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

[Dave Winchell <dwinchell@xxxxxxxxxxxxxxx>]

Keir,

The accuracy data I've collected for i/o loads for the
various time protocols follows. In addition, the data
for cpu loads is shown.

The loads labeled cpu and i/o-8 are on an 8 processor AMD box.
Two guests, red hat and sles 64 bit, 8 vcpu each.
The cpu load is usex -e36 on each guest.
(usex is available at http://people.redhat.com/anderson/usex.)
i/o load is 8 instances of dd if=/dev/hda6 of=/dev/null.

The loads labeled i/o-32 are 32 instances of dd.
Also, these are run on 4 cpu AMD box.
In addition, there is an idle rh-32bit guest.
All three guests are 8vcpu.

The loads labeled i/o-4/32 are the same as i/o-32
except that the redhat-64 guest has 4 instances of dd.

Date Duration Protocol sles, rhat error load

11/07 23 hrs 40 min ASYNC -4.96 sec, +4.42 sec -.006%, +.005% cpu
11/09 3 hrs 19 min ASYNC -.13 sec, +1.44 sec, -.001%, +.012% cpu

11/08 2 hrs 21 min SYNC -.80 sec, -.34 sec, -.009%, -.004% cpu
11/08 1 hr 25 min SYNC -.24 sec, -.26 sec, -.005%, -.005% cpu
11/12 65 hrs 40 min SYNC -18 sec, -8 sec, -.008%, -.003% cpu

11/08 28 min MIXED -.75 sec, -.67 sec -.045%, -.040% cpu
11/08 15 hrs 39 min MIXED -19. sec,-17.4 sec, -.034%, -.031% cpu


11/14 17 hrs 17 min ASYNC -6.1 sec,-55.7 sec, -.01%, -.09% i/o-8
11/15 2 hrs 44 min ASYNC -1.47 sec,-14.0 sec, -.015% -.14% i/o-8

11/13 15 hrs 38 min SYNC -9.7 sec,-12.3 sec, -.017%, -.022% i/o-8
11/14 48 min SYNC - .46 sec, - .48 sec, -.017%, -.018% i/o-8

11/14 4 hrs 2 min MIXED -2.9 sec, -4.15 sec, -.020%, -.029% i/o-8
11/20 16 hrs 2 min MIXED -13.4 sec,-18.1 sec, -.023%, -.031% i/o-8



11/21 28 min MIXED -2.01 sec, -.67 sec, -.12%, -.04% i/o-32
11/21 2 hrs 25 min SYNC -.96 sec, -.43 sec, -.011%, -.005% i/o-32
11/21 40 min ASYNC -2.43 sec, -2.77 sec -.10%, -.11% i/o-32

11/26 113 hrs 46 min MIXED -297. sec, 13. sec -.07%, .003% i/o-4/32
11/26 4 hrs 50 min SYNC -3.21 sec, 1.44 sec, -.017%, .01% i/o-4/32


Overhead measurements:

Progress in terms of number of passes through a fixed system workload
on an 8 vcpu red hat with an 8 vcpu sles idle.
The workload was usex -b48.


ASYNC 167 min 145 passes .868 passes/min
SYNC 167 min 144 passes .862 passes/min
SYNC 1065 min 919 passes .863 passes/min
MIXED 221 min 196 passes .887 passes/min


Conclusions:

The only protocol which meets the .05% accuracy requirement for ntp 
tracking under the loads
above is the SYNC protocol. The worst case accuracies for SYNC, MIXED, 
and ASYNC
are .022%, .12%, and .14%, respectively.

We could reduce the cost of the SYNC method by only scheduling the extra 
wakeups if a certain number
of ticks are missed.

Regards,
Dave

Keir Fraser wrote:

> On 9/11/07 19:22, "Dave Winchell" <dwinchell@xxxxxxxxxxxxxxx> wrote:
>
>  
>
> > Since I had a high error (~.03%) for the ASYNC method a couple of days ago,
> > I ran another ASYNC test. I think there may have been something
> > wrong with the code I used a couple of days ago for ASYNC. It may have been
> > missing the immediate delivery of interrupt after context switch in.
> >
> > My results indicate that either SYNC or ASYNC give acceptable accuracy,
> > each running consistently around or under .01%. MIXED has a fairly high
> > error of
> > greater than .03%. Probably too close to .05% ntp threshold for comfort.
> > I don't have an overnight run with SYNC. I plan to leave SYNC running
> > over the weekend. If you'd rather I can leave MIXED running instead.
> >
> > It may be too early to pick the protocol and I can run more overnight tests
> > next week.
> >    
> I'm a bit worried about any unwanted side effects of the SYNC+run_timer
> approach -- e.g., whether timer wakeups will cause higher system-wide CPU
> contention. I find it easier to think through the implications of ASYNC. I'm
> surprised that MIXED loses time, and is less accurate than ASYNC. Perhaps it
> delivers more timer interrupts than the other approaches, and each interrupt
> event causes a small accumulated error?
>
> Overall I would consider MIXED and ASYNC as favourites and if the latter is
> actually more accurate then I can simply revert the changeset that
> implemented MIXED.
>
> Perhaps rather than running more of the same workloads you could try idle
> VCPUs and I/O bound VCPUs (e.g., repeated large disc reads to /dev/null)? We
> don't have any data on workloads that aren't CPU bound, so that's really an
> obvious place to put any further effort imo.
>
> -- Keir
>
>
>  

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxxxxxxxx
http://lists.xensource.com/xen-devel