Re: [PATCH v2] loop: Add PF_LESS_THROTTLE to block/loop device thread.

[Ming Lei]

On Wed, Apr 5, 2017 at 12:33 PM, NeilBrown <neilb@xxxxxxxx> wrote:
>
> 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>

Reviewed-by: Ming Lei <tom.leiming@xxxxxxxxx>

> ---
>
> 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
>