Messages in this thread | | | Date | Sat, 25 Apr 2015 18:32:30 +0800 | Subject | Re: loop block-mq conversion scalability issues | From | Ming Lei <> |
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On Sat, Apr 25, 2015 at 5:46 AM, Justin M. Forbes <jforbes@fedoraproject.org> wrote: > On Fri, 2015-04-24 at 10:59 +0800, Ming Lei wrote: >> Hi Justin, >> >> Thanks for the report. >> >> On Thu, 23 Apr 2015 16:04:10 -0500 >> "Justin M. Forbes" <jforbes@redhat.com> wrote: >> >> > The block-mq conversion for loop in 4.0 kernels is showing us an >> > interesting scalability problem with live CDs (ro, squashfs). It was >> > noticed when testing the Fedora beta that the more CPUs a liveCD image >> > was given, the slower it would boot. A 4 core qemu instance or bare >> > metal instance took more than twice as long to boot compared to a single >> > CPU instance. After investigating, this came directly to the block-mq >> > conversion, reverting these 4 patches will return performance. More >> > details are available at >> > https://bugzilla.redhat.com/show_bug.cgi?id=1210857 >> > I don't think that reverting the patches is the ideal solution so I am >> > looking for other options. Since you know this code a bit better than I >> > do I thought I would run it by you while I am looking as well. >> >> I can understand the issue because the default @max_active for >> alloc_workqueue() is quite big(512), which may cause too much >> context switchs, then loop I/O performance gets decreased. >> >> Actually I have written the kernel dio/aio based patch for decreasing >> both CPU and memory utilization without sacrificing I/O performance, >> and I will try to improve and push the patch during this cycle and hope >> it can be merged(kernel/aio.c change is dropped, and only fs change is >> needed on fs/direct-io.c). >> >> But the following change should help for your case, could you test it? >> >> --- >> diff --git a/drivers/block/loop.c b/drivers/block/loop.c >> index c6b3726..b1cb41d 100644 >> --- a/drivers/block/loop.c >> +++ b/drivers/block/loop.c >> @@ -1831,7 +1831,7 @@ static int __init loop_init(void) >> } >> >> loop_wq = alloc_workqueue("kloopd", >> - WQ_MEM_RECLAIM | WQ_HIGHPRI | WQ_UNBOUND, 0); >> + WQ_MEM_RECLAIM | WQ_HIGHPRI | WQ_UNBOUND, 32); >> if (!loop_wq) { >> err = -ENOMEM; >> goto misc_out; >> > Patch tested, it made things work (I gave up after 5 minutes and boot > still seemed hung). I also tried values of 1, 16, 64, and 128). > Everything below 128 was much worse than the current situation. Setting > it at 128 seemed about the same as booting without the patch. I can do > some more testing over the weekend, but I don't think this is the > correct solution.
I saw you mention there are about 200 kworker threads waiting for run, that is why I thought it may be related with 'max active'
Also you can change the workqueue as bound by removing the flag of 'WQ_UNBOUND' to see if there is any difference?
Could you share me how loop is used during the Fedora live boot?
> I would be interested in testing your dio/aio patches as well though.
Recently there is big changes in loop, and I need to port the original patches to current loop, but I guess it shouldn't take much time.
Thanks, Ming Lei
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