Re: Time skew on HP DL785 (and possibly other boxes)

[Jeremy Fitzhardinge <jeremy@goop.org>]

Dan Magenheimer wrote:
> However, I'm told that its not possible to route a clocksource
> over hypertransport, so TSC's on processors on different
> motherboards may be VERY different and apparently the
> mechanisms for synchronizing Xen system time across
> motherboards may not be up to the challenge.  As a result,
> OS's and apps sensitive to time that are running on PV
> domains may be in for a rough ride on systems like this.
> (HVM domains may run into other problems because time will
> apparently stop for a "long time".)
>   

I don't see what the problem is.  If each individual cpu has well known 
tsc parameters (rate and offset), then a PV client will get those timing 
parameters and use it to compute its time.  It doesn't matter if they're 
syncronized between cpus or nodes.

Xen will need to calibrate each of them against a good reference 
(hpet?), but that's no different from now.  I guess its possible that 
this system has more variation and latency for hpet access, which may 
mean that the calibration algorithm needs tweaking.

Of course, if the tsc rates on each cpu is changing in some 
unpredictable way then that's a whole other barrel of problems.  Guests 
rely on Xen maintaing accurate tsc timing parameters.

> Since systems like this are targeted for consolidation
> and virtualization, I see this as a potentially big problem
> as it may appear to real Xen customers as bizarre
> non-reproducible problems, such as "make" failing,
> leading to questions about the stability and viability
> of using Xen.
>
> Comments?
>   

In Linux there's this function:

/*
 * apic_is_clustered_box() -- Check if we can expect good TSC
 *
 * Thus far, the major user of this is IBM's Summit2 series:
 *
 * Clustered boxes may have unsynced TSC problems if they are
 * multi-chassis. Use available data to take a good guess.
 * If in doubt, go HPET.
 */
__cpuinit int apic_is_clustered_box(void)
{...}


Which deals with Summit2 and ScaleSMP vsmp systems which also have 
unsynchronized tscs across nodes.  At the moment it assumes that no 
non-VSMP AMD system has unsynchronized tscs; sounds like it will need 
updating for this system.

    J