From: Ming Lei <tom.leiming@gmail.com>
To: NeilBrown <neilb@suse.com>
Cc: Jens Axboe <axboe@fb.com>,
linux-block <linux-block@vger.kernel.org>,
linux-mm <linux-mm@kvack.org>,
LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v2] loop: Add PF_LESS_THROTTLE to block/loop device thread.
Date: Wed, 5 Apr 2017 13:05:33 +0800 [thread overview]
Message-ID: <CACVXFVPLoKM3-eY7MHx7o1fcmsvc84xMxY+Ns9QO3POd9+NP8Q@mail.gmail.com> (raw)
In-Reply-To: <87wpazh3rl.fsf@notabene.neil.brown.name>
On Wed, Apr 5, 2017 at 12:33 PM, NeilBrown <neilb@suse.com> 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@lst.de>
> Acked-by: Michal Hocko <mhocko@suse.com>
> Signed-off-by: NeilBrown <neilb@suse.com>
Reviewed-by: Ming Lei <tom.leiming@gmail.com>
> ---
>
> 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
>
next prev parent reply other threads:[~2017-04-05 5:05 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
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 [this message]
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
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=CACVXFVPLoKM3-eY7MHx7o1fcmsvc84xMxY+Ns9QO3POd9+NP8Q@mail.gmail.com \
--to=tom.leiming@gmail.com \
--cc=axboe@fb.com \
--cc=linux-block@vger.kernel.org \
--cc=linux-kernel@vger.kernel.org \
--cc=linux-mm@kvack.org \
--cc=neilb@suse.com \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).