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    SubjectRe: [patch 3/8] per backing_dev dirty and writeback page accounting
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    > Only if the queue depth is not bound. Queue depths are bound and so
    > the distance we can go over the threshold is limited. This is the
    > fundamental principle on which the throttling is based.....
    >
    > Hence, if the queue is not full, then we will have either written
    > dirty pages to it (i.e wbc->nr_write != write_chunk so we will throttle
    > or continue normally if write_chunk was written) or we have no more
    > dirty pages left.
    >
    > Having no dirty pages left on the bdi and it not being congested
    > means we effectively have a clean, idle bdi. We should not be trying
    > to throttle writeback here - we can't do anything to improve the
    > situation by continuing to try to do writeback on this bdi, so we
    > may as well give up and let the writer continue. Once we have dirty
    > pages on the bdi, we'll get throttled appropriately.

    OK, you convinced me.

    How about this patch? I introduced a new wbc counter, that sums the
    number of dirty pages encountered, including ones already under
    writeback.

    Dave, big thanks for your insights.

    Miklos

    Index: linux/include/linux/writeback.h
    ===================================================================
    --- linux.orig/include/linux/writeback.h 2007-03-14 22:43:42.000000000 +0100
    +++ linux/include/linux/writeback.h 2007-03-14 22:58:56.000000000 +0100
    @@ -44,6 +44,7 @@ struct writeback_control {
    long nr_to_write; /* Write this many pages, and decrement
    this for each page written */
    long pages_skipped; /* Pages which were not written */
    + long nr_dirty; /* Number of dirty pages encountered */

    /*
    * For a_ops->writepages(): is start or end are non-zero then this is
    Index: linux/mm/page-writeback.c
    ===================================================================
    --- linux.orig/mm/page-writeback.c 2007-03-14 22:41:01.000000000 +0100
    +++ linux/mm/page-writeback.c 2007-03-14 23:00:20.000000000 +0100
    @@ -220,6 +220,17 @@ static void balance_dirty_pages(struct a
    pages_written += write_chunk - wbc.nr_to_write;
    if (pages_written >= write_chunk)
    break; /* We've done our duty */
    +
    + /*
    + * If just a few dirty pages were encountered, and
    + * the queue is not congested, then allow this dirty
    + * producer to continue. This resolves the deadlock
    + * that happens when one filesystem writes back data
    + * through another. It should also help when a slow
    + * device is completely blocking other writes.
    + */
    + if (wbc.nr_dirty < 8 && !bdi_write_congested(bdi))
    + break;
    }
    congestion_wait(WRITE, HZ/10);
    }
    @@ -612,6 +623,7 @@ retry:
    min(end - index, (pgoff_t)PAGEVEC_SIZE-1) + 1))) {
    unsigned i;

    + wbc->nr_dirty += nr_pages;
    scanned = 1;
    for (i = 0; i < nr_pages; i++) {
    struct page *page = pvec.pages[i];
    -
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