When a filesystem is mounted from a loop device, writes are throttled by balance_dirty_pages() twice: once when writing to the filesystem and once when the loop_handle_cmd() writes to the backing file. This double-throttling can trigger positive feedback loops that create significant delays. The throttling at the lower level is seen by the upper level as a slow device, so it throttles extra hard. The PF_LESS_THROTTLE flag was created to handle exactly this circumstance, though with an NFS filesystem mounted from a local NFS server. It reduces the throttling on the lower layer so that it can proceed largely unthrottled. To demonstrate this, create a filesystem on a loop device and write (e.g. with dd) several large files which combine to consume significantly more than the limit set by /proc/sys/vm/dirty_ratio or dirty_bytes. Measure the total time taken. When I do this directly on a device (no loop device) the total time for several runs (mkfs, mount, write 200 files, umount) is fairly stable: 28-35 seconds. When I do this over a loop device the times are much worse and less stable. 52-460 seconds. Half below 100seconds, half above. When I apply this patch, the times become stable again, though not as fast as the no-loop-back case: 53-72 seconds. There may be room for further improvement as the total overhead still seems too high, but this is a big improvement. Reviewed-by: Christoph Hellwig <hch@xxxxxx> Acked-by: Michal Hocko <mhocko@xxxxxxxx> Signed-off-by: NeilBrown <neilb@xxxxxxxx> --- I moved where the flag is set, thanks to suggestion from Ming Lei. I've preserved the *-by: tags I was offered despite the code being different, as the concept is identical. Thanks, NeilBrown drivers/block/loop.c | 1 + 1 file changed, 1 insertion(+) diff --git a/drivers/block/loop.c b/drivers/block/loop.c index 0ecb6461ed81..44b3506fd086 100644 --- a/drivers/block/loop.c +++ b/drivers/block/loop.c @@ -852,6 +852,7 @@ static int loop_prepare_queue(struct loop_device *lo) if (IS_ERR(lo->worker_task)) return -ENOMEM; set_user_nice(lo->worker_task, MIN_NICE); + lo->worker_task->flags |= PF_LESS_THROTTLE; return 0; } -- 2.12.2
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