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

[Paolo Bonzini <pbonzini@redhat.com>]

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.

Paolo