Re: [PATCH][5/?] count writeback pages in nr_scanned

[Nick Piggin]

Andrew Morton wrote:
> Nick Piggin <nickpiggin@xxxxxxxxxxxx> wrote:
>
> > > I suspect something might still be broken.  It may take a few
> > > days of continuous testing to trigger the bug, though ...
> >
> > It is possible to be those blk_congestion_wait paths, because
> > the queue simply won't be congested. So doing io_schedule_timeout
> > might help.
>
>
> If the queue is not congested, blk_congestion_wait() will still sleep.  See
> freed_request().

Hmm... doesn't look like it to me:

        if (rl->count[rw] < queue_congestion_off_threshold(q))
                clear_queue_congested(q, rw);

And clear_queue_congested does an unconditional wakeup (if there
is someone sleeping on the congestion queue).

> > I wonder if reducing the size of the write queue in CFQ would help
> > too? IIRC, it only really wants a huge read queue.
>
>
> Surely it will help - but we need to be able to handle the situation
> because memory can still become full of PageWriteback pages if there are
> many disks.

Yep.
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