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[Qemu-devel] Re: [kvm-devel] [PATCH 0/4] Rework alarm timer infrastrucur

From: Luca Tettamanti
Subject: [Qemu-devel] Re: [kvm-devel] [PATCH 0/4] Rework alarm timer infrastrucure - take2
Date: Mon, 20 Aug 2007 23:20:58 +0200
User-agent: Mutt/1.5.16 (2007-06-11)

Il Sun, Aug 19, 2007 at 10:31:26PM +0300, Avi Kivity ha scritto: 
> Luca wrote:
> > On 8/19/07, Luca Tettamanti <address@hidden> wrote:
> >   
> >> +static uint64_t qemu_next_deadline(void) {
> >> +    uint64_t nearest_delta_us = ULLONG_MAX;
> >> +    uint64_t vmdelta_us;
> >>     
> >
> > Hum, I introduced a bug here... those vars should be signed.
> >
> > On the overhead introduced: how do you measure it?
> >
> >   
> Run a 100Hz guest, measure cpu usage using something accurate like
> cyclesoak, with and without dynticks, with and without kvm.

Ok, here I've measured the CPU usage on the host when running an idle

At 100Hz

hpet            4.8%
dynticks        5.1%

Note: I've taken the mean over a period of 20 secs, but the difference
between hpet and dynticks is well inside the variability of the test.

hpet            2.2%
dynticks        1.0%

Hum... here the numbers jumps a bit, but dynticks is always below hpet.

At 1000Hz:

hpet            5.5%
dynticks       11.7%

hpet            3.4%
dynticks        7.3%

No surprises here, you can see the additional 1k syscalls per second. On
the bright side, keep in mind that with a tickless guest and dynticks
I've seen as little as 50-60 timer ticks per second.

Hackbench (hackbench -pipe 50) inside the guest:

QEMU: impossible to measure, the variance of the results is much bigger
than difference between dynticks and hpet.

Around 0.8s slower in case on dynticks; variance of the results is
about 0.3s in both cases.

"Chi parla in tono cortese, ma continua a prepararsi, potra` andare avanti;
 chi parla in tono bellicoso e avanza rapidamente dovra` ritirarsi" 
Sun Tzu -- L'arte della guerra

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