Re: [PATCH] [RFC] xfs: wire up aio_fsync method

Jens Axboe
2014-06-18T02:24:17Z

On 2014-06-17 17:28, Dave Chinner wrote:
> [cc linux-mm]
>
> On Tue, Jun 17, 2014 at 07:23:58AM -0600, Jens Axboe wrote:
>> On 2014-06-16 16:27, Dave Chinner wrote:
>>> On Mon, Jun 16, 2014 at 01:30:42PM -0600, Jens Axboe wrote:
>>>> On 06/16/2014 01:19 AM, Dave Chinner wrote:
>>>>> On Sun, Jun 15, 2014 at 08:58:46PM -0600, Jens Axboe wrote:
>>>>>> On 2014-06-15 20:00, Dave Chinner wrote:
>>>>>>> On Mon, Jun 16, 2014 at 08:33:23AM +1000, Dave Chinner wrote:
>>>>>>> FWIW, the non-linear system CPU overhead of a fs_mark test I've been
>>>>>>> running isn't anything related to XFS. The async fsync workqueue
>>>>>>> results in several thousand worker threads dispatching IO
>>>>>>> concurrently across 16 CPUs:
>>>>>>>
>>>>>>> $ ps -ef |grep kworker |wc -l
>>>>>>> 4693
>>>>>>> $
>>>>>>>
>>>>>>> Profiles from 3.15 + xfs for-next + xfs aio_fsync show:
>>>>>>>
>>>>>>> - 51.33% [kernel] [k] percpu_ida_alloc
>>>>>>> - percpu_ida_alloc
>>>>>>> + 85.73% blk_mq_wait_for_tags
>>>>>>> + 14.23% blk_mq_get_tag
>>>>>>> - 14.25% [kernel] [k] _raw_spin_unlock_irqrestore
>>>>>>> - _raw_spin_unlock_irqrestore
>>>>>>> - 66.26% virtio_queue_rq
>>>>>>> - __blk_mq_run_hw_queue
>>>>>>> - 99.65% blk_mq_run_hw_queue
>>>>>>> + 99.47% blk_mq_insert_requests
>>>>>>> + 0.53% blk_mq_insert_request
>>>>>>> .....
>>>>>>> - 7.91% [kernel] [k] _raw_spin_unlock_irq
>>>>>>> - _raw_spin_unlock_irq
>>>>>>> - 69.59% __schedule
>>>>>>> - 86.49% schedule
>>>>>>> + 47.72% percpu_ida_alloc
>>>>>>> + 21.75% worker_thread
>>>>>>> + 19.12% schedule_timeout
>>>>>>> ....
>>>>>>> + 18.06% blk_mq_make_request
>>>>>>>
>>>>>>> Runtime:
>>>>>>>
>>>>>>> real 4m1.243s
>>>>>>> user 0m47.724s
>>>>>>> sys 11m56.724s
>>>>>>>
>>>>>>> Most of the excessive CPU usage is coming from the blk-mq layer, and
>>>>>>> XFS is barely showing up in the profiles at all - the IDA tag
>>>>>>> allocator is burning 8 CPUs at about 60,000 write IOPS....
>>>>>>>
>>>>>>> I know that the tag allocator has been rewritten, so I tested
>>>>>>> against a current a current Linus kernel with the XFS aio-fsync
>>>>>>> patch. The results are all over the place - from several sequential
>>>>>>> runs of the same test (removing the files in between so each tests
>>>>>>> starts from an empty fs):
>>>>>>>
>>>>>>> Wall time sys time IOPS files/s
>>>>>>> 4m58.151s 11m12.648s 30,000 13,500
>>>>>>> 4m35.075s 12m45.900s 45,000 15,000
>>>>>>> 3m10.665s 11m15.804s 65,000 21,000
>>>>>>> 3m27.384s 11m54.723s 85,000 20,000
>>>>>>> 3m59.574s 11m12.012s 50,000 16,500
>>>>>>> 4m12.704s 12m15.720s 50,000 17,000
>>>>>>>
>>>>>>> The 3.15 based kernel was pretty consistent around the 4m10 mark,
>>>>>>> generally only +/-10s in runtime and not much change in system time.
>>>>>>> The files/s rate reported by fs_mark doesn't vary that much, either.
>>>>>>> So the new tag allocator seems to be no better in terms of IO
>>>>>>> dispatch scalability, yet adds significant variability to IO
>>>>>>> performance.
>>>>>>>
>>>>>>> What I noticed is a massive jump in context switch overhead: from
>>>>>>> around 250,000/s to over 800,000/s and the CPU profiles show that
>>>>>>> this comes from the new tag allocator:
>>>>>>>
>>>>>>> - 34.62% [kernel] [k] _raw_spin_unlock_irqrestore
>>>>>>> - _raw_spin_unlock_irqrestore
>>>>>>> - 58.22% prepare_to_wait
>>>>>>> 100.00% bt_get
>>>>>>> blk_mq_get_tag
>>>>>>> __blk_mq_alloc_request
>>>>>>> blk_mq_map_request
>>>>>>> blk_sq_make_request
>>>>>>> generic_make_request
>>>>>>> - 22.51% virtio_queue_rq
>>>>>>> __blk_mq_run_hw_queue
>>>>>>> ....
>>>>>>> - 21.56% [kernel] [k] _raw_spin_unlock_irq
>>>>>>> - _raw_spin_unlock_irq
>>>>>>> - 58.73% __schedule
>>>>>>> - 53.42% io_schedule
>>>>>>> 99.88% bt_get
>>>>>>> blk_mq_get_tag
>>>>>>> __blk_mq_alloc_request
>>>>>>> blk_mq_map_request
>>>>>>> blk_sq_make_request
>>>>>>> generic_make_request
>>>>>>> - 35.58% schedule
>>>>>>> + 49.31% worker_thread
>>>>>>> + 32.45% schedule_timeout
>>>>>>> + 10.35% _xfs_log_force_lsn
>>>>>>> + 3.10% xlog_cil_force_lsn
>>>>>>> ....
>>> .....
>>>> Can you try with this patch?
>>>
>>> Ok, context switches are back down in the realm of 400,000/s. It's
>>> better, but it's still a bit higher than that the 3.15 code. XFS is
>>> actually showing up in the context switch path profiles now...
>>>
>>> However, performance is still excitingly variable and not much
>>> different to not having this patch applied. System time is unchanged
