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

* 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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