[Qemu-devel] Re: [kvm-devel] [PATCH 0/4] Rework alarm timer infrastrucure - take2

[Avi Kivity <avi@qumranet.com>]

Luca Tettamanti wrote:
>>>   
>>>       
>> 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
> guest.
>
> At 100Hz
>
> QEMU
> 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.
>
> KVM
> hpet            2.2%
> dynticks        1.0%
>
> Hum... here the numbers jumps a bit, but dynticks is always below hpet.
>   

The differences here are small, so I'll focus on the 1000Hz case.

> At 1000Hz:
>
> QEMU
> hpet            5.5%
> dynticks       11.7%
>
> KVM
> hpet            3.4%
> dynticks        7.3%
>
> No surprises here, you can see the additional 1k syscalls per second. 

This is very surprising to me.  The 6.2% difference for the qemu case 
translates to 62ms per second, or 62us per tick at 1000Hz.  That's more 
than a hundred simple syscalls on modern processors.  We shouldn't have 
to issue a hundred syscalls per guest clock tick.

The difference with kvm is smaller (just 3.9%), which is not easily 
explained as the time for the extra syscalls should be about the same.  
My guess is that guest behavior is different; with dynticks the guest 
does about twice as much work as with hpet.

Can you verify this by running

    strace -c -p `pgrep qemu` & sleep 10; pkill strace

for all 4 cases, and posting the results?


-- 
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