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Re: [Xen-devel] [PATCH] Add a timer mode that disables pending missed ticks

Hi Keir,

I've added comments below.
See my next mail on some interesting performance numbers.


Keir Fraser wrote:

On 7/11/07 19:38, "Dave Winchell" <dwinchell@xxxxxxxxxxxxxxx> wrote:

My feeling is that we should go full SYNC. Yes, in theory the
guests should be able to handle ASYNC, but in reality it appears that
some do not. Since it is easy for us to give them SYNC,
lets just do it and not stress them out.

One problem with pure SYNC is there's a fair chance you won't deliver any
ticks at all for a long time, if the guest only runs in short bursts (e.g.,
I/O bound) and happens not to be running on any tick boundary. I'm not sure
how much that matters. It could cause time goes backwards if the time
extrapolation via the TSC is not perfectly accurate, or cause problems if
there are any assumptions that TSC delta since last tick fits in 32 bits
(less likely in x64 code I suppose). Anyway, my point is that only testing
VCPUs under full load may cause us to optimise in ways that have nasty
unexpected effects for other workloads.
I agree that this could be a problem. I have an idea that could give us full
SYNC and eliminate the long periods without clock interrupts.
In pt_process_missed_ticks() when missed_ticks > 0 set pt->run_timer = 1.
In pt_save_timer():

   list_for_each_entry ( pt, head, list )

And in pt_timer_fn():

   pt->run_timer = 0;

So, for a guest that misses a tick, we will interrupt him once from the
descheduled state and then leave him alone in the descheduled state.

For default mode as checked into unstable is now,
64 bit guests should run quite fast as missed is calculated and then a bunch
of additional interrupts are delivered. On the other hand
32bit guests very well in default mode.

For the original code, before we put in the constant tsc offset business,
64bit guests run poorly and 32bit quests very well time-wise.

The default mode hasn't changed. Are you under the impression that
missed-ticks-but-no-delay-of-tsc is the default mode now? I know x64 guests
run badly with that because they treat every one of the missed ticks they
receive as a full tick.
Sorry, I was confused.
However, the default mode will still run poorly for 64 bit guests because
of the pending_nr's accumulated while the guest has interrupts disabled.
As I recall, the effect is quite large, on the order of 10% error.
I'll get you a number later today.

-- Keir

Or is the lack of
synchronization of TSCs across VCPUs causing issues that you're trying to
This does cause issues, but its not the only contributor to poor timing.
Having TSCs synchronized across vcpus will help some of the time going
backwards problems we have seen, I think.


Keir Fraser wrote:

On 7/11/07 17:29, "Keir Fraser" <Keir.Fraser@xxxxxxxxxxxx> wrote:

So, you can see we send an interrupt immediately (and ASYNC) if any ticks
have been missed, but then successive ticks are delivered 'on the beat'. A
possible middleground? Or perhaps we should just go with SYNC after all...
How do these Linux x64 guests fare with the original and default timer mode,
by the way? I would expect that time should be accounted pretty accurately
in that mode, albeit with more interrupts than you'd like. Or is the lack of
synchronisation of TSCs across VCPUs causing issues that you're trying to

-- Keir

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