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Re: [Qemu-devel] Lock contention in QEMU


From: Stefan Hajnoczi
Subject: Re: [Qemu-devel] Lock contention in QEMU
Date: Thu, 15 Dec 2016 08:04:53 +0000
User-agent: Mutt/1.7.1 (2016-10-04)

On Wed, Dec 14, 2016 at 08:06:10PM -0500, Weiwei Jia wrote:
> Hi Stefan,
> 
> Thanks for your reply. Please see the inline replies.
> 
> On Wed, Dec 14, 2016 at 2:31 PM, Stefan Hajnoczi <address@hidden> wrote:
> > On Wed, Dec 14, 2016 at 12:58:11AM -0500, Weiwei Jia wrote:
> >> I find the timeslice of vCPU thread in QEMU/KVM is unstable when there
> >> are lots of read requests (for example, read 4KB each time (8GB in
> >> total) from one file) from Guest OS. I also find that this phenomenon
> >> may be caused by lock contention in QEMU layer. I find this problem
> >> under following workload.
> >>
> >> Workload settings:
> >> In VMM, there are 6 pCPUs which are pCPU0, pCPU1, pCPU2, pCPU3, pCPU4,
> >> pCPU5. There are two Kernel Virtual Machines (VM1 and VM2) upon VMM.
> >> In each VM, there are 5 vritual CPUs (vCPU0, vCPU1, vCPU2, vCPU3,
> >> vCPU4). vCPU0 in VM1 and vCPU0 in VM2 are pinned to pCPU0 and pCPU5
> >> separately to handle interrupts dedicatedly. vCPU1 in VM1 and vCPU1 in
> >> VM2 are pinned to pCPU1; vCPU2 in VM1 and vCPU2 in VM2 are pinned to
> >> pCPU2; vCPU3 in VM1 and vCPU3 in VM2 are pinned to pCPU3; vCPU4 in VM1
> >> and vCPU4 in VM2 are pinned to pCPU4. Besides vCPU0 in VM2 (pinned to
> >> pCPU5), other vCPUs all have one CPU intensive thread (while(1){i++})
> >> upon each of them in VM1 and VM2 to avoid the vCPU to be idle. In VM1,
> >> I start one I/O thread on vCPU2, which the I/O thread reads 4KB from
> >> one file each time (reads 8GB in total). The I/O scheduler in VM1 and
> >> VM2 is NOOP. The I/O scheduler in VMM is CFQ. I also pinned the I/O
> >> worker threads launched by QEMU to pCPU5 (note: there is no CPU
> >> intensive thread on pCPU5 so the I/O requests will be handled by QEMU
> >> I/O thread workers ASAP). The process scheduling class in VM and VMM
> >> is CFS.
> >
> > Did you pin the QEMU main loop to pCPU5?  This is the QEMU process' main
> > thread and it handles ioeventfd (virtqueue kick) and thread pool
> > completions.
> 
> No, I did not pin main loop to pCPU5. Do you mean If I pin QEMU main
> loop to pCPU5 under above workload, the timeslice of vCPU2 thread will
> be stable even though there are lots of I/O requests? I didn't use
> virtio for VM and I use SCSI. My whole VM xml configuration file is as
> follows.

Pinning the main loop will probably not solve the problem but it might
help a bit.  I just noticed it while reading your email because you
pinned everything carefully except the main loop, which is an important
thread.

