Re: [Qemu-devel] [RFC v1 4/4] util/oslib-win32: Recursivly pass the timeout

[Alistair Francis <alistair.francis@xilinx.com>]

On Thu, Jun 29, 2017 at 5:34 AM, Paolo Bonzini <pbonzini@redhat.com> wrote:
>
>
> On 28/06/2017 01:57, Alistair Francis wrote:
>> +        /* We only found one and we are waiting on more then one. Let's try
>> +         * again.
>>           */
>> -        if (timeout == 0 && nhandles > 1) {
>> +        if (nhandles > 1) {
>>              /* Remove the handle that fired */
>>              int i;
>>              if ((ready - WAIT_OBJECT_0) < nhandles - 1) {
>> @@ -444,7 +444,20 @@ static int poll_rest(gboolean poll_msgs, HANDLE *handles, gint nhandles,
>>                  }
>>              }
>>              nhandles--;
>> -            recursed_result = poll_rest(FALSE, handles, nhandles, fds, nfds, 0);
>> +
>> +            /* If we just had a very small timeout let's increase it when we
>> +             * recurse to ensure we don't just busy wait. This ensures we let
>> +             * the Windows threads block at least a little. If we previously
>> +             * had some wait let's set it to zero to avoid blocking for too
>> +             * long.
>> +             */
>> +            if (timeout < 10) {
>> +                timeout = timeout + 1;
>> +            } else {
>> +                timeout = 0;
>> +            }
>> +            recursed_result = poll_rest(FALSE, handles, nhandles, fds,
>> +                                        nfds, timeout);
>>              return (recursed_result == -1) ? -1 : 1 + recursed_result;
>
> I'm not sure I agree with this change, which is effectively delaying the
> processing of events.  The question to me is which handles are
> triggering so fast that QEMU effectively busy waits.

Yeah, that is what I was trying to figure out, but didn't make much headway.

I kept seeing zero timeouts, which means that the thread never blocks
and this patch helps a lot.

>
> Maybe your QEMUs can get some breath with commit 12f8def0e0 ("win32:
> replace custom mutex and condition variable with native primitives",
> 2017-03-27), since the native primitives are more efficient and TCG 2.8
> used condvars a lot for qemu_io_proceeded_cond.

Ok, I will try that.

Does this mean you don't see the same slowness on QEMU 2.9?

Thanks,
Alistair

>
> Paolo