Re: [2.6 patch] i386: always use 4k stacks

[Arjan van de Ven]

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)


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