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    SubjectRe: [PATCH 0/7] Per-bdi writeback flusher threads
    [CC Jens]

    On Tue, Apr 07, 2009 at 10:03:38PM +0800, Jos Houtman wrote:
    >
    > I tried the write-back branch from the 2.6-block tree.
    >
    > And I can atleast confirm that it works, atleast in relation to the
    > writeback not keeping up when the device was congested before it wrote a
    > 1024 pages.
    >
    > See: http://lkml.org/lkml/2009/3/22/83 for a bit more information.

    Hi Jos, you said that this simple patch solved the problem, however you
    mentioned somehow suboptimal performance. Can you elaborate that? So
    that I can push or improve it.

    Thanks,
    Fengguang
    ---
    fs/fs-writeback.c | 3 ++-
    1 file changed, 2 insertions(+), 1 deletion(-)

    --- mm.orig/fs/fs-writeback.c
    +++ mm/fs/fs-writeback.c
    @@ -325,7 +325,8 @@ __sync_single_inode(struct inode *inode,
    * soon as the queue becomes uncongested.
    */
    inode->i_state |= I_DIRTY_PAGES;
    - if (wbc->nr_to_write <= 0) {
    + if (wbc->nr_to_write <= 0 ||
    + wbc->encountered_congestion) {
    /*
    * slice used up: queue for next turn
    */
    > But the second problem seen in that thread, a write-starve-read problem does
    > not seem to solved. In this problem the writes of the writeback algorithm
    > starve the ongoing reads, no matter what io-scheduler is picked.
    >
    > For good measure I also applied the blk-latency patches on top of the
    > writeback branch, this did not improve anything. Nor did lowering
    > max_sectors_kb, as linus suggested in the IO latency thread.
    >
    >
    > As for a reproducible test-case: the simplest I could come up with was
    > modifying the fsync-tester not to fsync, but letting the normal writeback
    > handle it. And starting a separate process that tries to sequentially read a
    > file from the same device. The read performance drops to a bare minimum as
    > soon as the writeback algorithm kicks in.
    >
    >
    > Jos
    >
    >
    >
    >


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