Re: [Xen-devel] How to increase timer resolution?

[Keir Fraser <keir.fraser@xxxxxxxxxxxxx>]

On 14/9/08 16:23, "Robert Kaiser (FH)" <kaiser@xxxxxxxxxxxxxxxxxxxxxxxxxx>
wrote:

> Keir Fraser schrieb:
>> set_timer() and friends are already high-resolution, so long as your system
>> has a local APIC. This is undoubtedly the case if running a multi-processor
>> system. On a uni-processor system you might need to forcibly enable use of
>> the APIC with 'lapic' as a Xen boot parameter.
>> 
>> Either way, early during boot you should find a message saying local APIC is
>> found and enabled, or not found, or whatever.
>>   
> 
> Quote:
> 
> (XEN) Local APIC disabled by BIOS -- reenabling.
> (XEN) Could not enable APIC!
> 
> Seems like I have to switch to a different testing machine then, the old
> notebook I was using doesn't seem to have an APIC :-(
> 
> Do you know what resolution I can expect to get on an APIC-enabled machine?

The set_timer() infrastructure is not actually tick-based. With an APIC
you'll get timer accuracy of around 100us (this figure because we service
timer handlers up to 100us before the deadline specified to set_timer(),
just to avoid multiple timer interrupts for close-together events). Without
this extra fudge factor you'd get accuracy down to microsecond resolution.

 -- Keir



_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxxxxxxxx
http://lists.xensource.com/xen-devel