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    SubjectRe: [PATCH 01/45] writeback: reduce calls to global_page_state in balance_dirty_pages()
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    On Sun, 2009-10-11 at 18:50 +0800, Wu Fengguang wrote:
    >
    > Sorry for the confusion, but I mean, filesystems have to limit
    > nr_writeback (directly or indirectly via the block io queue),
    > otherwise it either hit nr_dirty to 0 (with the loop), or let
    > nr_writeback grow out of control (without the loop).

    Doesn't this require the writeback queue to have a limit < dirty_thresh?

    Or more specifically, for the bdi case:

    bdi_dirty + bdi_writeback + bdi_unstable <= bdi_thresh

    we require that the writeback queue be smaller than bdi_thresh, which
    could be quite difficult, since bdi_thresh can easily be 0.

    Without observing the bdi_thresh constraint we can have:

    \Sum_(over bdis) writeback_queue_size

    dirty pages outstanding, which could be significantly higher than
    dirty_thresh.

    Or am I confused again?



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