Messages in this thread | | | Date | Tue, 28 Sep 2010 07:41:07 +0900 | From | Jens Axboe <> | Subject | Re: Request starvation with CFQ |
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On 2010-09-28 07:35, Jan Kara wrote: > On Tue 28-09-10 07:04:40, Jens Axboe wrote: >> 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: > <snip> >>>> 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. > So no per-queue request limit? Since ioc is per-process if I'm right, > that would solve the problem quite nicely. Thanks for info.
Exactly, no more per-queue upper limit, or at least a very relaxed one if that. I want to get rid of some of that shared state.
-- Jens Axboe
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