Messages in this thread Patch in this message | | | Date | Fri, 15 Aug 2008 18:03:38 +0200 | From | Ingo Molnar <> | Subject | Re: pthread_create() slow for many threads; also time to revisit 64b context switch optimization? |
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* Jamie Lokier <jamie@shareable.org> wrote:
> > how about this then? > > > +#define MAP_64BIT_STACK 0x20000 /* give out 32bit addresses on old CPUs */ > > I think the flag makes sense but it's name is confusing - 64BIT for a > flag which means "maybe request 32-bit stack"! Suggest: > > +#define MAP_STACK 0x20000 /* 31bit or 64bit address for stack, */ > + /* whichever is faster on this CPU */
ok. I've applied the patch below to tip/x86/urgent.
> Also, is this _only_ useful for thread stacks, or are there other > memory allocations where 31-bitness affects execution speed on old > P4s?
just about anything i guess - but since those CPUs do not really matter anymore in terms of bleeding-edge performance, what we care about is the intended current use of this flag: thread stacks.
Ingo
--------------------> From 4812c2fddc7f5a3a4480d541a4cb2b7e4ec21dcb Mon Sep 17 00:00:00 2001 From: Ingo Molnar <mingo@elte.hu> Date: Wed, 13 Aug 2008 18:02:18 +0200 Subject: [PATCH] x86: add MAP_STACK mmap flag
as per this discussion:
http://lkml.org/lkml/2008/8/12/423
Pardo reported that 64-bit threaded apps, if their stacks exceed the combined size of ~4GB, slow down drastically in pthread_create() - because glibc uses MAP_32BIT to allocate the stacks. The use of MAP_32BIT is a legacy hack - to speed up context switching on certain early model 64-bit P4 CPUs.
So introduce a new flag to be used by glibc instead, to not constrain 64-bit apps like this.
glibc can switch to this new flag straight away - it will be ignored by the kernel. If those old CPUs ever matter to anyone, support for it can be implemented.
Signed-off-by: Ingo Molnar <mingo@elte.hu> --- include/asm-x86/mman.h | 1 + 1 files changed, 1 insertions(+), 0 deletions(-)
diff --git a/include/asm-x86/mman.h b/include/asm-x86/mman.h index c1682b5..e5852b5 100644 --- a/include/asm-x86/mman.h +++ b/include/asm-x86/mman.h @@ -12,6 +12,7 @@ #define MAP_NORESERVE 0x4000 /* don't check for reservations */ #define MAP_POPULATE 0x8000 /* populate (prefault) pagetables */ #define MAP_NONBLOCK 0x10000 /* do not block on IO */ +#define MAP_STACK 0x20000 /* give out 32bit stack addresses on old CPUs */ #define MCL_CURRENT 1 /* lock all current mappings */ #define MCL_FUTURE 2 /* lock all future mappings */
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