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

Dave Chinner
2014-06-18T00:28:59Z

[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.

> 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.

Cheers,

Dave.
--
Dave Chinner
david@fromorbit.com