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

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Message-ID: <20080815155457.GA5210@shareable.org>
Date:	Fri, 15 Aug 2008 16:54:57 +0100
From:	Jamie Lokier <jamie@...reable.org>
To:	Ingo Molnar <mingo@...e.hu>
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?

Ingo Molnar wrote:
> As unimplemented flags just get ignored by the kernel, if this flag goes 
> into v2.6.27 as-is and is ignored by the kernel (i.e. we just use a 
> plain old 64-bit [47-bit] allocation), then you could do the glibc 
> change straight away, correct? So then if people complain we can fix it 
> in the kernel purely.
> 
> 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 */

Also, is this _only_ useful for thread stacks, or are there other
memory allocations where 31-bitness affects execution speed on old P4s?

-- Jamie
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