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    SubjectRe: [PATCH] Optimize lock in queue unplugging
    On Tue, Apr 29 2008, Mike Anderson wrote:
    > Jens Axboe <jens.axboe@oracle.com> wrote:
    > > On Tue, Apr 29 2008, Mikulas Patocka wrote:
    > > > Hi
    > > >
    > > > Mike Anderson was doing an OLTP benchmark on a computer with 48 physical
    > > > disks mapped to one logical device via device mapper.
    > > >
    > > > He found that there was a slowdown on request_queue->lock in function
    > > > generic_unplug_device. The slowdown is caused by the fact that when some
    > > > code calls unplug on the device mapper, device mapper calls unplug on all
    > > > physical disks. These unplug calls take the lock, find that the queue is
    > > > already unplugged, release the lock and exit.
    > > >
    > > > With the below patch, performance of the benchmark was increased by 18%
    > > > (the whole OLTP application, not just block layer microbenchmarks).
    > > >
    > > > So I'm submitting this patch for upstream. I think the patch is correct,
    > > > because when more threads call simultaneously plug and unplug, it is
    > > > unspecified, if the queue is or isn't plugged (so the patch can't make
    > > > this worse). And the caller that plugged the queue should unplug it
    > > > anyway. (if it doesn't, there's 3ms timeout).
    > >
    > > Where were these unplug calls coming from? The block layer will
    > > generally only unplug when it is already unplugged, so if you are seeing
    > > so many unplug calls that the patch redues overhead by as much
    > > described, perhaps the callsite is buggy?
    >
    > I do not have direct access the the benchmark setup, but here is the data
    > I have received.
    >
    > The oprofile data was showing ll_rw_blk::generic_unplug_device() as a top
    > routine at 13% of the samples. Annotation of the samples shows hits on
    > spin_lock_irq(q->queue_lock).
    >
    > Here are some sample call traces:
    >
    > Call trace #1
    >
    > kernel: [<ffffffff80058c6c>] generic_unplug_device+0x5d/0xc6
    > kernel: [<ffffffff8820ea3e>] :dm_mod:dm_table_unplug_all+0x33/0x41
    > kernel: [<ffffffff8820cc85>] :dm_mod:dm_unplug_all+0x1d/0x28
    > kernel: [<ffffffff8005a78a>] blk_backing_dev_unplug+0x56/0x5b
    > kernel: [<ffffffff80014cdc>] sync_buffer+0x36/0x3f
    > kernel: [<ffffffff800629a4>] __wait_on_bit+0x40/0x6f
    > kernel: [<ffffffff80014ca6>] sync_buffer+0x0/0x3f
    > kernel: [<ffffffff80062a3f>] out_of_line_wait_on_bit+0x6c/0x78
    > kernel: [<ffffffff8009c474>] wake_bit_function+0x0/0x23
    > kernel: [<ffffffff88034c85>] :jbd:journal_commit_transaction+0x91f/0x1086
    > kernel: [<ffffffff8003d038>] lock_timer_base+0x1b/0x3c
    > kernel: [<ffffffff8803840e>] :jbd:kjournald+0xc1/0x213
    > kernel: [<ffffffff8009c446>] autoremove_wake_function+0x0/0x2e
    > kernel: [<ffffffff8009c283>] keventd_create_kthread+0x0/0x61
    > kernel: [<ffffffff8803834d>] :jbd:kjournald+0x0/0x213
    > kernel: [<ffffffff8009c283>] keventd_create_kthread+0x0/0x61
    > kernel: [<ffffffff800321d5>] kthread+0xfe/0x132
    > kernel: [<ffffffff8005cfb1>] child_rip+0xa/0x11
    > kernel: [<ffffffff8009c283>] keventd_create_kthread+0x0/0x61
    > kernel: [<ffffffff800320d7>] kthread+0x0/0x132
    > kernel: [<ffffffff8005cfa7>] child_rip+0x0/0x11
    >
    > Call trace #2
    > kernel: [<ffffffff80058c6c>] generic_unplug_device+0x5d/0xc6
    > kernel: [<ffffffff8820ea3e>] :dm_mod:dm_table_unplug_all+0x33/0x41
    > kernel: [<ffffffff8820cc85>] :dm_mod:dm_unplug_all+0x1d/0x28
    > kernel: [<ffffffff8005a78a>] blk_backing_dev_unplug+0x56/0x5b
    > kernel: [<ffffffff800e8bfe>] __blockdev_direct_IO+0x889/0xaa2
    > kernel: [<ffffffff88050800>] :ext3:ext3_direct_IO+0xf3/0x18b
    > kernel: [<ffffffff8804ec84>] :ext3:ext3_get_block+0x0/0xe3
    > kernel: [<ffffffff800be6bb>] generic_file_direct_IO+0xbd/0xfb
    > kernel: [<ffffffff8001e637>] generic_file_direct_write+0x60/0xf2
    > kernel: [<ffffffff80015cfd>] __generic_file_aio_write_nolock+0x2b7/0x3b8
    > kernel: [<ffffffff8002134f>] generic_file_aio_write+0x65/0xc1
    > kernel: [<ffffffff8804c192>] :ext3:ext3_file_write+0x16/0x91
    > kernel: [<ffffffff80017944>] do_sync_write+0xc7/0x104
    > kernel: [<ffffffff8009c446>] autoremove_wake_function+0x0/0x2e
    > kernel: [<ffffffff80111400>] free_msg+0x22/0x3c
    > kernel: [<ffffffff800161c4>] vfs_write+0xce/0x174
    > kernel: [<ffffffff8004194c>] sys_pwrite64+0x50/0x70
    > kernel: [<ffffffff8005cde9>] error_exit+0x0/0x84
    > kernel: [<ffffffff8005c116>] system_call+0x7e/0x83

    So it's basically dm calling into blk_unplug() all the time, which
    doesn't check if the queue is plugged. The reason why I didn't like the
    initial patch is that ->unplug_fn() really should not be called unless
    the queue IS plugged. So how about this instead:

    http://git.kernel.dk/?p=linux-2.6-block.git;a=commit;h=c44993018887e82abd49023e92e8d8b6000e03ed

    That's a lot more appropriate, imho.

    --
    Jens Axboe



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