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

[malc <av1474@comtv.ru>]

On Tue, 21 Aug 2007, Luca Tettamanti wrote:

> Avi Kivity ha scritto:
>> Luca Tettamanti wrote:
>>> 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.
>
[..snip preulde..]

> I've also tried APC which was suggested by malc[1] and:
> - readings are far more stable
> - the gap between dynticks and non-dynticks seems not significant

[..dont snip the obvious fact and snip the numbers..]

>
> Luca
> [1] copy_to_user inside spinlock is a big no-no ;)
>

[..notice a projectile targeting at you and rush to see the code..]

Mixed feelings about this... But in principle the code ofcourse is
dangerous, thank you kindly for pointing this out.

I see two ways out of this:

a. moving the lock/unlock inside the loop with unlock preceding
    sometimes sleep deprived copy_to_user

b. fill temporaries and after the loop is done copy it in one go

Too late, too hot, i wouldn't mind beying on a receiving side of
a good advice.

-- 
vale