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

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Message-ID: <20080815171913.GB23600@elte.hu>
Date:	Fri, 15 Aug 2008 19:19:13 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Ulrich Drepper <drepper@...il.com>
Cc:	Jamie Lokier <jamie@...reable.org>,
	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?


* Ulrich Drepper <drepper@...il.com> wrote:

> -----BEGIN PGP SIGNED MESSAGE-----
> Hash: SHA1
> 
> Jamie Lokier wrote:
> > Suggest:
> > 
> > +#define MAP_STACK       0x20000         /* 31bit or 64bit address for stack, */
> > +                                        /* whichever is faster on this CPU */
> 
> I agree.  Except for the comment.
> 
> 
> > Also, is this _only_ useful for thread stacks, or are there other
> > memory allocations where 31-bitness affects execution speed on old P4s?
> 
> Actually, I would define the flag as "do whatever is best assuming the
> allocation is used for stacks".
> 
> For instance, minimally the /proc/*/maps output could show "[user
> stack]" or something like this.  For security, perhaps, setting of
> PROC_EXEC can be prevented.

makes sense. Updated patch below. I've also added your Acked-by. Queued 
it up in tip/x86/urgent, for v2.6.27 merging.

( also, just to make sure: all Linux kernel versions will ignore such 
  extra flags, so you can just update glibc to use this flag 
  unconditionally, correct? )

	Ingo

--------------------------->
>From 2fdc86901d2ab30a12402b46238951d2a7891590 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>
Acked-by: Ulrich Drepper <drepper@...il.com>
---
 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..90bc410 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 an address that is best suited for process/thread stacks */
 
 #define MCL_CURRENT	1		/* lock all current mappings */
 #define MCL_FUTURE	2		/* lock all future mappings */
--
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