[Xen-devel] Re: [PATCH] clocksource=tsc

[Keir Fraser <keir.fraser@xxxxxxxxxxxxx>]

I'm sympathetic to the patch, but it looks like an ugly hack right now. Most
of the clocksource_is_tsc special cases will fall away if you, e.g., have
platform_timer_stamp = stime_platform_stamp = local_tsc_stamp =
stime_local_stamp = 0; and provide a plt->read_counter() function; and
accept that for now you will unnecessarily take the platform-timer spinlock;
etc. Also it is platform timer 5, not 4!

Clean it up and it'll go in. Since it should clearly only affect behaviour
when the new clocksource is explicitly selected, I'll take this patch
further into 3.3 freeze than usual -- it's more important to me that it's
clean.

 Thanks,
 Keir

On 12/7/08 22:38, "Dan Magenheimer" <dan.magenheimer@xxxxxxxxxx> wrote:

> Attached patch adds clocksource=tsc boot option that
> uses TSC as clocksource.  This option should only be
> used on machines where TSC is known to be synchronized
> across all processors.  A future TODO is to dynamically
> determine if this is the case.
> 
> TSC may "beat" with another clocksource, resulting in
> cross-processor Xen system time skew.  This skew can
> be visible in PV guests and can appear as "time is stopped"
> in hvm guests.  On some systems, this patch can reduce skew
> by 30x or more.
> 
> Signed-off-by: Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>
> 
> ===================================
> Thanks... for the memory
> I really could use more / My throughput's on the floor
> The balloon is flat / My swap disk's fat / I've OOM's in store
> Overcommitted so much
> (with apologies to the late great Bob Hope)



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