On Wed, 2009-10-07 at 15:38 +0800, Wu Fengguang wrote:
> plain text document attachment (writeback-nfs-request-queue.patch)
> The generic writeback routines are departing from congestion_wait()
> in preferance of get_request_wait(), aka. waiting on the block queues.
>
> Introduce the missing writeback wait queue for NFS, otherwise its
> writeback pages may grow out of control.
>
> In perticular, balance_dirty_pages() will exit after it pushes
> write_chunk pages into the PG_writeback page pool, _OR_ when the
> background writeback work quits. The latter is new behavior, and could
> not only quit (normally) after below background threshold, but also
> quit when it finds _zero_ dirty pages to write. The latter case gives
> rise to the number of PG_writeback pages if it is not explicitly limited.
>
> CC: Jens Axboe <jens.axboe@xxxxxxxxxx>
> CC: Chris Mason <chris.mason@xxxxxxxxxx>
> CC: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
> CC: Trond Myklebust <Trond.Myklebust@xxxxxxxxxx>
> Signed-off-by: Wu Fengguang <fengguang.wu@xxxxxxxxx>
> ---
>
> The wait time and network throughput varies a lot! this is a major problem.
> This means nfs_end_page_writeback() is not called smoothly over time,
> even when there are plenty of PG_writeback pages on the client side.
Could that be some ack batching from the nfs protocol/server?
> +static void nfs_wakeup_congested(long nr, long limit,
> + struct backing_dev_info *bdi,
> + wait_queue_head_t *wqh)
> +{
> + if (nr < 2*limit - min(limit/8, NFS_WAIT_PAGES)) {
> + if (test_bit(BDI_sync_congested, &bdi->state))
> + clear_bdi_congested(bdi, BLK_RW_SYNC);
> + if (waitqueue_active(&wqh[BLK_RW_SYNC])) {
> + smp_mb__after_clear_bit();
> + wake_up(&wqh[BLK_RW_SYNC]);
> + }
> + }
> + if (nr < limit - min(limit/8, NFS_WAIT_PAGES)) {
> + if (test_bit(BDI_async_congested, &bdi->state))
> + clear_bdi_congested(bdi, BLK_RW_ASYNC);
> + if (waitqueue_active(&wqh[BLK_RW_ASYNC])) {
> + smp_mb__after_clear_bit();
> + wake_up(&wqh[BLK_RW_ASYNC]);
> + }
> + }
> +}
>
wakeup implies a full memory barrier.
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