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Re: [Qemu-devel] [Qemu-block] [PATCH v2 11/11] block: Only poll block la


From: Fam Zheng
Subject: Re: [Qemu-devel] [Qemu-block] [PATCH v2 11/11] block: Only poll block layer fds in bdrv_aio_poll
Date: Fri, 11 Sep 2015 19:12:55 +0800
User-agent: Mutt/1.5.23 (2014-03-12)

On Fri, 09/11 13:02, Paolo Bonzini wrote:
> 
> 
> On 11/09/2015 13:01, Fam Zheng wrote:
> > On Fri, 09/11 12:46, Paolo Bonzini wrote:
> >>
> >>
> >> On 11/09/2015 12:40, Fam Zheng wrote:
> >>> On Fri, 09/11 11:54, Paolo Bonzini wrote:
> >>>>
> >>>>
> >>>> On 11/09/2015 11:44, Fam Zheng wrote:
> >>>>>>>> That would be a step back.  Using GSource is useful because it lets
> >>>>>>>> you integrate libraries such as GTK+.
> >>>>>>>
> >>>>>>> Can we move GTK to a separate GSource thread?
> >>>>>>
> >>>>>> I think that GTK should always run in the main thread, or at least the
> >>>>>> one running the default main loop / GMainContext.
> >>>>>
> >>>>> Yeah it's basically GMainContext staying in the main thread and
> >>>>> block/net/chardev I/O put in a new AioContext thread.
> >>>>
> >>>> Why?  The point of an event loop is that you can multiplex everything on
> >>>> the same thread.  Unless we have specific needs (e.g. scalability) one
> >>>> thread is the way to go and keep things simple.
> >>>
> >>> The reason is scalability. :)
> >>
> >> Scalability of what?  If virtio-net or virtio-serial needs to be more
> >> scalable, putting all of them into a non-main-loop thread will not make
> >> things more scalable, because you have a single thread anyway.  You'd
> >> need to go BQL-free and allow an arbitrary number.
> >>
> >>> Moving things to AIO isn't deviation, it's more about enabling of 
> >>> dataplane and
> >>> epoll. That's why block was moved to AioContext, and I think we can do 
> >>> similar
> >>> for net and serial, the difference is that as a start, they don't need to 
> >>> be
> >>> fully BQL-free like virtio-blk and scsi. But by running in an aio_poll() 
> >>> loop,
> >>> they can better performance because of epoll.
> >>
> >> Isn't that what your "iohandler.c with AioHandler" already does?  True,
> >> it would be epoll-within-poll, not pure poll.  But if you need epoll,
> >> you might as well go BQL-free.
> > 
> > epoll-within-poll? Do you mean change the main event loop from:
> > 
> >     qemu_poll_ns([..., ioeventfd1, ioeventfd2, ..., ioeventfd99], ...)
> > 
> > to
> > 
> >     qemu_poll_ns([epollfd], ...)
> > 
> > where epollfd watches all the fds, and let the handler of epollfd do
> > epoll_wait()?
> 
> I mean that glib's main loop (or os_host_main_loop_wait in main-loop.c)
> uses poll, and then the AioContext GSource uses epoll.  Still, you have
> a constant (and low) number of file descriptors in the outer poll.
> 

OK, I think I get your idea, and it sounds good to me, thank you very much! ;-)

Fam



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