Optimizing Heap Allocations in Go: A Case Study

https://www.dolthub.com/blog/2025-04-18-optimizing-heap-allocations/

by ingve • 1 year ago

54 10 1 year ago

It's possible that this is a compiler bug. It's also possible that there's some fringe case where the reference actually can escape via that method call, and the compiler doesn't have enough context to rule it out.

Here's an example, I think: suppose the method spawns a new goroutine that contains a reference to `chunkStore`. This goroutine can outlive the `ReadBytes` function call, and thus Go has to heap allocate the thing being referenced.

In general, this kind of example makes me suspect that Go's escape analysis algorithm treats any method call as a black box and heap allocates anything being passed to it by reference.

athorax2• 1 year ago

  The notion of stack vs heap allocation isn't something that even exists in the language. Users are expected to not worry about it... until, of course, until you're optimizing performance and you need to worry about it.

This is one of the best and worst aspects with Go. Anyone can write pretty performant code without having to understand the underlying memory model. If you get to the point where you are trying to optimize at this level, the benefits of using a more approachable language start to fall apart and you spend more time chasing implementation details.

nu11ptr• 1 year ago

In general, it is a win, since it lets you code faster and 80-90% the performance doesn't matter. Over time, you learn generally what leads to heap allocs and what doesn't. In rare hot spot, using -m will show you the allocations and you can optimize.

athorax2• 1 year ago

I would generally agree. It's good enough performance for most applications. For those that it isn't fast enough for (even with optimizations like these), it still allows for rapid prototyping to arrive at that conclusion.

Ygg2• 1 year ago

I think same applies to any GC language. Ride is fun until GC starts either taking too much time, too much memory or taking too much of CPU.

Thaxll• 1 year ago

At least you have the tools to understand where things get allocated.

38• 1 year ago

instead of this:

    t.Buf = []byte{}

you can just do:

    t.Buf = nil

rsc• 1 year ago

Those are semantically different (one is nil and one is not) but neither allocates.

virexene• 1 year ago

I wonder if the reason the escape analysis fails could be that, for small enough types, the concrete value is directly inlined inside the interface value, instead of the latter being "a smart pointer" as the author said. So when the compiler needs to take a reference to the concrete value in `vs.chunkStore`, that ends up as an internal pointer inside the `vs` allocation, requiring it to be on the heap.

Either that or the escape analysis just isn't smart enough; taking a pointer to an internal component of an interface value seems like a bit of a stretch.

Snawoot• 1 year ago

I had an attempt to improve performance of memory allocation with the use of arenas in Go and I chose freelist datastructure[1]

It almost doesn't use unsafe except one line to cast pointer types. I measured practical performance boost with "container/list" implementation hooked to my allocator. All in all it performs 2-5 times faster or up to 10 times faster if we can get rid[2] of any and allocations implied by the use of it.

All in all, heap allocations can be not that bad at all if you approach them from another angle.

[1] https://github.com/Snawoot/freelist

[2] https://github.com/Snawoot/list

Optimizing Heap Allocations in Go: A Case Study

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