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

To: "dan.magenheimer@xxxxxxxxxx" <dan.magenheimer@xxxxxxxxxx>
From: Dave Winchell <dwinchell@xxxxxxxxxxxxxxx>
Date: Wed, 30 Jan 2008 10:25:00 -0500
Cc: "akira.ijuin@xxxxxxxxxx" <akira.ijuin@xxxxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxx>, "deepak.patel@xxxxxxxxxx" <deepak.patel@xxxxxxxxxx>, Dave Winchell <dwinchell@xxxxxxxxxxxxxxx>
Delivery-date: Wed, 30 Jan 2008 09:19:10 -0800
List-id: Xen developer discussion <xen-devel.lists.xensource.com>

Hi Dan,

I agree with you on disabling and the need for fixing hpet.
If no one else wants to fix it, I could give it a try.
I think we could make the hpet work better than pit/tsc.

While I monitor the skew over time, I'm not as methodical as you.
Its possible that I missed this behaviour.

I've been thinking about what you've seen and have no clue as to the cause.
Let me ask you a few questions:

Is the graph for RHEL5u1-64? (I've never tested this one.)
What was the behaviour of the other guests running?
If they had spikes, were they at the same wall time?
Were the other guests running ltp as well?

How are you measuring skew?

Are you running ntpd?

Anything that you can discover that would be in sync with
the spikes would be very helpful!

The code that I test with is our product code, which is based
on 3.1. So it is possible that something in 3.2 other than vpt.c
is the cause. I can test with 3.2, if necessary.

thanks,
Dave



Dan Magenheimer wrote:

Hi Dave (Keir, see suggestion below) --

Thanks!

Turning off vhpet certainly helps a lot (though see below).

I wonder if timekeeping with vhpet is so bad that it should be
turned off by default (in 3.1, 3.2, and unstable) until it is
fixed?  (I have a patch that defaults it off, can post it if
there is agreement on the above point.)  The whole point of an
HPET is to provide more precise timekeeping and if vhpet is
worse than vpit, it can only confuse users.  Comments?

In your testing, are you just measuring % skew over a long

period of time?

 We are graphing the skew continuously and
seeing periodic behavior that is unsettling, even with pit.
See attached.  Though your algorithm recovers, the "cliffs"
could still cause real user problems.  I wonder if there is
anything that can be done to make the "recovery" more
responsive?

We are looking into what part(s) of LTP is causing the cliffs.

Thanks,
Dan

-----Original Message-----
From: Dave Winchell [mailto:dwinchell@xxxxxxxxxxxxxxx]
Sent: Monday, January 28, 2008 8:21 AM
To: dan.magenheimer@xxxxxxxxxx

Cc: Keir Fraser; xen-devel@xxxxxxxxxxxxxxxxxxx;deepak.patel@xxxxxxxxxx;

akira.ijuin@xxxxxxxxxx; Dave Winchell

Subject: Re: [Xen-devel] [PATCH] Add a timer mode thatdisables pending

missed ticks


Dan,

I guess I'm a bit out of date calling for clock= usage.
Looking at linux 2.6.20.4 sources, I think you should specify
"clocksource=pit nohpet" on the linux guest bootline.

You can leave the xen and dom0 bootlines as they are.
The xen and guest clocksources do not need to be the same.
In my tests, xen is using the hpet for its timekeeping and
that appears to be the default.

When you boot the guests you should see
   time.c: Using PIT/TSC based timekeeping.
on the rh4u5-64 guest, and something similar on the others.

> (xm dmesg shows 8x Xeon 3.2GHz stepping 04, Platform timer
> 14.318MHz HPET.)

This appears to be the xen state, which is fine.
I was wrongly assuming that this was the guest state.

You might want to look in your guest logs and see what theywere picking

for a clock source.

Regards,
Dave




Dan Magenheimer wrote:

Thanks, I hadn't realized that!  No wonder we didn't see the same
improvement you saw!

Try specifying clock=pit on the linux boot line...

I'm confused... do you mean "clocksource=pit" on the Xen

command line or

"nohpet" / "clock=pit" / "clocksource=pit" on the guest (or

dom0?) command

line? Or both places? Since the tests take awhile, it

would be nice

to get this right the first time. Do the Xen and guest

clocksources need

to be the same?

