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SubjectRe: pthread_create() slow for many threads; also time to revisit 64b context switch optimization?

* 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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