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author | NeilBrown <neilb@suse.com> | 2017-06-16 08:02:09 +0300 |
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committer | Jens Axboe <axboe@fb.com> | 2017-06-18 18:07:42 +0300 |
commit | b2ee7d46befc43e355ffaf7bfabb00e7a901b3a0 (patch) | |
tree | e2c05bc23d7c4de20b441f13781bd01686e94bee /virt | |
parent | c27b2d634f81d1472a8107fc857481b67555d9bd (diff) | |
download | linux-b2ee7d46befc43e355ffaf7bfabb00e7a901b3a0.tar.xz |
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>
Reviewed-by: Ming Lei <tom.leiming@gmail.com>
Suggested-by: Michal Hocko <mhocko@suse.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Signed-off-by: NeilBrown <neilb@suse.com>
Signed-off-by: Jens Axboe <axboe@fb.com>
Diffstat (limited to 'virt')
0 files changed, 0 insertions, 0 deletions