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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 17:22:44 +0000
User-agent: Mutt/1.7.1 (2016-10-04)

On Tue, Dec 06, 2016 at 11:12:45AM +0100, Christian Borntraeger wrote:
> On 12/06/2016 10:20 AM, Stefan Hajnoczi wrote:
> > 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
> 
> I used 50000ns as poll value. When using 500000ns it is polling again.
> 
> > and how long is run_poll_handlers_end - run_poll_handlers_begin?
> 
> Too long. I looked again and I realized that I used cache=none without
> io=native. After adding io=native things are better. Even with 4 disks
> polling still happens. So it seems that the mileage will vary depending
> on the settings

Okay, it could be the things I mentioned.  The self-tuning can be too
conservative and poll less than we'd hope for.  On the other hand it's
hard to predict the actual ideal polling amount so I guess being
conservative is alright.  Maybe the algorithm can be refined later.

Stefan

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