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[Qemu-devel] 答复: Question about aio_poll and glib aio_ctx_dispatch

From: Li Qiang
Subject: [Qemu-devel] 答复: Question about aio_poll and glib aio_ctx_dispatch
Date: Thu, 20 Dec 2018 06:58:50 +0800

Hello Paolo

Thanks for your kind reply.

Yes, aio_poll and aio_ctx_dispatch mostly run in different threads, though
Sometimes they can run in a thread nested from Fam’s slides:

So you mean we can poll the same fd in two threads? If so , the ‘fd’ in 
‘qemu_aio_context’ will be run twice, I think there’s 
something I understand wrong.

Let’s take an example. In the ‘v9fs_reset’.

void v9fs_reset(V9fsState *s)
    VirtfsCoResetData data = { .pdu = { .s = s }, .done = false };
    Coroutine *co;

    while (!QLIST_EMPTY(&s->active_list)) {
        aio_poll(qemu_get_aio_context(), true);

    co = qemu_coroutine_create(virtfs_co_reset, &data);

    while (!data.done) {
        aio_poll(qemu_get_aio_context(), true);

Here we use aio_poll to wait the pending action to complete.
Here aio_poll will poll the fds in ‘qemu_aio_context’ in vcpu thread,
However, the main loop is also poll the fds in ‘qemu_aio_context’.
If some ‘fd’ in ‘qemu_aio_context’ has events, the two thread will be wakeup?
You say both will call aio_dispatch_handlers and timerlistgroup_run_timers.
But the are the same fd, how can this happen?

Li Qiang

发件人: Paolo Bonzini
发送时间: 2018年12月20日 4:42
收件人: Li Qiang; address@hidden; address@hidden; Qemu Developers; 李强
主题: Re: Question about aio_poll and glib aio_ctx_dispatch

On 19/12/18 11:05, Li Qiang wrote:
> Sent it to qemu-devel.
> Li Qiang <address@hidden <mailto:address@hidden>> 于2018年12月19日周
> 三 下午6:04写道:
>     Hello Paolo, Stefan, Fam and all,
>     Here I have a question about 'aio_poll'.
>     IIUC the 'aio_poll' is (mostly) used for synchronous IO
>     as I see a lot of code like this: 
>     while(condition)
>      aio_poll();
>     However it seems the 'aio_poll' and 'aio_ctx_dispatch' both poll the fd.
>     So what happened when the 'fd' has events, which function will be
>     wakeup?

Roughly speaking, aio_poll is used for synchronous IO and within I/O
threads; aio_ctx_dispatch is used within the main thread.

Both end up calling aio_dispatch_handlers and timerlistgroup_run_timers.


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