Re: [PATCH] [RFC] Optimize select/poll by putting small data sets on the stack

[Arnd Bergmann <arnd@arndb.de>]

On Tuesday 03 January 2006 20:58, Andi Kleen wrote:
> 
> This is a RFC for now. I would be interested in testing
> feedback. Patch is for 2.6.15.
> 
> Optimize select and poll by a using stack space for small fd sets
> 
> This brings back an old optimization from Linux 2.0. Using
> the stack is faster than kmalloc. On a Intel P4 system
> it speeds up a select of a single pty fd by about 13%
> (~4000 cycles -> ~3500)

Hmm, can you include the same change for compat_sys_select()?
When that was introduced, sys_select and compat_sys_select were
basically identical in their code, which makes it a lot easier
to verify that the compat_ version is correct.

Interestingly, doing a diff between sys_select and compat_sys_select
in the current kernel seems to suggest that they are both buggy
in that they miss checks for failing __put_user, but in /different/
places.

	Arnd <><