> >>
> >> Linux Kernel version for VMM is: 3.16.39
> >> Linux Kernel version for VM1 and VM2 is: 4.7.4
> >> QEMU emulator version is: 2.0.0
> >>
> >> When I test above workload, I find the timeslice of vCPU2 thread
> >> jitters very much. I suspect this is triggered by lock contention in
> >> QEMU layer since my debug log in front of VMM Linux Kernel's
> >> schedule->__schedule->context_switch is like following. Once the
> >> timeslice jitters very much, following debug information will appear.
> >>
> >> 7097537 Dec 13 11:22:33 mobius04 kernel: [39163.015789] Call Trace:
> >> 7097538 Dec 13 11:22:33 mobius04 kernel: [39163.015791]
> >> [<ffffffff8176b2f0>] dump_stack+0x64/0x84
> >> 7097539 Dec 13 11:22:33 mobius04 kernel: [39163.015793]
> >> [<ffffffff8176bf85>] __schedule+0x5b5/0x960
> >> 7097540 Dec 13 11:22:33 mobius04 kernel: [39163.015794]
> >> [<ffffffff8176c409>] schedule+0x29/0x70
> >> 7097541 Dec 13 11:22:33 mobius04 kernel: [39163.015796]
> >> [<ffffffff810ef4d8>] futex_wait_queue_me+0xd8/0x150
> >> 7097542 Dec 13 11:22:33 mobius04 kernel: [39163.015798]
> >> [<ffffffff810ef6fb>] futex_wait+0x1ab/0x2b0
> >> 7097543 Dec 13 11:22:33 mobius04 kernel: [39163.015800]
> >> [<ffffffff810eef00>] ? get_futex_key+0x2d0/0x2e0
> >> 7097544 Dec 13 11:22:33 mobius04 kernel: [39163.015804]
> >> [<ffffffffc0290105>] ? __vmx_load_host_state+0x125/0x170 [kv
> >> m_intel]
> >> 7097545 Dec 13 11:22:33 mobius04 kernel: [39163.015805]
> >> [<ffffffff810f1275>] do_futex+0xf5/0xd20
> >> 7097546 Dec 13 11:22:33 mobius04 kernel: [39163.015813]
> >> [<ffffffffc0222690>] ? kvm_vcpu_ioctl+0x100/0x560 [kvm]
> >> 7097547 Dec 13 11:22:33 mobius04 kernel: [39163.015816]
> >> [<ffffffff810b06f0>] ? __dequeue_entity+0x30/0x50
> >> 7097548 Dec 13 11:22:33 mobius04 kernel: [39163.015818]
> >> [<ffffffff81013d06>] ? __switch_to+0x596/0x690
> >> 7097549 Dec 13 11:22:33 mobius04 kernel: [39163.015820]
> >> [<ffffffff811f9f23>] ? do_vfs_ioctl+0x93/0x520
> >> 7097550 Dec 13 11:22:33 mobius04 kernel: [39163.015822]
> >> [<ffffffff810f1f1d>] SyS_futex+0x7d/0x170
> >> 7097551 Dec 13 11:22:33 mobius04 kernel: [39163.015824]
> >> [<ffffffff8116d1b2>] ? fire_user_return_notifiers+0x42/0x50
> >> 7097552 Dec 13 11:22:33 mobius04 kernel: [39163.015826]
> >> [<ffffffff810154b5>] ? do_notify_resume+0xc5/0x100
> >> 7097553 Dec 13 11:22:33 mobius04 kernel: [39163.015828]
> >> [<ffffffff81770a8d>] system_call_fastpath+0x1a/0x1f
> >>
> >>
> >> If true, I think this may be a scalability problem caused by QEMU I/O
> >> part. Do we have a feature in QEMU to avoid this? Would you please
> >> give me some suggestions about how to make the timeslice of vCPU2
> >> thread stable even though there are lots of I/O Read requests on it.
> >
> > Yes, there is a way to reduce jitter caused by the QEMU global mutex:
> >
> > qemu -object iothread,id=iothread0 \
> >      -drive if=none,id=drive0,file=test.img,format=raw,cache=none \
> >      -device virtio-blk-pci,iothread=iothread0,drive=drive0
> >
> > Now the ioeventfd and thread pool completions will be processed in
> > iothread0 instead of the QEMU main loop thread.  This thread does not
> > take the QEMU global mutex so vcpu execution is not hindered.
> >
> > This feature is called virtio-blk dataplane.
> >
> > You can query IOThread thread IDs using the query-iothreads QMP command.
> > This will allow you to pin iothread0 to pCPU5.
> >
> > Please let us know if this helps.
> 
> Does this feature only work for VirtIO? Does it work for SCSI or IDE?

This only works for virtio-blk and virtio-scsi.  The virtio-scsi
dataplane support is more recent and I don't remember if it is complete.
I've CCed Fam and Paolo who worked on virtio-scsi dataplane.

Now that you have mentioned that you aren't using virtio devices, there
is another source of lock contention that you will encounter.  I/O
request submission takes place in the vcpu thread when ioeventfd is not
used.  Only virtio uses ioeventfd so your current QEMU configuration is
unable to let the vcpu continue execution during I/O request submission.

If you care about performance then using virtio devices is probably the
best choice.  Try comparing against virtio-scsi dataplane - you should
see a lot less jitter.

Stefan

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