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

["Dan Magenheimer" <dan.magenheimer@xxxxxxxxxx>]

Anyone make measurements on the final patch?

I just ran a 64-bit RHEL5.1 pvm kernel and saw a loss of about 0.2% with no 
load.  This was xen-unstable tip today with no options specified.  32-bit was 
about 0.01%.

I think I missed something... how do I run the various accounting choices and 
which ones are known to be appropriate for which kernels?

Thanks,
Dan

> -----Original Message-----
> From: xen-devel-bounces@xxxxxxxxxxxxxxxxxxx
> [mailto:xen-devel-bounces@xxxxxxxxxxxxxxxxxxx]On Behalf Of Keir Fraser
> Sent: Thursday, December 06, 2007 4:57 AM
> To: Dave Winchell
> Cc: Shan, Haitao; xen-devel@xxxxxxxxxxxxxxxxxxx; Dong, Eddie; Jiang,
> Yunhong
> Subject: Re: [Xen-devel] [PATCH] Add a timer mode that 
> disables pending
> missed ticks
> 
> 
> Please take a look at xen-unstable changeset 16545.
> 
>  -- Keir
> 
> On 26/11/07 20:57, "Dave Winchell" <dwinchell@xxxxxxxxxxxxxxx> wrote:
> 
> > 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
> >>
> >>
> >>
> >>
> >
> 
> 
> 
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>


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