Re: [Lsf-pc] Linux kernel impact on PostgreSQL performance

On Wed, Jan 15, 2014 at 07:13:27PM -0500, Tom Lane wrote:
> Dave Chinner <david(at)fromorbit(dot)com> writes:
> > On Wed, Jan 15, 2014 at 02:29:40PM -0800, Jeff Janes wrote:
> >> And most importantly, "Also, please don't freeze up everything else in the
> >> process"
>
> > If you hand writeback off to the kernel, then writeback for memory
> > reclaim needs to take precedence over "metered writeback". If we are
> > low on memory, then cleaning dirty memory quickly to avoid ongoing
> > allocation stalls, failures and potentially OOM conditions is far more
> > important than anything else.....
>
> I think you're in violent agreement, actually. Jeff's point is exactly
> that we'd rather the checkpoint deadline slid than that the system goes
> to hell in a handbasket for lack of I/O cycles. Here "metered" really
> means "do it as a low-priority task".

No, I meant the opposite - in low memory situations, the system is
going to go to hell in a handbasket because we are going to cause a
writeback IO storm cleaning memory regardless of these IO
priorities. i.e. there is no way we'll let "low priority writeback
to avoid IO storms" cause OOM conditions to occur. That is, in OOM
conditions, cleaning dirty pages becomes one of the highest priority
tasks of the system....

Cheers,

Dave.
--
Dave Chinner
david(at)fromorbit(dot)com