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 >
WARNING: multiple messages have this Message-ID (diff)
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 > -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>
next prev parent reply other threads:[~2017-04-05 5:05 UTC|newest] Thread overview: 24+ 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 2017-04-04 7:10 ` Christoph Hellwig 2017-04-05 4:27 ` NeilBrown 2017-04-05 5:13 ` Ming Lei 2017-04-05 5:13 ` Ming Lei 2017-04-04 11:23 ` Michal Hocko 2017-04-04 11:23 ` Michal Hocko 2017-04-04 14:24 ` Ming Lei 2017-04-04 14:24 ` Ming Lei 2017-04-05 4:31 ` NeilBrown 2017-04-05 4:33 ` [PATCH v2] " NeilBrown 2017-04-05 4:33 ` NeilBrown 2017-04-05 5:05 ` Ming Lei [this message] 2017-04-05 5:05 ` Ming Lei 2017-04-05 7:19 ` Michal Hocko 2017-04-05 7:19 ` Michal Hocko 2017-04-05 7:32 ` 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 6:53 ` Michal Hocko 2017-04-06 23:47 ` [PATCH v3] " NeilBrown 2017-04-06 23:47 ` NeilBrown
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