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

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

Hi Dave --

Did you get your correction ported?  If so, it would be nice to see this get 
into 3.1.3.

Note that I just did some very limited testing with timer_mode=2(=SYNC=no 
missed ticks pending)
on tip of xen-3.1-testing (64-bit Linux hv guest) and the worst error I've seen 
so far
is 0.012%.  But I haven't tried any exotic loads, just LTP.

Thanks,
Dan

> -----Original Message-----
> From: Dave Winchell [mailto:dwinchell@xxxxxxxxxxxxxxx]
> Sent: Wednesday, December 19, 2007 12:33 PM
> To: dan.magenheimer@xxxxxxxxxx
> Cc: Keir Fraser; Shan, Haitao; xen-devel@xxxxxxxxxxxxxxxxxxx; Dong,
> Eddie; Jiang, Yunhong; Dave Winchell
> Subject: Re: [Xen-devel] [PATCH] Add a timer mode that 
> disables pending
> missed ticks
> 
> 
> Dan,
> 
> I did some testing with the constant tsc offset SYNC method 
> (now called
> no_missed_ticks_pending)
> and found the error to be very high, much larger than 1 %, as 
> I recall.
> I have not had a chance to submit a correction. I will try to 
> do it later
> this week or the first week in January. My version of constant tsc
> offset SYNC method
> produces .02 % error, so I just need to port that into the 
> current code.
> 
> The error you got for both of those kernels is what I would expect
> for the default mode, delay_for_missed_ticks.
> 
> I'll let Keir answer on how to set the time mode.
> 
> Regards,
> Dave
> 
> Dan Magenheimer wrote:
> 
> >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
> >>>>
> >>>>
> >>>>
> >>>>
> >>>>
> >>>>
> >>
> >>_______________________________________________
> >>Xen-devel mailing list
> >>Xen-devel@xxxxxxxxxxxxxxxxxxx
> >>http://lists.xensource.com/xen-devel
> >>
> >>
> >>
> >>
> >
> >
> >
> 
>


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