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SubjectRe: CFQ: async queue blocks the whole system
From
2011/6/9 Vivek Goyal <vgoyal@redhat.com>:
> On Thu, Jun 09, 2011 at 10:47:43PM +0800, Tao Ma wrote:
>> Hi Vivek,
>>       Thanks for the quick response.
>> On 06/09/2011 10:14 PM, Vivek Goyal wrote:
>> > On Thu, Jun 09, 2011 at 06:49:37PM +0800, Tao Ma wrote:
>> >> Hi Jens and Vivek,
>> >>    We are current running some heavy ext4 metadata test,
>> >> and we found a very severe problem for CFQ. Please correct me if
>> >> my statement below is wrong.
>> >>
>> >> CFQ only has an async queue for every priority of every class and
>> >> these queues have a very low serving priority, so if the system
>> >> has a large number of sync reads, these queues will be delayed a
>> >> lot of time. As a result, the flushers will be blocked, then the
>> >> journal and finally our applications[1].
>> >>
>> >> I have tried to let jbd/2 to use WRITE_SYNC so that they can checkpoint
>> >> in time and the patches are sent. But today we found another similar
>> >> block in kswapd which make me think that maybe CFQ should be changed
>> >> somehow so that all these callers can benefit from it.
>> >>
>> >> So is there any way to let the async queue work timely or at least
>> >> is there any deadline for async queue to finish an request in time
>> >> even in case there are many reads?
>> >>
>> >> btw, We have tested deadline scheduler and it seems to work in our test.
>> >>
>> >> [1] the message we get from one system:
>> >> INFO: task flush-8:0:2950 blocked for more than 120 seconds.
>> >> "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
>> >> flush-8:0       D ffff88062bfde738     0  2950      2 0x00000000
>> >>  ffff88062b137820 0000000000000046 ffff88062b137750 ffffffff812b7bc3
>> >>  ffff88032cddc000 ffff88062bfde380 ffff88032d3d8840 0000000c2be37400
>> >>  000000002be37601 0000000000000006 ffff88062b137760 ffffffff811c242e
>> >> Call Trace:
>> >>  [<ffffffff812b7bc3>] ? scsi_request_fn+0x345/0x3df
>> >>  [<ffffffff811c242e>] ? __blk_run_queue+0x1a/0x1c
>> >>  [<ffffffff811c57cc>] ? queue_unplugged+0x77/0x8e
>> >>  [<ffffffff813dbe67>] io_schedule+0x47/0x61
>> >>  [<ffffffff811c512c>] get_request_wait+0xe0/0x152
>> >
>> > Ok, so flush slept on trying to get a "request" allocated on request
>> > queue. That means all the ASYNC request descriptors are already consumed
>> > and we are not making progress with ASYNc requests.
>> >
>> > A relatively recent patch allowed sync queues to always preempt async queues
>> > and schedule sync workload instead of async. This had the potential to
>> > starve async queues and looks like that's what we are running into.
>> >
>> > commit f8ae6e3eb8251be32c6e913393d9f8d9e0609489
>> > Author: Shaohua Li <shaohua.li@intel.com>
>> > Date:   Fri Jan 14 08:41:02 2011 +0100
>> >
>> >     block cfq: make queue preempt work for queues from different workload
>> >
>> > Do you have few seconds of blktrace. I just wanted to verify that this
>> > is what we are running into.
>> We are using the latest kernel, so the patch is already there. :(
>>
>> You are right that all the requests have been allocated and the flusher
>> is waiting for requests to be available. But the root cause is that in
>> heavy sync read, the async queue in cfq is delayed too much. I have
>> added some traces in the cfq codes path and after several investigation,
>> I found several interesting things and tried to improve it. But I am not
>> sure whether it is bug or it is designed intentionally.
>>
>> 1. In cfq_dispatch_requests we select a sync queue to serve, but if the
>> queue has too much requests in flight, the cfq_slice_used_soon may be
>> true and the cfqq isn't allowed to send and will waste some timeslice.
>> Then why choose this cfqq? Why not choose a qualified one?
>
> CFQ in general tries not to drive too deep a queue depth in an effort
> to improve latencies. CFQ is generally recommened for slow SATA drives
> and dispatching too many requests from a single queue can only serve to
> increase the latency.
>
>>
>> 2. async queue isn't allowed to be sent if there is some sync request in
>> fly, but as now most of the devices has a greater depth, should we
>> improve it somehow? I guess queue_depth should be a valid number maybe?
>
> We seem to be running this batching thing in cfq_may_dispatch() where
> we drain sync requests before async is dispatched and vice-a-versa.
> I am not sure how does this batching thing helps. I think Jens should
> be a better person to comment on that.
>
> I ran a fio job with few readers and few writers. I do see that few times
> we have schedule ASYNC workload/queue but did not dispatch a request
> from that. And reason being that there are sync requests in flight. And
> by the time sync requests finish, async queue gets preempted.
>
> So async queue does it scheduled but never gets a chance to dispatch
> a request because there was sync IO in flight.
>
> If there is no major advantage of draining sync requests before async
> is dispatched, I think this should be an easy fix.
I thought this is to avoid sync latency if we switch from an async
queue to sync queue later.

>> 3. Even there is no sync i/o, the async queue isn't allowed to send too
>> much requests because of the check in cfq_may_dispatch "Async queues
>> must wait a bit before being allowed dispatch", so in my test the async
>> queue has several chances to be selected, but it is only allowed
>> todispatch one request at a time. It is really amazing.
>
> Again heavily loaded to improve sync latencies. Say you have queue
> depth of 128 and you fill that all with async requests because right
> now there is no sync request around. Then a sync request comes in.
> We don't have a way to give it a priority and it might happen that
> it gets executed after 128 async requests have finished (driver and
> drive dependent though).
>
> So in an attempt to improve sync latencies we don't drive too
> high queue depths.
>
> Its latency vs throughput tradeoff.
The current cfq do be able to stave async queue, because we want to give small
latency to sync queue.
I agree we should do something to improve async starvation, but the
problem is how
long async queue slice should be. A sd card I tested has very high
latency for write. A 4k write can take > 300ms. Just dispatching a
singe write can dramatically impact
read throughput. Even in modern SSD, read is several times faster than write.

Thanks,
Shaohua
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