Skip to content
  • Darrick J. Wong's avatar
    bdi: allow block devices to say that they require stable page writes · 7d311cda
    Darrick J. Wong authored
    
    
    This patchset ("stable page writes, part 2") makes some key
    modifications to the original 'stable page writes' patchset.  First, it
    provides creators (devices and filesystems) of a backing_dev_info a flag
    that declares whether or not it is necessary to ensure that page
    contents cannot change during writeout.  It is no longer assumed that
    this is true of all devices (which was never true anyway).  Second, the
    flag is used to relaxed the wait_on_page_writeback calls so that wait
    only occurs if the device needs it.  Third, it fixes up the remaining
    disk-backed filesystems to use this improved conditional-wait logic to
    provide stable page writes on those filesystems.
    
    It is hoped that (for people not using checksumming devices, anyway)
    this patchset will give back unnecessary performance decreases since the
    original stable page write patchset went into 3.0.  Sorry about not
    fixing it sooner.
    
    Complaints were registered by several people about the long write
    latencies introduced by the original stable page write patchset.
    Generally speaking, the kernel ought to allocate as little extra memory
    as possible to facilitate writeout, but for people who simply cannot
    wait, a second page stability strategy is (re)introduced: snapshotting
    page contents.  The waiting behavior is still the default strategy; to
    enable page snapshotting, a superblock flag (MS_SNAP_STABLE) must be
    set.  This flag is used to bandaid^Henable stable page writeback on
    ext3[1], and is not used anywhere else.
    
    Given that there are already a few storage devices and network FSes that
    have rolled their own page stability wait/page snapshot code, it would
    be nice to move towards consolidating all of these.  It seems possible
    that iscsi and raid5 may wish to use the new stable page write support
    to enable zero-copy writeout.
    
    Thank you to Jan Kara for helping fix a couple more filesystems.
    
    Per Andrew Morton's request, here are the result of using dbench to measure
    latencies on ext2:
    
    3.8.0-rc3:
       Operation      Count    AvgLat    MaxLat
       ----------------------------------------
       WriteX        109347     0.028    59.817
       ReadX         347180     0.004     3.391
       Flush          15514    29.828   287.283
    
      Throughput 57.429 MB/sec  4 clients  4 procs  max_latency=287.290 ms
    
    3.8.0-rc3 + patches:
       WriteX        105556     0.029     4.273
       ReadX         335004     0.005     4.112
       Flush          14982    30.540   298.634
    
      Throughput 55.4496 MB/sec  4 clients  4 procs  max_latency=298.650 ms
    
    As you can see, for ext2 the maximum write latency decreases from ~60ms
    on a laptop hard disk to ~4ms.  I'm not sure why the flush latencies
    increase, though I suspect that being able to dirty pages faster gives
    the flusher more work to do.
    
    On ext4, the average write latency decreases as well as all the maximum
    latencies:
    
    3.8.0-rc3:
       WriteX         85624     0.152    33.078
       ReadX         272090     0.010    61.210
       Flush          12129    36.219   168.260
    
      Throughput 44.8618 MB/sec  4 clients  4 procs  max_latency=168.276 ms
    
    3.8.0-rc3 + patches:
       WriteX         86082     0.141    30.928
       ReadX         273358     0.010    36.124
       Flush          12214    34.800   165.689
    
      Throughput 44.9941 MB/sec  4 clients  4 procs  max_latency=165.722 ms
    
    XFS seems to exhibit similar latency improvements as ext2:
    
    3.8.0-rc3:
       WriteX        125739     0.028   104.343
       ReadX         399070     0.005     4.115
       Flush          17851    25.004   131.390
    
      Throughput 66.0024 MB/sec  4 clients  4 procs  max_latency=131.406 ms
    
    3.8.0-rc3 + patches:
       WriteX        123529     0.028     6.299
       ReadX         392434     0.005     4.287
       Flush          17549    25.120   188.687
    
      Throughput 64.9113 MB/sec  4 clients  4 procs  max_latency=188.704 ms
    
    ...and btrfs, just to round things out, also shows some latency
    decreases:
    
    3.8.0-rc3:
       WriteX         67122     0.083    82.355
       ReadX         212719     0.005     2.828
       Flush           9547    47.561   147.418
    
      Throughput 35.3391 MB/sec  4 clients  4 procs  max_latency=147.433 ms
    
    3.8.0-rc3 + patches:
       WriteX         64898     0.101    71.631
       ReadX         206673     0.005     7.123
       Flush           9190    47.963   219.034
    
      Throughput 34.0795 MB/sec  4 clients  4 procs  max_latency=219.044 ms
    
    Before this patchset, all filesystems would block, regardless of whether
    or not it was necessary.  ext3 would wait, but still generate occasional
    checksum errors.  The network filesystems were left to do their own
    thing, so they'd wait too.
    
    After this patchset, all the disk filesystems except ext3 and btrfs will
    wait only if the hardware requires it.  ext3 (if necessary) snapshots
    pages instead of blocking, and btrfs provides its own bdi so the mm will
    never wait.  Network filesystems haven't been touched, so either they
    provide their own wait code, or they don't block at all.  The blocking
    behavior is back to what it was before 3.0 if you don't have a disk
    requiring stable page writes.
    
    This patchset has been tested on 3.8.0-rc3 on x64 with ext3, ext4, and
    xfs.  I've spot-checked 3.8.0-rc4 and seem to be getting the same
    results as -rc3.
    
    [1] The alternative fixes to ext3 include fixing the locking order and
    page bit handling like we did for ext4 (but then why not just use
    ext4?), or setting PG_writeback so early that ext3 becomes extremely
    slow.  I tried that, but the number of write()s I could initiate dropped
    by nearly an order of magnitude.  That was a bit much even for the
    author of the stable page series! :)
    
    This patch:
    
    Creates a per-backing-device flag that tracks whether or not pages must
    be held immutable during writeout.  Eventually it will be used to waive
    wait_for_page_writeback() if nothing requires stable pages.
    
    Signed-off-by: default avatarDarrick J. Wong <darrick.wong@oracle.com>
    Reviewed-by: default avatarJan Kara <jack@suse.cz>
    Cc: Adrian Hunter <adrian.hunter@intel.com>
    Cc: Andy Lutomirski <luto@amacapital.net>
    Cc: Artem Bityutskiy <dedekind1@gmail.com>
    Cc: Joel Becker <jlbec@evilplan.org>
    Cc: Mark Fasheh <mfasheh@suse.com>
    Cc: Steven Whitehouse <swhiteho@redhat.com>
    Cc: Jens Axboe <axboe@kernel.dk>
    Cc: Eric Van Hensbergen <ericvh@gmail.com>
    Cc: Ron Minnich <rminnich@sandia.gov>
    Cc: Latchesar Ionkov <lucho@ionkov.net>
    Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    7d311cda