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

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Date:	Wed, 13 Aug 2008 17:40:43 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Ulrich Drepper <drepper@...hat.com>
Cc:	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@...hat.com> wrote:

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> Ingo Molnar wrote:
> > i find it pretty unacceptable these days that we limit any aspect of 
> > pure 64-bit apps in any way to 4GB (or any other 32-bit-ish limit). 
> 
> Sure, but if we can pin-point the sub-archs for which it is the 
> problem then a flag to optionally request it is even easier to handle.  
> You'd simply ignore the flag for anything but the P4 architecture.

i suspect you are talking about option #2 i described. It is the option 
which will take the most time to trickle down to people.

> I personally have no problem removing the whole thing because I have 
> no such machine running anymore.  But there are people out there who 
> have.

hm, i think the set of people running on such boxes _and_ then upgrading 
to a new glibc and expecting everything to be just as fast to the 
microsecond as before should be miniscule. Those P4 derived 64-bit boxes 
were astonishingly painful in 64-bit mode - most of that hw is running 
32-bit i suspect, because 64-bit on it was really a joke.

Btw., can you see any problems with option #1: simply removing MAP_32BIT 
from 64-bit stack allocations in glibc unconditionally? It's the fastest 
to execute and also the most obvious solution. +1 usecs overhead in the 
64-bit context-switch path on those old slow boxes wont matter much. 

10 _millisecs_ to start a single thread on top-of-the-line hw is quite 
unaccepable. (and there's little sane we can do in the kernel about 
allocation overhead when we have an imperfectly filled 4GB box for all 
allocations)

	Ingo
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