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Date: Tue, 28 Sep 2010 07:04:40 +0900
From: Jens Axboe <jaxboe@...ionio.com>
To: Vivek Goyal <vgoyal@...hat.com>
CC: Jan Kara <jack@...e.cz>, LKML <linux-kernel@...r.kernel.org>,
"jmoyer@...hat.com" <jmoyer@...hat.com>,
Lennart Poettering <lennart@...ttering.net>
Subject: Re: Request starvation with CFQ
On 2010-09-28 05:02, Vivek Goyal wrote:
> On Mon, Sep 27, 2010 at 09:00:24PM +0200, Jan Kara wrote:
>> Hi,
>>
>> when helping Lennart with answering some questions, I've spotted the
>> following problem (at least I think it's a problem ;): The thing is that
>> CFQ schedules how requests should be dispatched but does not in any
>> significant way limit to whom requests get allocated. Given we have a
>> quite limited pool of available requests it can happen that processes
>> will be actually starved not waiting for disk but waiting for requests
>> getting allocated and any IO scheduling priorities or classes will not
>> have serious effect.
>> A pathological example I've tried below:
>> #include <fcntl.h>
>> #include <stdio.h>
>> #include <stdlib.h>
>> #include <sys/stat.h>
>>
>> int main(void)
>> {
>> int fd = open("/dev/vdb", O_RDONLY);
>> int loop = 0;
>>
>> if (fd < 0) {
>> perror("open");
>> exit(1);
>> }
>> while (1) {
>> if (loop % 100 == 0)
>> printf("Loop %d\n", loop);
>> posix_fadvise(fd, (random() * 4096) % 1000204886016ULL, 4096, POSIX_FADV_WILLNEED);
>> loop++;
>> }
>> }
>>
>> This program will just push as many requests as possible to the block
>> layer and does not wait for any IO. Thus it will basically ignore any
>> decisions about when requests get dispatched. BTW, don't get distracted
>> by the fact that the program operates directly on the device, that is just
>> for simplicity. Large enough file would work the same way.
>> Even though I run this program with ionice -c 3, I still see that any
>> other IO to the device is basically stalled. When I look at the block
>> traces, I indeed see that what happens is that the above program submits
>> requests until there are no more available:
>> ...
>> 254,16 2 802 1.411285520 2563 Q R 696733184 + 8 [random_read]
>> 254,16 2 803 1.411314880 2563 G R 696733184 + 8 [random_read]
>> 254,16 2 804 1.411338220 2563 I R 696733184 + 8 [random_read]
>> 254,16 2 805 1.411415040 2563 Q R 1006864600 + 8 [random_read]
>> 254,16 2 806 1.411441620 2563 S R 1006864600 + 8 [random_read]
>>
>> during and after that IO happens:
>> 254,16 3 31 1.417898030 0 C R 345134640 + 8 [0]
>> 254,16 3 32 1.418171910 0 D R 1524771568 + 8 [swapper]
>> 254,16 0 33 1.432317140 0 C R 1524771568 + 8 [0]
>> 254,16 0 34 1.432597000 0 D R 1077270768 + 8 [swapper]
>> ...
>> 254,16 0 35 1.503238050 0 C R 33633744 + 8 [0]
>> 254,16 0 36 1.503558290 0 D R 22178968 + 8 [swapper]
>>
>> and the other program comes with IO and gets stalled:
>> 254,16 1 39 1.508843180 2564 A RM 12346 + 8 <- (254,17) 12312
>> 254,16 1 40 1.508876520 2564 Q RM 12346 + 8 [ls]
>> 254,16 1 41 1.508905140 2564 S RM 12346 + 8 [ls]
>> ...
>> IO is still running:
>> 254,16 2 807 1.512081560 0 C R 22178968 + 8 [0]
>> 254,16 2 808 1.512365010 0 D R 475025688 + 8 [swapper]
>> 254,16 3 35 1.522113270 0 C R 475025688 + 8 [0]
>> 254,16 3 36 1.522390779 0 D R 697010128 + 8 [swapper]
>> 254,16 4 33 1.531443760 0 C R 697010128 + 8 [0]
>> ...
>> random reader even gets to submitting more requests:
>> 254,16 2 815 1.785734950 2563 G R 1006864600 + 8 [random_read]
>> 254,16 2 816 1.785752290 2563 I R 1006864600 + 8 [random_read]
>> 254,16 2 817 1.785825880 2563 Q R 832683552 + 8 [random_read]
>> 254,16 2 818 1.785850890 2563 G R 832683552 + 8 [random_read]
>> 254,16 2 819 1.785874610 2563 I R 832683552 + 8 [random_read]
>> ...
>> and finally our program gets to adding it's request as well:
>> 254,16 1 60 2.160884040 2564 G RM 12346 + 8 [ls]
>> 254,16 1 61 2.160914700 2564 I R 12346 + 8 [ls]
>> 254,16 1 62 2.161142170 2564 D R 12346 + 8 [ls]
>> 254,16 1 63 2.161233670 2564 U N [ls] 128
>>
>> I can provide the full traces for download if someone is interested
>> in some part I didn't include here. The kernel is 2.6.36-rc4.
>> Now I agree that the above program is about as bad as it can get but
>> Lennart would like to implement readahead during boot on background and
>> I believe that could starve other IO in a similar way. So any idea how
>> to solve this? To me it seems as if we also needed to somehow limit the
>> number of allocated requests per cfqq but OTOH we have to be really careful
>> to not harm common workloads where we benefit from having lots of requests
>> queued...
>
> Hi Jan,
>
> True that during request allocation, there is no consideration for ioprio.
> I think the whole logic is round robin, where after getting a bunch of
> request each process is put to sleep in the queue and then we do round
> robin on all waiters. This should in general be an issue with request
> queue and not just CFQ.
>
> So if there are bunch of threads which are very bullish on doing IO, and
> there is a dependent reader, read latencies will shoot up.
>
> In fact current implementation of blkio controller also suffers with this
> limitation because we don't yet have per group request descriptors and
> once request queue is congested, requests from one group can get stuck
> behind the requests from other group.
>
> One way forward could be to implement per cgroup request descriptors and
> put this readahead thread into a separate cgroup of low weight.
>
> Other could be to implemnet some kind of request quota per priority level.
> This is similar to per cgroup quota I talked above, just one level below.
>
> Third could be ad-hoc way of putting some limit on per cfqq. But I think a
> process can easily circumvent that by forking off child which are not
> sharing cfq context and then we are back to same situaiton.
>
> A very hackish solution could be to try to increase nr_requests on the
> queue to say 1024. This will work only if you know that read-ahead process
> does some limited amount of read-ahead and does not overwhelm the queue
> with more than 1024 requets. And then use ioprio with low prio for
> read-ahead process.
I don't think that is necessarily hackish. The current rq allocation
batching and accounting is pretty horrible imho, in fact in recent
patches I ripped that out. The vm copes a lot better with larger depths
these days, so what I want to add is just a per-ioc queue limit instead.
--
Jens Axboe
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