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Date: Fri, 10 Jun 2011 09:19:12 +0800 From: Shaohua Li <shaohua.li@...el.com> To: Vivek Goyal <vgoyal@...hat.com> Cc: Tao Ma <tm@....ma>, linux-kernel@...r.kernel.org, Jens Axboe <axboe@...nel.dk> Subject: Re: CFQ: async queue blocks the whole system 2011/6/9 Vivek Goyal <vgoyal@...hat.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@...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? > > 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 -- 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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