4 ms·

There's a patch to make the current conservative GC a precise GC, which will totally fix this problem. It's in the process of being reviewed and integrated into

• pcwalton 14y ago
  That's awesome, kudos. How do you deal with maintaining the distinction between pointers and values on the stack and in registers? I didn't see any code to do that, skimming through that patch. Since it's a precise GC, you need to maintain the distinction. In Rust we have a very large patchset to LLVM to allow us to do this; it involves modifying nearly every stage of the LLVM pipeline. Curious how you did that in gccgo. Edit: I just looked at the source and it still seems conservative for the stack and registers. So it's not a precise GC yet and will not eliminate false positives. (This is of course totally fine, I just wanted to clarify for myself.)
  • enneff 14y ago
    This isn't my patch, so I can't speak to the specifics of it. Also, it isn't for gccgo, but the gc compiler.
    • lobster_johnson 14y ago
      That's too bad. Hopefully someone will improve `gccgo` as well. I actually started out experimenting with Go using the `gc` compiler, and found performance (basically manipulating large arrays sequentially) atrocious. Moving to `gccgo` improved performance four-fold, making my unoptimized Go test code in some cases faster than my unoptimized C test code.
      • luriel 14y ago
        How long ago was that? The gc compilers have seen lots of improvement recently, and more will be merged soon. Also, if the problem is so pathological, it might be worth filling an issue.
        • lobster_johnson 14y ago
          Go 1.0.2, which is the newest stable release. I will see if I have some extra time to file a report. Slightly off topic (but relevant to my comment above), do you know why this: func TraverseInline() { var ints := make([]int, N) for i := range ints { sum += i } } should be slower than: func TraverseArray(ints []int) int { sum := 0 for i := range ints { sum += i } return sum } func TraverseWithFunc() { var ints := make([]int, N) sum := TraverseArray(ints) } Seeing a huge difference here. With an array of 300000000 ints, TraverseInline() takes about 750ms, whereas TraverseWithFunc() takes 394ms. With gccgo (GCC 4.7.1), the different is much slighter, but there's still a difference: 196ms compared to 172ms. (These are best-case figures, I'm doing lots of loops.) Go is compiled, not JIT, so I don't see why a function should offer better performance.
          • agentS 14y ago
            Not sure why the difference in performance between function and non-function, particularly considering this looks inline-able. 2 theories on why gcc is doing better. Maybe its unrolling the loop, and avoids a bunch of jumps. Alternatively, it could be using SSE, but this theory is a much less likely, because I would expect it to be 4 times faster not twice as fast. gc doesn't use SSE instructions yet.
          • genbattle 14y ago
            I don't have any immediate insight for the difference in performance you're seeing (my hunch is that it's to do with where and how memory is being allocated), but I would suggest you ask about it on the Go-Nuts mailing list: https://groups.google.com/forum/?fromgroups#!forum/golang-nuts https://groups.google.com/forum/?fromgroups#!forum/golang-nu... The Go authors should be very receptive and informative on such an issue.
  • sausagefeet 14y ago
    Is this patch going to be part of LLVM? Is one out of luck if they want to implement a precise collector in LLVM today?
    • pcwalton 14y ago
      I've talked with Chris Lattner about upstreaming it and the LLVM developers seem open to it when it's ready. In the meantime you can find the most recent work here: https://github.com/elliottslaughter/llvm/tree/noteroots-ir https://github.com/elliottslaughter/llvm/tree/noteroots-ir Currently, you are mostly out of luck if you try to use mainline, unless you're okay with the llvm.gcroot intrinsics, which pin things to the stack so they can never live in registers (causing a corresponding performance loss). Probably your best bet is to do what Go will do once the patch enneff was referring to is merged: scan conservatively on the stack and precisely on the heap. You can still get false positives and leaks, and you can't do moving GC that way, but it's better than fully conservative GC. By far, the hardest part of GC is precisely marking the stack and registers. Yet you have to be able to do it if you want to prevent leaks. Our goal is to put in the hard work so that new languages will be able to have proper GC without having to roll a custom code generator or target the JVM or CLR.
      • sausagefeet 14y ago
        > Our goal is to put in the hard work so that new languages will be able to have proper GC without having to roll a custom code generator or target the JVM or CLR. As someone who has ambitions of writing a fun little language this is exactly what I want out of LLVM. Can't wait for your patches to hopefully get to mainline.
• thebluesky 14y ago
  32bit memleak is by far the main reason I stopped looking at go. Many people still have plenty of 32 bit hosts. When will this be in the official release??
  • enneff 14y ago
    It'll be in Go 1.1, around the end of the year.