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Re: [Xen-devel] PV guest timings

On 26/11/2013 14:50, "Simon Martin" <smartin@xxxxxxxxxxxx> wrote:

> Hi all,
> I have finally got my little PV guest running. My critical error was not
> setting __XEN_INTERFACE_VERSION__. After tearing to pieces the code and put it
> back together a few times I decided to debug my Makefile script comparing it
> to Mini-OS.
> I am now starting to do some simple timings and the numbers are so bad I'm
> wondering what could be wrong with my test.
> As mentioned previously I have created a CPU pool with one CPU and one domU. I
> am using the standard credit scheduler. I set timer_slop=0 on the Xen command
> line.
> I initialise console, traps, events, and TSC clock in my PV and then start a
> periodic operation running. I then calculate latency as (clock time -
> deadline) and period as (clock time - previous deadline).At the moment I'm
> just displaying min/max values. I also increment a tick count on every cycle.
> Looking at the statistics I see that I get the expected number of ticks per
> second, however I get latencies in the range [-1ms, +25us] and the periods in
> the range of [3us, 1.02ms]. The upper bounds look OK, but the lower bounds are
> all over the place.
> This makes me think that I'm not the only thing using VIRQ_TIMER. I seem to
> remember that this is called nominally every 10ms by the Hypervisor.
> Is there any way to recognize the origin of the timer?
> Can anyone suggest other things to try?

Are you using the single-shot timer or the periodic timer?

How are you calculating clock time?

 -- Keir

> Regards.
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