mm: filemap: Avoid unnecessary calls to lock_page when waiting for IO to complete during a read
In the generic read paths the kernel looks up a page in the page cache
and if it's up to date, it is used. If not, the page lock is acquired to
wait for IO to complete and then check the page. If multiple processes
are waiting on IO, they all serialise against the lock and duplicate the
checks. This is unnecessary.
The page lock in itself does not give any guarantees to the callers about
the page state as it can be immediately truncated or reclaimed after the
page is unlocked. It's sufficient to wait_on_page_locked and then continue
if the page is up to date on wakeup.
It is possible that a truncated but up-to-date page is returned but the
reference taken during read prevents it disappearing underneath the caller
and the data is still valid if PageUptodate.
The overall impact is small as even if processes serialise on the lock,
the lock section is tiny once the IO is complete. Profiles indicated that
unlock_page and friends are generally a tiny portion of a read-intensive
workload. An artifical test was created that had instances of dd access
a cache-cold file on an ext4 filesystem and measure how long the read took.
paralleldd
4.4.0 4.4.0
vanilla avoidlock
Amean Elapsd-1 5.28 ( 0.00%) 5.15 ( 2.50%)
Amean Elapsd-4 5.29 ( 0.00%) 5.17 ( 2.12%)
Amean Elapsd-7 5.28 ( 0.00%) 5.18 ( 1.78%)
Amean Elapsd-12 5.20 ( 0.00%) 5.33 ( -2.50%)
Amean Elapsd-21 5.14 ( 0.00%) 5.21 ( -1.41%)
Amean Elapsd-30 5.30 ( 0.00%) 5.12 ( 3.38%)
Amean Elapsd-48 5.78 ( 0.00%) 5.42 ( 6.21%)
Amean Elapsd-79 6.78 ( 0.00%) 6.62 ( 2.46%)
Amean Elapsd-110 9.09 ( 0.00%) 8.99 ( 1.15%)
Amean Elapsd-128 10.60 ( 0.00%) 10.43 ( 1.66%)
The impact is small but intuitively, it makes sense to avoid unnecessary
calls to lock_page.
Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
Reviewed-by: Jan Kara <jack@suse.cz>
1 file changed
