Re: [PATCH 2/3] writeback: Record if the congestion was unnecessary

[Johannes Weiner <hannes@cmpxchg.org>]

On Thu, Aug 26, 2010 at 04:14:15PM +0100, Mel Gorman wrote:
> If congestion_wait() is called when there is no congestion, the caller
> will wait for the full timeout. This can cause unreasonable and
> unnecessary stalls. There are a number of potential modifications that
> could be made to wake sleepers but this patch measures how serious the
> problem is. It keeps count of how many congested BDIs there are. If
> congestion_wait() is called with no BDIs congested, the tracepoint will
> record that the wait was unnecessary.

I am not convinced that unnecessary is the right word.  On a workload
without any IO (i.e. no congestion_wait() necessary, ever), I noticed
the VM regressing both in time and in reclaiming the right pages when
simply removing congestion_wait() from the direct reclaim paths (the
one in __alloc_pages_slowpath and the other one in
do_try_to_free_pages).

So just being stupid and waiting for the timeout in direct reclaim
while kswapd can make progress seemed to do a better job for that
load.

I can not exactly pinpoint the reason for that behaviour, it would be
nice if somebody had an idea.

So personally I think it's a good idea to get an insight on the use of
congestion_wait() [patch 1] but I don't agree with changing its
behaviour just yet, or judging its usefulness solely on whether it
correctly waits for bdi congestion.