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    SubjectRe: [PATCH 2/3] writeback: Record if the congestion was unnecessary
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    On Fri, 2010-08-27 at 04:31 +0800, Mel Gorman wrote:
    > On Thu, Aug 26, 2010 at 08:29:04PM +0200, Johannes Weiner wrote:
    > > 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.
    > >
    >
    > There is a possibility that the behaviour in that case was due to flusher
    > threads doing the writes rather than direct reclaim queueing pages for IO
    > in an inefficient manner. So the stall is stupid but happens to work out
    > well because flusher threads get the chance to do work.
    If this is the case, we already have queue congested. removing
    congestion_wait() might cause regression but either your change or the
    congestion_wait_check() should not have the regression, as we do check
    if the bdi is congested.

    Thanks,
    Shaohua



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