Re: regression in page writeback

Wu Fengguang
2009-09-23T03:07:17Z

On Wed, Sep 23, 2009 at 10:09:15AM +0800, Andrew Morton wrote:
> On Wed, 23 Sep 2009 10:01:04 +0800 Wu Fengguang <fengguang.wu@intel.com> wrote:
>
> > > If there's still outstanding dirty data for any of those queues, both
> > > wb_kupdate() and background_writeout() will take a teeny sleep and then
> > > will re-poll the queues.
> > >
> > > Did that logic get broken?
> >
> > No, but the "teeny sleep" is normally much smaller. When io queue is
> > not congested, every io completion event will wakeup the congestion
> > waiters. Also A's event could wake up B's waiters.
> >
> > __freed_request() always calls blk_clear_queue_congested() if under
> > congestion threshold which in turn wakes up congestion waiters:
> >
> > if (rl->count[sync] < queue_congestion_off_threshold(q))
> > blk_clear_queue_congested(q, sync);
> >
>
> Yes. Have any problems been demonstrated due to that?

Hmm, I was merely clarifying a fact.

Chris Mason listed some reasons to convert the congestion_wait() based
polls to the some queue waiting:

http://lkml.org/lkml/2009/9/8/210

My impression is, it changed obviously too fast without enough discussions.

> And what's _sufficiently_ wrong with that to justify adding potentially
> thousands of kernel threads? It was always a design objective to avoid
> doing that.

Yeah, the number of threads could be a problem.

I guess the per-bdi threads and congestion_wait are mostly two
independent changes. congestion_wait is not the reason to do per-bdi
threads. Just that Jens piggy backed some changes to congestion_wait.

Thanks,
Fengguang