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[Xen-devel] RE: [PATCH] rendezvous-based local time calibration WOW!

To: "Keir Fraser" <keir.fraser@xxxxxxxxxxxxx>, "Xen-Devel (E-mail)" <xen-devel@xxxxxxxxxxxxxxxxxxx>
Subject: [Xen-devel] RE: [PATCH] rendezvous-based local time calibration WOW!
From: "Dan Magenheimer" <dan.magenheimer@xxxxxxxxxx>
Date: Mon, 4 Aug 2008 09:24:46 -0600
Cc: Ian Pratt <Ian.Pratt@xxxxxxxxxxxxx>, Dave Winchell <dwinchell@xxxxxxxxxxxxxxx>
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OK, how about this version.  The rendezvous only collects
the key per-cpu time data then sets up a per-cpu 1ms timer
to later update the timestamp record and vcpu system time,
so neither should have racing issues.

I've only run it for about an hour but still haven't seen
any skew over 600nsec so apparently it is the collection of
the key time data that must be closely synchronized (probably
to ensure the slope is correct) while exact synchronization
of setting the timestamp records is less important.

Note that I'm not positive I got the clocksource=tsc part
correct... but am interested in your opinion on whether
clocksource=tsc can now be eliminated anyway (as the
main reason I pushed for it was because of unacceptable
skew which with this patch appears to be fixed).

Signed-off-by: Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>

> -----Original Message-----
> From: Keir Fraser [mailto:keir.fraser@xxxxxxxxxxxxx]
> Sent: Sunday, August 03, 2008 11:25 AM
> To: dan.magenheimer@xxxxxxxxxx; Xen-Devel (E-mail)
> Cc: Ian Pratt; Dave Winchell
> Subject: Re: [PATCH] rendezvous-based local time calibration WOW!
> It's not safe to poke a new timestamp record from an interrupt handler
> (which is what the smp_call_function() callback functions 
> are). Users of the
> timestamp records (e.g., get_s_time) need 
> local_irq_save/restore() or an
> equivalent of the Linux seqlock. The latter is likely faster. 
> I'm dubious
> about update_vcpu_system_time() from an interrupt handler 
> too. It needs
> thought about how it might race with a context switch (change 
> of 'current')
> or if it interrupts an existing invocation of 
> update_vcpu_system_time().
>  -- Keir
> On 3/8/08 17:50, "Dan Magenheimer" <dan.magenheimer@xxxxxxxxxx> wrote:
> > The synchronization of local_time_calibration (l_t_c) via
> > round-to-nearest-epoch provided some improvement, but I was
> > still seeing skew up to 16usec and higher.  I measured the
> > temporal distance between the rounded-epoch vs when ltc
> > was actually running to ensure there wasn't some kind of
> > bug and found that l_t_c was running up to 150us after the
> > round-epoch and sometimes up to 50us before.  I guess this
> > is the granularity of setting a Xen timer.  While it seemed
> > that +/- 100us shouldn't cause that much skew, I finally
> > decided to try synchronization-via-rendezvous, as suggested
> > by Ian here:
> >
> > 
> http://lists.xensource.com/archives/html/xen-devel/2008-07/msg
> http://lists.xensource.com/archives/html/xen-devel/2008-07/msg01080.html
> The result is phenomenal... using this approach (in attached
> patch), I have yet to see a skew exceed 1usec!!!  So this is
> about a 10-fold increase in accuracy vs the rounded-epoch
> method and about 20-fold over the one-epoch-from-NOW() method.
> The platform time is now read once for all processors rather
> than once per processor.  (Actually, it is read once again
> in platform_time_calibration()... by "inlining" that routine
> into master_local_time_calibration() that extra read can
> be -- and probably should be -- avoided too.)
> It may be too late to get this into 3.3.0 but, if so, please
> consider it asap for 3.3.1 rather than just xen-unstable/3.4.
> Dan
> ===================================
> 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)

Attachment: rendezcalib3.patch
Description: Binary data

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