linux-kernel - Re: pthread_create() slow for many threads; also time to revisit 64b context switch optimization?

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Date:	Fri, 15 Aug 2008 18:03:38 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Jamie Lokier <jamie@...reable.org>
Cc:	Ulrich Drepper <drepper@...hat.com>,
	Arjan van de Ven <arjan@...radead.org>,
	akpm@...ux-foundation.org, hugh@...itas.com, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, briangrant@...gle.com,
	cgd@...gle.com, mbligh@...gle.com,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: pthread_create() slow for many threads; also time to revisit
	64b context switch optimization?


* Jamie Lokier <jamie@...reable.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@...e.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@...e.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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