Re: [Qemu-devel] spawning and killing threads in qemu

[Stefan Hajnoczi]

2012/2/7 陳韋任 <address@hidden>:
> Hi Stefan,
>
>  Thanks for the explanation. :)
>
>> >   Qemu 1.0 enable IO thread by default, I think that's why you can see 
>> > there are
>> > two threads. You can check block/raw-posix-aio.h, posix-aio-compat.c and 
>> > linux-aio.c.
>>
>> --enable-io-thread is different from posix-aio-compat.c's thread pool.
>
>  The option "--enable-io-thread" should have been gone sine QEMU 1.0, right?
> Or you just use the term to mean IO thread?

Yes, I just used that term to describe the "IO thread".

>> --enable-io-thread means there is a dedicated thread (created at
>> startup) which runs the event loop.
>
>  I want to know where IO thread is lauched but I am lost in vl.c. Would you
> mind to shed some light on that?

The IO thread is actually the main thread of the process.  vcpus are
created as separate threads but the main loop in vl.c becomes the IO
thread.

>> The temporary threads you are seeing are indeed posix-aio-compat.c
>> worker threads.  They execute blocking I/O system calls so that the QEMU
>> event loop can continue to process events while I/O operations are
>> running.
>
>  So, IO thread runs the event loop (I think it's main_loop_wait in 
> main-loop.c,
> right?), and it leave blocking I/O system calls to posix-aio-compat.c worker
> threads? Is that correct?

Yes.  The posix-aio-compat.c only handles block I/O, so subsystems
like networking, VNC, and character devices use other mechanisms to
avoid tying up the main loop.

Stefan