Interning in Go

https://medium.com/google-cloud/interning-in-go-4319ea635002

by todsacerdoti • 2 years ago

140 46 2 years ago

I missed the initial blogpost about this; thanks for the solid explanation and the links. Probably won't make much of a difference for my use cases but cool to know this is now in the stdlib.

liotier• 2 years ago

Is this essentially dictionary compression ?

nine_k• 2 years ago

It's similar. It's just not by word, but by entire string, assuming that strings are immutable, long enough, and the same string values are referenced often enough.

On 64-bit machines though strings shorter than 8 bytes are better off not interned.

jfoutz2• 2 years ago

as I read it, it's any struct! Not just strings. which is cool.

recursive3• 2 years ago

No. This is avoiding keeping multiple copies of the same value. There's no compression.

VWWHFSfQ• 2 years ago

That's pretty much the definition of compression

morkalork• 2 years ago

This is new for Go? I remember learning about Java string interning decades ago in the context of xml parsers. If I remember correctly, there were even some memory leaks associated with it and thread locals?

pphysch• 2 years ago

You could've implemented bespoke interning at any point in Go; it was added to the standard library only recently, though, likely because it may leverage Go's relatively recent support for generics.

jfoutz2• 2 years ago

Interning is neat. Most of my experience is really dated. Primarily in the JVM, and mostly for class names, for reflection and class loaders. It's sort of surprising seeing this added to go, with its desires for minimalism. But when you can use it, it can be a big win.

Look past the "loading the whole book in memory" the author gets to the point soon enough.

The ip address example is ok. It's true, and highlights some important points. But keep in mind pointers are 64 bit. If you're not ipv6, and you're shuffling a lot of them, you're probably better off just keeping the uint64 and converting to string and allocating the struct as needed. interning doesn't appear to be much of a win in that narrow case. but if you do care about ipv6, and you're connecting to millions of upstreams, it's not unreasonable.

It's neat it's available. it's good to be aware of interning, but it's generally not a huge win. For a few special cases, it can be really awesome.

** edit uint32 for ipv4. bit counting is hard.

oefrha• 2 years ago

No, the example isn't about IP address octets, it's about IPv6 zones — we're talking strings like "eth0".

wjholden2• 2 years ago

Fun fact: in Go, an IPv4 address is internally represented as an IPv6 address, starting with ten zeroes and two 0xffs. The IPv4 address is copied in the last four bytes.

https://cs.opensource.google/go/go/+/refs/tags/go1.23.1:src/...

This is called an IPv4-Mapped Address.

https://www.rfc-editor.org/rfc/rfc5156#section-2.2

User23• 2 years ago

For reference, the term comes from Lisp’s INTERN. [1]

[1] http://clhs.lisp.se/Body/f_intern.htm

survivedurcode• 2 years ago

Beware the trade-offs of interning affecting GC behavior. Now you can’t have a stack-allocation optimization, for example.

tapirl2• 2 years ago

The interning feature should be only used for some rare cases, it is not intended to be used widely.

pjot• 2 years ago

  > I have a very large plaintext file and I’m loading it fully in memory. This results in a string variable book that occupies 282 MiB of memory.

At what point does something become (very) large?

sethammons• 2 years ago

With not really big numbers, like this one, I think of having n workers each having to do a thing. Every time you save on the memory footprint, you can grow the number of workers. Sometimes that is really really good. Usually it is probably meh

smellybigbelly• 2 years ago

Couldn’t you read in the book a bit smarter by deduplicating the io stream?

guappa2• 2 years ago

Let's say that you didn't read the book, but merely mmaped it.

favflam• 2 years ago

Is this the same as grpc.SharedBufferPool? The gRPC implementation does a lot of memory allocation.

cherryteastain• 2 years ago

Cool idea, but sounds detrimental in terms of cache efficiency. Typically processing a string by reading it sequentially is quite cache efficient as the processor will prefetch, but with this method it seems like the string will not be contiguous in memory which will lead to more cache misses.