Thanks,
Dan

-----Original Message-----
*From:* Dave Winchell [mailto:dwinchell@xxxxxxxxxxxxxxx]
*Sent:* Sunday, January 27, 2008 2:22 PM
*To:* dan.magenheimer@xxxxxxxxxx; Keir Fraser
*Cc:* xen-devel@xxxxxxxxxxxxxxxxxxx; deepak.patel@xxxxxxxxxx;
akira.ijuin@xxxxxxxxxx; Dave Winchell
*Subject:* RE: [Xen-devel] [PATCH] Add a timer mode that disables
pending missed ticks

   Hi Dan,

   Hpet timer does have a fairly large error, as I was trying this
   one recently.

I don't remember what I got for error, but 1% sounds

about right.

The problem is that hpet is not built on top of vpt.c,

the module

   Keir and I did
   all the recent work in, for its periodic timer needs. Try
   specifying clock=pit
   on the linux boot line. If it still picks the hpet, which it
   might, let me know
   and I'll tell you how to get around this.

   Regards,
   Dave

--------------------------------------------------------------
----------

   *From:* Dan Magenheimer [mailto:dan.magenheimer@xxxxxxxxxx]
   *Sent:* Fri 1/25/2008 6:50 PM
   *To:* Dave Winchell; Keir Fraser
   *Cc:* xen-devel@xxxxxxxxxxxxxxxxxxx; deepak.patel@xxxxxxxxxx;
   akira.ijuin@xxxxxxxxxx

*Subject:* RE: [Xen-devel] [PATCH] Add a timer mode

that disables

   pending missed ticks
Sorry for the very late followup on this but we finally

were able

   to get our testing set up again on stable 3.1 bits and have
   seen some very bad results on 3.1.3-rc1, on the order of 1%.

   Test enviroment was a 4-socket dual core machine with 24GB of
   memory running six two-vcpu 2GB domains, four hvm plus two pv.
   All six guests were running LTP simultaneously.  The four hvm
   guests were: RHEL5u1-64, RHEL4u5-32, RHEL5-64, and RHEL4u5-64.

Timer_mode was set to 2 for 64-bit guests and 0 for

32-bit guests.

   All four hvm guests experienced skew around -1%, even the 32-bit
   guest.  Less intensive testing didn't exhibit much skew at all.

   A representative graph is attached.

   Dave, I wonder if some portion of your patches didn't end up in
   the xen trees?

   (xm dmesg shows 8x Xeon 3.2GHz stepping 04, Platform timer
   14.318MHz HPET.)

   Thanks,
   Dan

   P.S. Many thanks to Deepak and Akira for running tests.

   > -----Original Message-----
   > From: xen-devel-bounces@xxxxxxxxxxxxxxxxxxx
   > [mailto:xen-devel-bounces@xxxxxxxxxxxxxxxxxxx]On Behalf Of
   > Dave Winchell
   > Sent: Wednesday, January 09, 2008 9:53 AM
   > To: Keir Fraser

> Cc: dan.magenheimer@xxxxxxxxxx;

xen-devel@xxxxxxxxxxxxxxxxxxx; Dave

   > Winchell
   > Subject: Re: [Xen-devel] [PATCH] Add a timer mode that
   > disables pending
   > missed ticks
   >
   >
   > Hi Keir,
   >
   > The latest change, c/s 16690, looks fine.
   > I agree that the code in c/s 16690 is equivalent to
   > the code I submitted. Also, your version is more
   > concise.
   >

> The error tests confirm the equivalence. With

overnight cpu loads,

   > the checked in version was accurate to +.048% for sles
   > and +.038% for red hat. My version was +.046% and +.032% in a
   > 2 hour test.
   > I don't think the difference is significant.
   >
   > i/o loads produced errors of +.01%.
   >
   > Thanks for all your efforts on this issue.
   >
   > Regards,
   > Dave
   >
   >
   >
   > Keir Fraser wrote:
   >
   > >Applied as c/s 16690, although the checked-in patch is
   > smaller. I think the
   > >only important fix is to pt_intr_post() and the only bit of
   > the patch I
   > >totally omitted was the change to pt_process_missed_ticks().
   > I don't think

> >that change can be important, but let's see what

happens to the

   error
   > >percentage...
   > >
   > > -- Keir
   > >
> >On 4/1/08 23:24, "Dave Winchell"

<dwinchell@xxxxxxxxxxxxxxx> wrote:

   > >
   > >
   > >
   > >>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;
   > >>
   > >>
   > >
   > >
   > >
   > >
   >
   >
   > _______________________________________________
   > Xen-devel mailing list
   > Xen-devel@xxxxxxxxxxxxxxxxxxx
   > http://lists.xensource.com/xen-devel
   >



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

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