Re: [Qemu-block] [Qemu-devel] [PATCH v2 11/11] block: Only poll block layer fds in bdrv_aio_poll

[Fam Zheng]

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()?

Fam