Re: select/poll optimization

[David Laight <david%l8s.co.uk@localhost>]

On Sun, Feb 24, 2008 at 02:51:40AM +0000, Andrew Doran wrote:
> Hi,
> 
> Currently select() and poll() synchronize using a global mutex. The below
> diff changes them to use per-CPU state, and removes the locking in
> selrecord(). In the best case there is no lock contention internal to
> select() or poll() with the patch, and in the worse case, all state is
> hashed out by a factor of $ncpu so it should scale as CPUs are added.
> 
> In the few tests I have done there is either no lock contention or it's just
> background noise. rmind@ and I have been discussing an approach using a
> 'thread x descriptor' array to store state, and do fine grained locking
> based on that, but I think the per-CPU approach is better because a lot less
> synchronization is needed, and no changes are needed to drivers.
> 
>       http://www.netbsd.org/~ad/percpu-select.diff
> 
> Comments?

I think it doesn't work !

You've not allowed for more than one lwp (using different mutex)
being in selrecord() for the same device at the same time.
Also:
+               /* Multiple waiters: record a collision. */
+               sip->sel_collision |= sc->sc_mask;
needs to be locked - or you lose bits in sel_collision.

        David

-- 
David Laight: david%l8s.co.uk@localhost