On Tue, Apr 29 2008, Mike Anderson wrote:
quoted text
> 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=c44993018887e82abd49023e92e8d8b...

That's a lot more appropriate, imho.

-- 
Jens Axboe

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