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Re: [Qemu-devel] [PATCH v4 12/13] aio: self-tune polling time
From: |
Stefan Hajnoczi |
Subject: |
Re: [Qemu-devel] [PATCH v4 12/13] aio: self-tune polling time |
Date: |
Tue, 6 Dec 2016 09:20:29 +0000 |
User-agent: |
Mutt/1.7.1 (2016-10-04) |
On Mon, Dec 05, 2016 at 09:06:17PM +0100, Christian Borntraeger wrote:
> On 12/01/2016 08:26 PM, Stefan Hajnoczi wrote:
> > This patch is based on the algorithm for the kvm.ko halt_poll_ns
> > parameter in Linux. The initial polling time is zero.
> >
> > If the event loop is woken up within the maximum polling time it means
> > polling could be effective, so grow polling time.
> >
> > If the event loop is woken up beyond the maximum polling time it means
> > polling is not effective, so shrink polling time.
> >
> > If the event loop makes progress within the current polling time then
> > the sweet spot has been reached.
> >
> > This algorithm adjusts the polling time so it can adapt to variations in
> > workloads. The goal is to reach the sweet spot while also recognizing
> > when polling would hurt more than help.
> >
> > Two new trace events, poll_grow and poll_shrink, are added for observing
> > polling time adjustment.
> >
> > Signed-off-by: Stefan Hajnoczi <address@hidden>
>
> Not sure way, but I have 4 host ramdisks with the same iothread as guest
> virtio-blk. running fio in the guest on one of these disks will poll, as
> soon as I have 2 disks in fio I almost always see shrinks (so polling
> stays at 0) and almost no grows.
Shrinking occurs when polling + ppoll(2) time exceeds poll-max-ns.
What is the value of poll-max-ns and how long is run_poll_handlers_end -
run_poll_handlers_begin?
I wonder if polling both disks takes longer than poll-max-ns once you
have two disks. The "polling" activity includes processing the I/O
requests, so I imagine the time extends significantly as more disks have
I/O requests ready for processing.
Maybe the block_ns timing calculation should exclude processing time to
avoid false shrinking?
It also strikes me that there's a blind spot to the self-tuning
algorithm: imagine virtqueue kick via ppoll(2) + ioeventfd takes N
nanoseconds. Detecting new virtqueue buffers via polling takes M
nanoseconds. When M <= poll-max-ns < N the algorithm decides there is
no point in polling but it would actually be faster to poll. The reason
is that the algorithm only looks at block_ns, which is N, not M.
This seems difficult to tackle because the algorithm has no way of
predicting M unless it randomly tries to poll longer.
Stefan
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- Re: [Qemu-devel] [PATCH v4 04/13] virtio: poll virtqueues for new buffers, (continued)
- [Qemu-devel] [PATCH v4 06/13] iothread: add polling parameters, Stefan Hajnoczi, 2016/12/01
- [Qemu-devel] [PATCH v4 07/13] virtio-blk: suppress virtqueue kick during processing, Stefan Hajnoczi, 2016/12/01
- [Qemu-devel] [PATCH v4 05/13] linux-aio: poll ring for completions, Stefan Hajnoczi, 2016/12/01
- [Qemu-devel] [PATCH v4 08/13] virtio-scsi: suppress virtqueue kick during processing, Stefan Hajnoczi, 2016/12/01
- [Qemu-devel] [PATCH v4 09/13] virtio: turn vq->notification into a nested counter, Stefan Hajnoczi, 2016/12/01
- [Qemu-devel] [PATCH v4 11/13] virtio: disable virtqueue notifications during polling, Stefan Hajnoczi, 2016/12/01
- [Qemu-devel] [PATCH v4 10/13] aio: add .io_poll_begin/end() callbacks, Stefan Hajnoczi, 2016/12/01
- [Qemu-devel] [PATCH v4 12/13] aio: self-tune polling time, Stefan Hajnoczi, 2016/12/01
[Qemu-devel] [PATCH v4 13/13] iothread: add poll-grow and poll-shrink parameters, Stefan Hajnoczi, 2016/12/01
Re: [Qemu-devel] [PATCH v4 00/13] aio: experimental virtio-blk polling mode, Paolo Bonzini, 2016/12/02
Re: [Qemu-devel] [PATCH v4 00/13] aio: experimental virtio-blk polling mode, Stefan Hajnoczi, 2016/12/02
Re: [Qemu-devel] [PATCH v4 00/13] aio: experimental virtio-blk polling mode, Fam Zheng, 2016/12/05
Re: [Qemu-devel] [PATCH v4 00/13] aio: experimental virtio-blk polling mode, Christian Borntraeger, 2016/12/05