Date: Mon, 14 Nov 2005 12:34:42 +0100
From: Jens Axboe <axboe@suse.de>
To: Arjan van de Ven <arjan@infradead.org>
Cc: Pierre Ossman <drzeus-list@drzeus.cx>, Adrian Bunk <bunk@stusta.de>,
       Andrew Morton <akpm@osdl.org>, linux-kernel@vger.kernel.org
Subject: Re: [2.6 patch] i386: always use 4k stacks
Message-ID: <20051114113442.GU3699@suse.de>
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On Mon, Nov 14 2005, Arjan van de Ven wrote:
> On Mon, 2005-11-14 at 12:24 +0100, Jens Axboe wrote:
> > > 
> > > not sure; I do know that it very much helps java (many more threads
> > > possible) and the VM (far less order 1 allocs). In addition the 4Kb
> > > allocation can be satisfied with the per cpu list of free 4Kb pages,
> > > while obviously an order 1 cannot and has to go global.
> > 
> > I realize it has nice advantages in theory, just wondering if anyone has
> > done a performance analysis of 4kb vs 8kb stacks lately (or at all?).
> 
> I don't think at least anyone at RH has done any; the functionality gain
> was already enough for us. One item I missed: in the many-thread cases,
> you also save a lot of memory that can now be used for pagecache; 
> this won't of course be visible in a microbenchmark but should help
> system wide.
> 
> Also in the implementation I don't see any way 4Kb stacks could show up
> in any benchmarks as negative; there are only 4 or 5 extra instructions
> in any path, and afaics no cache downsides (in fact the same irq stack
> memory is now reused for irqs instead of threadstack-du-jour, so less
> footprint/hotter caches)

The only downside is the potential crashes due to overflowing the stack,
I'm not worried about 4kb stacks performing worse.

-- 
Jens Axboe

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