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

[Dave Winchell <dwinchell@xxxxxxxxxxxxxxx>]

Hi Dan and Keir,

Attached is a patch that fixes some issues with the SYNC policy 
(no_missed_ticks_pending).
I have not tried to make the change the minimal one, but, rather, just 
ported into
the new code what I know to work well. The error for 
no_missed_ticks_pending goes from
over 3% to .03% with this change according to my testing.

Regards,
Dave

Dan Magenheimer wrote:

> 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
> > > >
> > > >
> > > >
> > > >
> > > >        
> > >      
> >    
>  

diff -r cfdbdca5b831 xen/arch/x86/hvm/vpt.c
--- a/xen/arch/x86/hvm/vpt.c    Thu Dec 06 15:36:07 2007 +0000
+++ b/xen/arch/x86/hvm/vpt.c    Fri Jan 04 17:58:16 2008 -0500
@@ -58,7 +58,7 @@ static void pt_process_missed_ticks(stru
 
     missed_ticks = missed_ticks / (s_time_t) pt->period + 1;
     if ( mode_is(pt->vcpu->domain, no_missed_ticks_pending) )
-        pt->do_not_freeze = !pt->pending_intr_nr;
+        pt->do_not_freeze = 1;
     else
         pt->pending_intr_nr += missed_ticks;
     pt->scheduled += missed_ticks * pt->period;
@@ -127,7 +127,12 @@ static void pt_timer_fn(void *data)
 
     pt_lock(pt);
 
-    pt->pending_intr_nr++;
+    if ( mode_is(pt->vcpu->domain, no_missed_ticks_pending) ) {
+        pt->pending_intr_nr = 1;
+       pt->do_not_freeze = 0;
+    }
+    else
+       pt->pending_intr_nr++;
 
     if ( !pt->one_shot )
     {
@@ -221,8 +226,6 @@ void pt_intr_post(struct vcpu *v, struct
         return;
     }
 
-    pt->do_not_freeze = 0;
-
     if ( pt->one_shot )
     {
         pt->enabled = 0;
@@ -235,6 +238,10 @@ void pt_intr_post(struct vcpu *v, struct
             pt->last_plt_gtime = hvm_get_guest_time(v);
             pt->pending_intr_nr = 0; /* 'collapse' all missed ticks */
         }
+       else if ( mode_is(v->domain, no_missed_ticks_pending) ) {
+           pt->pending_intr_nr--;
+           pt->last_plt_gtime = hvm_get_guest_time(v);
+       }
         else
         {
             pt->last_plt_gtime += pt->period_cycles;

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