved this problem, and the solution is to reorganize things into smaller manageable packages.

r15habh2• 14 years ago

What I mean is, even if they manage to solve this problem with some tweaks, they will again hit the bottleneck in an year or two, so they should rethink their source code management

redstone• 14 years ago

This is Joshua (who posted the original email). I'm glad to see so much interest in source control scalability. If there are others who have ever contemplated investing a bit of time to improving git, it'd be great to coordinate and see what makes sense to do - even if it turns out that the right answer is just to make the tools that manage multiple repos so good that it feels as easy as a single repo.

railsmax• 14 years ago

Hey do you know a lot of sites with such needs? Facebook is the first, and probably all sites with such needs I can count with fingers on my one hand. I don't think it's git issue - everyone use this system and all are happy using it. This is like a new feature, but not issue.

MikeOnFire• 14 years ago

My first thought, as suggested by some on the list, was modularization. Redstone's response (that the 1.3 million files are essentially all interdependent) terrifies me.

Facebook hit git performance issue on large repository

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