Re: [Qemu-devel] Re: [PATCH 2/7] Enable I/O thread and VNC threads by default

[Anthony Liguori <anthony@codemonkey.ws>]

On 02/08/2011 05:46 AM, Aurelien Jarno wrote:
> On Tue, Feb 08, 2011 at 12:07:02PM +0100, Tristan Gingold wrote:
>    
>> On Feb 8, 2011, at 6:58 PM, Anthony Liguori wrote:
>>
>>      
>>> On 02/08/2011 04:06 AM, Aurelien Jarno wrote:
>>>        
>>>> Yes, it's slow. But is it a problem? You assume that people use QEMU
>>>> only for emulating SMP platforms. This is a wrong assumption. Beside the
>>>> x86 target, only sparc really supports SMP emulation.
>>>>
>>>>          
>>> It's *not* just about performance.
>>>
>>> TCG requires a signal to break out of a tight chained TB loop.  If you have a guest in a tight loop waiting for something external (like polling on a in-memory flag), the device emulation will not get to run until a signal is fired.
>>>
>>> Unless you set SIGIO on every file descriptor that selects polls on (and you can't because there are a number that just don't support SIGIO), then you have a race condition.
>>>        
>> A race condition ?  Looks like you are describing a dead-lock.
>>
>> But the dead lock doesn't happen because of the timer which periodically exits from TCG.  Hence the performance issue.
>>      

With dynticks, you don't always have a periodic timer (unless the guest 
has a periodic timer enabled).  There's a good bit of early startup code 
that runs without a periodic timer enabled.

Now that said, we never truly sleep forever.  We'll set something like a 
5 second timeout.  But 5 seconds might as well be forever and this is 
certainly a giant hack.

Regards,

Anthony Liguori

>>> This can be fixed by running TCG in a separate thread than select() and sending a signal to the TCG VCPU when select() returns (effectively SIGIO in userspace).
>>>
>>> This is exactly what the I/O thread does.
>>>        
>>
>> (Nobody was able to make it working on Windows - or nobody was interested in ?)
>>
>>      
> Given the I/O thread is disabled by default, my guess is that nobody
> really see an interest in looking at that.
>
>