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Message-ID: <4DF0DD0F.8090407@tao.ma> Date: Thu, 09 Jun 2011 22:47:43 +0800 From: Tao Ma <tm@....ma> To: Vivek Goyal <vgoyal@...hat.com> CC: linux-kernel@...r.kernel.org, Jens Axboe <axboe@...nel.dk> Subject: Re: CFQ: async queue blocks the whole system 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@...el.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? 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? 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. 4. We have nr_requests = 128 for the async queues, but have so many limitations for it to be carried out, and once the processes get the chances, it will batch the requests, and in my tests, the total async requests can be accumulated to more than 180, So it takes a really long time for the process to wait for the requests to be lower than 127 again to request more I/O and causes the livelock. So the number also be improved somehow? Maybe it should be changed dynamically? anyway, if the async queue can be dispatched in time and in batches in cfq, it shouldn't be a problem. btw, I uses a SAS which has a queue_depth=128 and the D2C time is very small, so the whole system throughout is really really low. Regards, Tao -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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