Re: [Qemu-devel] [PATCH v2 0/6] aio: Support epoll by introducing qemu_poll abstraction

[Stefan Hajnoczi <stefanha@gmail.com>]

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On Tue, Dec 16, 2014 at 10:04:38AM +0800, Fam Zheng wrote:
> On Thu, 12/04 11:43, Fam Zheng wrote:
> > v2: Emulate nanoseconds precison of timeout with ppoll and timerfd.
> >     Their performance is on par with each other, but both much better than
> >     qemu.git:
> > 
> >     syscall         high # of fd      low # of fd
> >     -------------------------------------------------
> >     qemu.git(ppoll) 44                96
> >     ppoll+epoll     85                101
> >     timerfd+epoll   87                109
> 
> More data points.
> 
> Xiaomei tested this series (applied on top of RHEL 7 qemu-kvm-rhev) and found
> that:
> 
> 0) when # of fds is high, epoll solutions are much better (+30%).
> 
> 1) timerfd+epoll is slightly better than ppoll+epoll, but the difference is
> minimal.
> 
> 2) original code is 2%~5% faster than the new implementations when # of fds is
> low.

What is "high" and "low"?

I'd like to understand whether they are extremes that almost no users
will encounter or whether they are plausible in the real world.

> This leads to the conclusion that that we'll have a small performance
> degradation if merge this series. I'm thinking about possible optimizations.
> Options in my mind are:
> 
> 1) Remove 1ns PR_SET_TIMERSLACK in timerfd+epoll, this doesn't make qemu_poll
> faster than the old qemu_poll_ns, but may have other positive effects that
> compensate the cost.

Sounds like a random hack.  What is the reasoning for messing with timer
slack?

Perhaps it is worth investigating timer slack as an independent issue
though.

> 2) Use dynamic switch between ppoll and timerfd+epoll. In poll-linux.c, We
> start with pure ppoll, while keeping track of elapsed time in ppoll. And
> periodically, we try "timerfd+epoll" for a few iterations, so that we can
> compare if it is faster than pure ppoll. If it is, swap them, use timerfd+epoll
> and and periodically try "ppoll".
> 
> That said, I'll also look at the kernel side. Maybe optimizing ppoll or just
> add EPOLL_NANOSECOND_TIMEOUT to epoll_create1 is a better place for
> engineering.

I agree that a kernel fix would be good.  Even if the patch is rejected,
we might get good ideas on how applications can optimize.

Stefan

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