>>> (still around the 11m20s +/- 1m) and IOPS, wall time and files/s all
>>> show significant variance (at least +/-25%) from run to run. The
>>> worst case is not as slow as the unpatched kernel, but it's no
>>> better than the 3.15 worst case.
>>>
>>> Profiles on a slow run look like:
>>>
>>> - 43.43% [kernel] [k] _raw_spin_unlock_irq
>>> - _raw_spin_unlock_irq
>>> - 64.23% blk_sq_make_request
>>> generic_make_request
>>> submit_bio ¿
>>> + 26.79% __schedule
>>> ...
>>> - 15.00% [kernel] [k] _raw_spin_unlock_irqrestore
>>> - _raw_spin_unlock_irqrestore
>>> - 39.81% virtio_queue_rq
>>> __blk_mq_run_hw_queue
>>> + 24.13% complete
>>> + 17.74% prepare_to_wait_exclusive
>>> + 9.66% remove_wait_queue
>>>
>>> Looks like the main contention problem is in blk_sq_make_request().
>>> Also, there looks to be quite a bit of lock contention on the tag
>>> wait queues given that this patch made prepare_to_wait_exclusive()
>>> suddenly show up in the profiles.
>>>
>>> FWIW, on a fast run there is very little time in
>>> blk_sq_make_request() lock contention, and overall spin lock/unlock
>>> overhead of these two functions is around 10% each....
>>>
>>> So, yes, the patch reduces context switches but doesn't really
>>> reduce system time, improve performance noticably or address the
>>> run-to-run variability issue...
>>
>> OK, so one more thing to try. With the same patch still applied,
>> could you edit block/blk-mq-tag.h and change
>>
>> BT_WAIT_QUEUES = 8,
>>
>> to
>>
>> BT_WAIT_QUEUES = 1,
>>
>> and see if that smoothes things out?
>
> Ok, that smoothes things out to the point where I can see the
> trigger for the really nasty variable performance. The trigger is
> the machine running out of free memory. i.e. direct reclaim of clean
> pages for the data in the new files in the page cache drives the
> performance down by 25-50% and introduces significant variability.
>
> So the variability doesn't seem to be solely related to the tag
> allocator; it is contributing some via wait queue contention,
> but it's definitely not the main contributor, nor the trigger...
>
> MM-folk - the VM is running fake-numa=4 and has 16GB of RAM, and
> each step in the workload is generating 3.2GB of dirty pages (i.e.
> just on the dirty throttling threshold). It then does a concurrent
> asynchronous fsync of the 800,000 dirty files it just created,
> leaving 3.2GB of clean pages in the cache. The workload iterates
> this several times. Once the machine runs out of free memory (2.5
> iterations in) performance drops by about 30% on average, but the
> drop varies between 20-60% randomly. I'm not concerned by a 30% drop
> when memory fills up - I'm concerned by the volatility of the drop
> that occurs. e.g:
>
> FSUse% Count Size Files/sec App Overhead
> 0 800000 4096 29938.0 13459475
> 0 1600000 4096 28023.7 15662387
> 0 2400000 4096 23704.6 16451761
> 0 3200000 4096 16976.8 15029056
> 0 4000000 4096 21858.3 15591604
>
> Iteration 3 is where memory fills, and you can see that performance
> dropped by 25%. Iteration 4 drops another 25%, then iteration 5
> regains it. If I keep running the workload for more iterations, this
> is pretty typical of the iteration-to-iteration variability, even
> though every iteration is identical in behaviour as are the initial
> conditions (i.e. memory full of clean, used-once pages).
>
> This didn't happen in 3.15.0, but the behaviour may have been masked
> by the block layer tag allocator CPU overhead dominating the system
> behaviour.

OK, that's reassuring. I'll do some testing with the cyclic wait queues,
but probably not until Thursday. Alexanders patches might potentially
fix the variability as well, but if we can make-do without the multiple
wait queues, I'd much rather just kill it.

Did you see any spinlock contention with BT_WAIT_QUEUES = 1?

>> On the road the next few days, so might take me a few days to get
>> back to this. I was able to reproduce the horrible contention on the
>> wait queue, but everything seemed to behave nicely with just the
>> exclusive_wait/batch_wakeup for me. Looks like I might have to fire
>> up kvm and set it you like you.
>
> I'm guessing that the difference is that you weren't driving the
> machine into memory reclaim at the same time.

My testing was purely O_DIRECT, so no, no reclaim at all. I was only
focused on the lock contention in there, and the patch completely got
rid of that.

--
Jens Axboe