Re: 2.5.57 IO slowdown with CONFIG_PREEMPT enabled

[Andrew Morton <akpm@digeo.com>]

Robert Macaulay <robert_macaulay@dell.com> wrote:
>
> On Wed, 15 Jan 2003, Andrew Morton wrote:
> > if you could please test that with CONFIG_PREEMPT=y
> 
> Reverting that brings the speed back up

OK.  How irritating.

Presumably there's a fairness problem - once a CPU goes in there to start
spinning on the lock, the length of the loop is such that it's easy for
non-holders to zoom in and claim it first.  Or something.

Unless another way of solving the problem which that patch solves presents
itself we may need to revert it.

Or not.  Should a CONFIG_PREEMPT SMP kernel compromise its latency because of
overused locking??


I'd appreciate it if you could take a closer look at the mysterious
self-reverting .config, please.  See if you can work out what is causing it,
tell me how you're rebooting (reboot -f?  -fn?  Big Red Button?) and whether
the offending file is on the root filesystem, etc.


Thanks.