Re: [Qemu-devel] [PATCH] main-loop: For tools, initialize timers as part of qemu_init_main_loop()

[Michael Roth <mdroth@linux.vnet.ibm.com>]

On 01/22/2012 06:32 AM, Paolo Bonzini wrote:
> On 01/21/2012 09:39 PM, Jamie Lokier wrote:
>> Is this a timer that need to fire soon after setting, every time?
>>
>> I wonder if a different kind of Windows timer, lower-resolution, could
>> be used if the timeout is longer. If it has insufficient resolution,
>> it could be set to trigger a little early, then set a high-resolution
>> timer at that point.
>>
>> Maybe that could help for Linux CONFIG_NOHZ guests?
>
> No, it's an implementation detail of Windows multimedia timers. Just
> enabling them apparently burns CPU.
>
> There is another kind of timers for Windows but it didn't work reliably.
> Finding out why would be the right fix, but anyway

I'd looked into timer queues, which the developer docs suggested had 
deprecated the use of mm timers, but I came across this which I figured 
was why mm was preferred (%30 average error for a 50ms periodic/oneshot) 
by the QEMU code:

http://www.virtualdub.org/blog/pivot/entry.php?id=272

Apparently Windows 7 has some fixes for drift that makes them reasonable 
for a clock source, but still fairly low-res for an event timer.

I suppose we could use some historical timer data to adaptively 
determine a reasonable minimum resolution to set...basically if the last 
timer fired X ns ago, we'd configure the resolution to be 1/10th of that 
periodic or whatever, and start them off low-res as James suggested to 
avoid unnecessary CPU burn, but it's a pretty loose heuristic.

>
> Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
>
> Paolo
>
>