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

[Dave Winchell <dwinchell@xxxxxxxxxxxxxxx>]

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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