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

[Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>]

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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