SwiftyBug• 2 years ago

Seems like the classic trade-off of compute time x memory.

pimeys• 2 years ago

It's quite common if you need to parse a schema and keep it in memory for a long time. We're working on GraphQL federation gateway and interning strings is really useful there due to schemas sometimes being hundreds of megabytes.

Writing your own interner is under hundred lines of code and takes about 15 minutes. Writing a thread-safe interner is a bit trickier problem, but there are good libraries for that.

ctz• 2 years ago

Lots of small strings sparsely spread over 282MB is pretty cache inefficient?

Whereas the separate small allocations will likely end up close together, which gives a better chance that a given cache line will contain several rather than just one.

pjmlp3• 2 years ago

Interestingly enough, by following up some references of the article I discovered that Go is also following up on Java and .NET design decisions, that maybe could be there early on.

- Deprecating finalizers and using cleaner queues (https://openjdk.org/jeps/421)

- Weak references (https://learn.microsoft.com/en-us/dotnet/api/system.weakrefe..., https://docs.oracle.com/javase/8/docs/api/java/lang/ref/Weak...)

Related tickets,

"runtime: add AddCleanup and deprecate SetFinalizer" - https://github.com/golang/go/issues/67535

"weak: new package providing weak pointers" - https://github.com/golang/go/issues/67552

One day Go will eventually have all those features that they deemed unnecessary from "complex" languages.

nasretdinov• 2 years ago

Note that both of those features use generics, which weren't present in the language before, so IMO Go just prefers to wait a bit before implementing useful features instead of having too many of them (ahem, Swift?)

pjmlp3• 2 years ago

They use generics because they are available now, otherwise most likely they would be magic functions like before.

badhombres• 2 years ago

I think that’s a bit of a stretch to say go will implement all the features of c# and Java because of a few new features. Go isn’t a frozen language, they just take a lot of time and discussion before committing to a major change.

pjmlp3• 2 years ago

The point isn't implementing all the features of c# and Java, rather doubling down on their simplicity mantra against all programming language complexity, only to revisit such decisions years later, because after all the other programming languages had good reasons to have such features in first place.

nickcw• 2 years ago

The unique package is my top feature for go1.23. I've been experimenting with it in rclone.

People often want to have millions of S3 objects in memory and reducing the memory used would be very desirable.

I interned all the strings used - there are a lot of duplicates like Content Type and it reduced the memory usage by about 30% which is great.

I wonder how much difference this little fix mentioned in the article for go1.23.2 will make? https://github.com/golang/go/issues/69370

The article also mentions strings.Clone which has been around for a while. Using that is very easy and it stops big strings being pinned into memory. I had a problem with this in the S3 backend where some string was pinning the entire XML response from the server which strings.Clone fixed.

tapirl2• 2 years ago

Be aware that there is a bug in the current implementation (1.23.0 and 1.23.1) https://github.com/golang/go/issues/69370

guappa2• 2 years ago

Eh? Seems rather trivial to me. In python one could implement it by themselves with a set, a list, and a for loop.

It hardly seems worthy having a library over such a specific use case.

Someone• 2 years ago

You can’t do exactly this if you’re not in control of all code.

With this, if a third-party library interns a string whose content is “Foo” they get a Handle. If you also intern a string whose content is “Foo”, you get the same Handle back.

Having this in the standard product likely will make all code doing what you propose migrate to use unique.

Also, the implementation likely (I haven’t checked this implementation) will move any interned objects out of the memory range(s) inspected when doing garbage collection. That makes garbage collection do a bit less work.

dewey• 2 years ago

It hardly seems worthy having a library over such a specific use case.

Seems much better to have it in the stdlib like they do right now than having to import a third party dependency. Especially as such a low level package will probably be used by _other_ external packages you would be using.

saylisteins2• 2 years ago

Will this work across different goroutines?

val_deleplace3• 2 years ago

Yes, the package is designed to be thread-safe

val_deleplace3• 2 years ago

I tested with many concurrent goroutines, and confirmed that calling `unique.Make` does not cause any data race condition

saylisteins2• 2 years ago

Thanks a lot! I also found this blog post where they mention it's thread safe thanks to using HashTrieMap internally

Link: https://go.dev/blog/unique

Interning in Go

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