Messages in this thread | | | Date | Wed, 13 Aug 2008 12:44:45 +0200 | From | Ingo Molnar <> | Subject | Re: pthread_create() slow for many threads; also time to revisit 64b context switch optimization? |
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* Pardo <pardo@google.com> wrote:
> As example, in one case creating new threads goes from about 35,000 > cycles up to about 25,000,000 cycles -- which is under 100 threads per > second. [...]
> Various things would address the slow pthread_create(). Choices > include: > - Be more platform-aware about when to use MAP_32BIT. > - Abandon use of MAP_32BIT entirely, with worse performance on some machines. > - Change the mmap() algorithm to be faster on allocation failure > (avoid a linear search of vmas).
Sigh, unfortunately MAP_32BIT use in 64-bit apps for stacks was apparently created without foresight about what would happen in the MM when thread stacks exhaust 4GB.
The problem is that MAP_32BIT is used both as a performance hack for 64-bit apps and as an ABI compat mechanism for 32-bit apps. So we cannot just start disregarding MAP_32BIT in the kernel - we'd break 32-bit compat apps and/or compat 32-bit libraries.
There are various other options to solve the (severe!) performance breakdown:
1- glibc could start not using MAP_32BIT for 64-bit thread stacks (the boxes where context-switching is slow probably do not matter all that much anymore - they were very slow at everything 64-bit anyway)
Pros: easiest solution. Cons: slows down the affected machines and needs a new glibc.
2- We could introduce a new MAP_64BIT_STACK flag which we could propagate it into MAP_32BIT on those old CPUs. It would be disregarded on modern CPUs and thread stacks would be 64-bit.
Pros: cleanest solution. Cons: needs both new glibc and new kernel to take advantage of.
3- We could detect the first-4G-is-full condition and cache it. Problem is, there will likely be small holes in it so it's rather hard to do it in a sane way. Also, every munmap() of a thread stack will invalidate this - triggering a slow linear search every now and then.
Pros: only needs a new kernel to take advantage of. Cons: is the most complex and messiest solution with no clear benefit to other workloads. Also, does not 100% solve the performance problem and prolongues the 4GB stack threads hack.
i'd go for 1) or 2).
Ingo
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