Re: [Xen-devel] [RFC] Correct/fast timestamping in apps under Xen [1 of 4]: Reliable TSC

[Jeremy Fitzhardinge <jeremy@xxxxxxxx>]

On 10/09/09 02:34, Tim Deegan wrote:
> Your plans for usermode<-->hypervisor direct TSC integration seem to me
> to be an unpleasant hack.  I understand that you have good business
> reasons for wanting it (even if you're not allowed to tell us explicitly
> what they are) and we've seen the justifications enough times that we
> don't need to cover them again here, but it's still a hack.
>
> I'm unhappy with the idea of kicking around the Xen timekeeping code
> (and introducing the usual bug-tail) to support introducing a usermode
> TSC.  If there is to be a new mode for this, it should default to the
> current (works for everyone except the engineering team of a
> not-to-be-named enterprise application) behaviour.
>   

I'm seeing an approx 12x performance improvement with gettimeofday() and
clock_gettime() on systems with my vsyscall support patches
(~1200ns/call -> ~100ns[*]).  I think that should go a long way towards
mitigating the performance concerns using standard APIs.  

There's probably some scope for improving those numbers on systems with
better-than-baseline tsc support (ie rdtscp and/or guaranteed synced
tscs), but I think its enough to get started with, especially given the
broad applicability and relatively simple engineering.

[*] With native tsc; emulated tsc makes that 1700 -> 500, or only ~3.3x
improvement.

    J

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