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    SubjectRe: [PATCH] bdi_congestion_funp (was: Re: [PATCH] per process request limits (was Re: IO scheduler, queue depth, nr_requests))
    On Fri, Feb 20 2004, Miquel van Smoorenburg wrote:
    > > > Even if it isn't happening
    > > > a lot, and something isn't bust it might be a good idea to
    > > > do this.
    > >
    > > Seems OK from a quick check. pdflush will block in get_request_wait()
    > > occasionally, but not at all often. Perhaps we could move the
    > > write_congested test into the mpage_writepages() inner loop but it hardly
    > > seems worth the risk.
    > >
    > > Maybe things are different on Miquel's clockwork controller.
    > I haven't tested it yet because of the "This patch isn't actually so good"
    > comment, but I found another explanation.
    > > drivers/block/ll_rw_blk.c | 2 ++
    > > fs/fs-writeback.c | 2 ++
    > > 2 files changed, 4 insertions(+)
    > *Lightbulb on* .. I just read fs-writeback.c. As I said, this happens
    > with an LVM device. Could it be that because LVM and the actual device
    > have different struct request_queue's things go awry ?
    > In fs-writeback.c, your're looking at the LVM device (and its
    > request_queue, and its backing_dev_info). In__make_request, you're
    > looking at the SCSI device.

    In principle, the lvm/md queues themselves will never be congested. But
    the underlying queues can be, of course.

    Now this approach is _much_ better, imo. I don't particularly care very
    much for how you solved it, though, I'd much rather just see both
    setting and testing passed down (and kill the ->aux as well).

    Regardless of the initial hw depth vs block depth (which is also a
    generic device problem, not just dm related), this would be a good
    addition to the congestion logic.

    Jens Axboe

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