Re: windows acpi time drift

[Anthony Liguori <anthony@codemonkey.ws>]

Dor Laor wrote:
> On Wed, 2008-03-19 at 01:09 +0200, Dor Laor wrote:
>   
>> After some research of time drift while using window windows acpi hal I
>> discovered it uses the ... rtc timer as a source clock. 
>> Not the apic, acpi nor the pit. The acpi timer is not used by the time
>> keeping clock, the apic & pit timer irqs are masked.
>>
>> In order to fix the time drift we need to fix the rtc emulation.
>> The problem is that like the pit and the apic timers in userspace, the
>> rtc also has inaccurate timer, thus leading to irq coalescing before
>> getting acknowledged by the guest interrupt controller.
>>
>> We have two options:
>> 1. Bring another device to the kernel
>> 	- It's a simple device
>> 	- It will make the rtc clock more accurate (hrtimer)
>> 	- Easy time drift fix like apic/pic
>> 	- It has very minor performance improvment of canceling the 
>>           need to go to userspace after vmexit, thus not syncing vmcs.
>>           But it's only 15msec * 2 rate.
>>     
>
> Hmm, when doing multimedia, windows increases the frequency of rtc from
> 64HZ to 1024HZ, thus in-kernel device will save 2048 user-space exits.
> This might be a real improvement - small but measurable.
>   

Well, let's calculate it.  I measure a lightweight exit at 3590 cycles 
and a heavyweight exit at 8548.  If we look at the cost of dropping to 
userspace an extra 2048 times, since I have a 2.2 GHz chip, we're 
looking at an additional .0046 seconds to transition to userspace.  This 
is on top of the base .0033 seconds that it takes to take these exits in 
the first place.  This are dummy exits though so when you add in the 
cost of processing this (which I think is roughly equal whether in 
kernelspace or userspace) I think those values will quickly be overwhelmed.

So, at the end of the day, you're not getting even a 1% performance 
boost and you're adding a lot of complexity to the kernel.  This 
strongly suggests to me that we should be doing this in userspace.

Regards,

Anthony Liguori


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