linux-kernel - Re: Does process need to have a kernel-side stack all the time?

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Message-Id: <200804161620.43536.vda.linux@googlemail.com>
Date:	Wed, 16 Apr 2008 16:20:43 +0200
From:	Denys Vlasenko <vda.linux@...glemail.com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Andi Kleen <andi@...stfloor.org>, linux-kernel@...r.kernel.org
Subject: Re: Does process need to have a kernel-side stack all the time?

On Wednesday 16 April 2008 14:47, Ingo Molnar wrote:
> * Andi Kleen <andi@...stfloor.org> wrote:
> 
> > Denys Vlasenko <vda.linux@...glemail.com> writes:
> > 
> > > A lot of effort went into minimizing of stack usage.
> > > If I understand it correctly, one of the reasons for this
> > > was to be efficient and not have lots of pages
> > > used for stacks when we have a lot of threads
> > > (tens of thousands).
> > 
> > Actually the real reason the 4K stacks were introduced IIRC was that 
> > the VM is not very good at allocation of order > 0 pages and that only 
> > using order 0 and not order 1 in normal operation prevented some 
> > stalls.
> 
> no, the primary motivation Arjan and me started working on 4K stacks and 
> implemented it was what Denys mentioned: i had a testcase that ran 
> 50,000 threads before it ran out of memory - i wanted it to run 100,000 
> threads. The improved order-0 behavior was just icing on the cake.

And my "random thought" occurred along the lines "Ingo once ran
100000 threads and then ran out of space for stack... wait a minute!
Why every single one of those threads need to have a stack at the same time?
They sure as hell can't actively use more than NR_CPUS stacks at once!".

Pity it wouldn't work.
--
vda
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