The curious case of slow downloads

https://blog.cloudflare.com/the-curious-case-of-slow-downloads/

by jgrahamc • 10 years ago

207 55 10 years ago

blog.cloudflare.com

speps• 10 years ago

Original title doesn't mention "NGINX".

Dylan168078• 10 years ago

Context in a title is useful.

oarsinsync• 10 years ago

Not an NGINX problem either

gruez• 10 years ago

You're correct, but the title is way less helpful without the "nginx". Unless the reader knows that cloudflare uses nginx, he won't know the article pertains to nginx

masklinn8• 10 years ago

Is it now? nginx seems to have pretty normal behaviour here, so the core issue is write-polling buffers on linux rather than nginx itself. It seems to me nginx was the messenger of sorts, but the issue doesn't really pertain to nginx.

In a nearby comment, LoSboccacc notes that they've hit that issue [removed: on S3].

bigethan4• 10 years ago

This is why open source (and controlling your whole stack) matters in big business.

For example, Microsoft may be changing their image, but their core software is closed source. <cloud provider> may be great, but can you pull off something like this when you've got an issue? The importance of being able to debug and patch your mission critical systems is hard to overstate.

Please encourage your employer to fiscally support the open source code that they rely upon!

masklinn8• 10 years ago

For example, Microsoft may be changing their image, but their core software is closed source. <cloud provider> may be great, but can you pull off something like this when you've got an issue? The importance of being able to debug and patch your mission critical systems is hard to overstate.

Not OSS != no source access. A cloud provider on top of the MS stack would most likely have Shared Source Initiative licenses.

bigethan4• 10 years ago

What's that cost? How quickly can it be done? If I was a developer would I have to ask my manager to look into the details of our support plan? What about the ability to patch the source?

`git clone` is an incredible power. Companies should reward those who are generous with their code.

rakoo• 10 years ago

Cloudflare was allowed to patch their nginx with the correct solution. Would the Shared Source Initiative allow you to do that, ie patch and run the modified version ?

userbinator• 10 years ago

Not OSS != no source access.

Furthermore, no source access != no way to fix problems.

People have been fixing things with no source for ages, EULAs be damned. If the fix is literally flipping a single bit, the decision is easy. Just do it. The vendor doesn't need to know nor care what we did to fix it, but often giving a detailed description to them of the problem --- one that comes out of the process of fixing it --- will help them fix it quickly and distribute to their other customers too. I've had this experience a few times.

Obviously, "the byte at 0x73F441 of somefile.dll should be 31, not 32" is not how you should communicate such things to the vendor, but they do appreciate your effort in debugging the issue instead of immediately blaming it on a fault in their software. ;-)

(And relatedly, no source access != no way to find problems either, as the security community shows quite plainly...)

rkangel• 10 years ago

There's a different model in this case. You pay for support, and then the original developers go away and fix it for you.

When this works well, it's great. You've got the experts on the problem. Not all support is good though, obviously, and if you have a mission critical problem that you need to fix overnight, nothing beats the 'self-support' that you can do with open source software.

bigethan4• 10 years ago

Totally! Pay for support works when it's not urgent. But when your biz is hurting, it's hard to translate that urgency to another company that has _entirely_ different priorities than you do.

Reminds me of the heroku shaming posts that (then) rap genius had to do regarding dyanamo scaling and performance - http://genius.com/James-somers-herokus-ugly-secret-annotated They had no other way forward, even though they were paying the "experts"

This is why companies should pay open source contributors more frequently :-)

LoSboccacc• 10 years ago

ah! got bit by this and had to workaround serving stuff from s3. nice to see there was an actual problem and wasn't just some issue with our stack.

BugsBunnySan3• 10 years ago

Related story time: At university, we ran a local quakeworld server (yes, this was in the stone age XD). And I wanted to write a tool to allow users to control to server. It would listen on a network port and pipe its input into the quakeworld server running as a child subprocess.

Sounds simple enough, right? Disregarding accept(2)ing connections,etc... it's just going to sit in an endless loop, select(2) on the sockets then read what comes in, and write that to the quakeworld server.

To debug this, everytime someone sent some data over the network, it would write a simple message ("blah") into a file on my home drive, called "~/blah" (very sophisticated debugging, I know).

Now, the semantics of select(2), cousin of poll(2), are interesting.

My understanding: select(2) blocks until you can read data from the socket.

