From: Michal Hocko <mhocko@kernel.org>
To: NeilBrown <neilb@suse.com>
Cc: Jens Axboe <axboe@fb.com>,
linux-block@vger.kernel.org, linux-mm@kvack.org,
LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] loop: Add PF_LESS_THROTTLE to block/loop device thread.
Date: Tue, 4 Apr 2017 13:23:07 +0200 [thread overview]
Message-ID: <20170404112307.GA15490@dhcp22.suse.cz> (raw)
In-Reply-To: <871staffus.fsf@notabene.neil.brown.name>
On Mon 03-04-17 11:18:51, NeilBrown 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.
Yes this makes sense to me
> Signed-off-by: NeilBrown <neilb@suse.com>
Acked-by: Michal Hocko <mhocko@suse.com>
one nit below
> ---
> drivers/block/loop.c | 3 +++
> 1 file changed, 3 insertions(+)
>
> diff --git a/drivers/block/loop.c b/drivers/block/loop.c
> index 0ecb6461ed81..a7e1dd215fc2 100644
> --- a/drivers/block/loop.c
> +++ b/drivers/block/loop.c
> @@ -1694,8 +1694,11 @@ static void loop_queue_work(struct kthread_work *work)
> {
> struct loop_cmd *cmd =
> container_of(work, struct loop_cmd, work);
> + int oldflags = current->flags & PF_LESS_THROTTLE;
>
> + current->flags |= PF_LESS_THROTTLE;
> loop_handle_cmd(cmd);
> + current->flags = (current->flags & ~PF_LESS_THROTTLE) | oldflags;
we have a helper for this tsk_restore_flags(). It is not used
consistently and maybe we want a dedicated api like we have for the
scope NOIO/NOFS but that is a separate thing. I would find
tsk_restore_flags easier to read.
--
Michal Hocko
SUSE Labs
next prev parent reply other threads:[~2017-04-04 11:23 UTC|newest]
Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-04-03 1:18 [PATCH] loop: Add PF_LESS_THROTTLE to block/loop device thread NeilBrown
2017-04-04 7:10 ` Christoph Hellwig
2017-04-05 4:27 ` NeilBrown
2017-04-05 5:13 ` Ming Lei
2017-04-04 11:23 ` Michal Hocko [this message]
2017-04-04 14:24 ` Ming Lei
2017-04-05 4:31 ` NeilBrown
2017-04-05 4:33 ` [PATCH v2] " NeilBrown
2017-04-05 5:05 ` Ming Lei
2017-04-05 7:19 ` Michal Hocko
2017-04-05 7:32 ` Michal Hocko
2017-04-06 2:23 ` NeilBrown
2017-04-06 6:53 ` Michal Hocko
2017-04-06 23:47 ` [PATCH v3] " NeilBrown
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