Re: [Qemu-devel] QEMU event loop optimizations

[Stefan Hajnoczi]

On Fri, Apr 05, 2019 at 06:29:49PM +0200, Sergio Lopez wrote:
> 
> Stefan Hajnoczi writes:
> 
> > Hi Sergio,
> > Here are the forgotten event loop optimizations I mentioned:
> >
> >   https://github.com/stefanha/qemu/commits/event-loop-optimizations
> >
> > The goal was to eliminate or reorder syscalls so that useful work (like
> > executing BHs) occurs as soon as possible after an event is detected.
> >
> > I remember that these optimizations only shave off a handful of
> > microseconds, so they aren't a huge win.  They do become attractive on
> > fast SSDs with <10us read/write latency.
> >
> > These optimizations are aggressive and there is a possibility of
> > introducing regressions.
> >
> > If you have time to pick up this work, try benchmarking each commit
> > individually so performance changes are attributed individually.
> > There's no need to send them together in a single patch series, the
> > changes are quite independent.
> 
> It took me a while to find a way to get meaningful numbers to evaluate
> those optimizations. The problem is that here (Xeon E5-2640 v3 and EPYC
> 7351P) the cost of event_notifier_set() is just ~0.4us when the code
> path is hot, and it's hard differentiating it from the noise.
> 
> To do so, I've used a patched kernel with a naive io_poll implementation
> for virtio_blk [1], an also patched QEMU with poll-inflight [2] (just to
> be sure we're polling) and ran the test on semi-isolated cores
> (nohz_full + rcu_nocbs + systemd_isolation) with idle siblings. The
> storage is simulated by null_blk with "completion_nsec=0 no_sched=1
> irqmode=0".
> 
> # fio --time_based --runtime=30 --rw=randread --name=randread \
>  --filename=/dev/vdb --direct=1 --ioengine=pvsync2 --iodepth=1 --hipri=1
> 
> | avg_lat (us) | master | qbsn* |
> |   run1       | 11.32  | 10.96 |
> |   run2       | 11.37  | 10.79 |
> |   run3       | 11.42  | 10.67 |
> |   run4       | 11.32  | 11.06 |
> |   run5       | 11.42  | 11.19 |
> |   run6       | 11.42  | 10.91 |
>  * patched with aio: add optimized qemu_bh_schedule_nested() API
> 
> Even though there's still some variance in the numbers, the 0.4us
> improvement can be clearly appreciated.
> 
> I haven't tested the other 3 patches, as their optimizations only have
> effect when the event loop is not running in polling mode. Without
> polling, we get an additional overhead of, at least, 10us, in addition
> to a lot of noise, due to both direct costs (ppoll()...) and indirect
> ones (re-scheduling and TLB/cache pollution), so I don't think we can
> reliable benchmark them. Probably their impact won't be significant
> either, due to the costs I've just mentioned.

Thanks for benchmarking them.  We can leave them for now, since there is
a risk of introducing bugs and they don't make a great difference.

Stefan

> Sergio.
> 
> [1] 
> https://github.com/slp/linux/commit/d369b37db3e298933e8bb88c6eeacff07f39bc13
> [2] https://lists.nongnu.org/archive/html/qemu-devel/2019-04/msg00447.html

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