Actual behaviour: select(2) blocks until a subsequent read on the socket does not block.

Fun fact: if there's not data available on the socket, read does not block, but returns with an error immediatly.

Result: Over the weekend, the process was spinning in an endless loop, writing 'blah' into a file on the nfs-home drive as quickly as the filesystem allowed. This being a university scale solaris NFS server, that's pretty fast.

End Result: A test file called 'blah', filled with about 800 GB of 'blah'. Good thing the admins where nice people and didn't shoot me when I went to explain the next monday why the home drive was filled with "blah" XD

EDIT: read(2) doesn't actually error on EOF, just returns 0, effect is the same though.

ajnin• 10 years ago

Read will return 0 when the peer has closed the connection, that's how you detect it. When there is no data, the call will block instead until some data arrives (unless the socket is set to non-block mode).

BugsBunnySan3• 10 years ago

Hm yeah, it could've been that I disconnected from the server when I went home for the weekend and didn't check for that in the code. It was a long time ago :)

brazzy• 10 years ago

Good thing the admins where nice people and didn't shoot me when I went to explain the next monday why the home drive was filled with "blah" XD

They should be the ones explaining why they don't have disk quotas set up on a shared system.

BugsBunnySan3• 10 years ago

AFAIR, they had one, which was so flakey they had to disable it all the time.

But yeah, maybe them being somewhat at fault as well helped me out a bit :)

colejohnson66• 10 years ago

You have QuakeWorld and almost a terabyte in the same story. It seriously managed to write 800 gigs!? How did they have that much storage?

mjevans• 10 years ago

Wikipedia pegs QW as existing around 1996-2000...

https://en.wikipedia.org/wiki/QuakeWorld

Now, 800Gigs sounds like a lot, but I distinctly also recall that time as the era when we ran across 8GB clip issues. It wouldn't be unheard of for drives to actually be 20-60GB in size around this era, maybe larger for the expensive SCSI kind.

Getting 800GB online storage with something like RAID5 or RAID6 would probably take two fully loaded controllers; lets say 10 drives of usable storage across two arrays. (20 drives). That back of the envelope math is about 40GB per drive.

I could see this being an easy case at a LARGE university in some tech centrist area at about the timeframe of 16-18 years ago.

bluesign• 10 years ago

I guess dropping very slow connections are also kind of protection against DoS or DDOS attacks, where attacker can drain resources.

scurvy4• 10 years ago

Slow reads are not a great attack vector on modern OS releases.

Slow requests (like slowloris) were much more effective.

scurvy4• 10 years ago

I really wish people would stop misusing TCP resets. It's not just a fast way to close a connection. That's not what it's for at all. If you're ever thinking about sending a reset, please read RFC 793 and RFC 3360 first.

I'm looking at you Arbor.

Dylan168078• 10 years ago

Well the goal here seems to be aborting a connection, not just closing it. They specifically don't want to empty the buffer, because time is up. Is that a misuse of reset?

scurvy4• 10 years ago

Yes, it is a misuse of a TCP reset. The RFCs are very clear about when to use a TCP reset.

You can't use a reset just because you want to be fast or don't feel like sending any more data. Resets are for something abnormal happening. A timeout isn't abnormal to a TCP connection. It's part of the process.

Yrlec• 10 years ago

Great that they solved it! My company (degoo.com) had to use CloudFront instead of CloudFlare for our binaries because of this bug. Our conversion rate went down about 10% whenever we used CloudFlare instead. Perhaps time to switch back.

Animats• 10 years ago

They just rediscovered silly window syndrome, but at the application level rather than the TCP level.

In early TCP implementations, if you read from a TCP connection one byte at a time, and the sender was writing faster than the receiver, the sender TCP would get an ACK opening one byte of window. The sender TCP would then transmit a packet with only one byte. This often came up if a TCP connection was driving a slow device. The fix was to not send a window update until there was at least one full packet worth of window available.

This is the same problem, but at the OS buffering level. Linux has a similar solution. The problem is 1) servers today have to protect themselves against readers that are "too slow", because that can be used as a denial of service attack, and 2) the approach to doing this was not well designed.

This is a tough one for Cloudflare, because they're in the DDOS-prevention business. They have to decide whether a slow reader is an attacker or just someone on a slow connection. Still, insisting that a reader consume at least 5MB/minute is a bit much. Some people are still on dialup.

The curious case of slow downloads

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