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

[Tristan Gingold]

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.

> 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 ?)

Tristan.