lkml.org 
[lkml]   [2017]   [Apr]   [5]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
Patch in this message
/
From
Date
Subject[PATCH v2] loop: Add PF_LESS_THROTTLE to block/loop device thread.

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>
---

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
[unhandled content-type:application/pgp-signature]
\
 
 \ /
  Last update: 2017-04-05 06:35    [W:0.381 / U:0.748 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site