Re: [PATCH 0/6] In kernel PIT patch

[Anthony Liguori <anthony@codemonkey.ws>]

Dor Laor wrote:
> On Tue, 2008-03-04 at 18:50 -0600, Anthony Liguori wrote:
>   
>> Dor Laor wrote:
>>     
>> I thought there was some discussion about whether -tdf was every useful 
>> in practice?
>>     
>
> It works.
> Just try to play a movie in windows standard HAL with and w/o -tdf
> --no-irq-chip and you'll see the difference.
>   

I don't have a VM with the standard HAL but I'll take your word for it.

>>
>> So how do we measure the benefits of an in-kernel PIT?
>>     
>
> Play the same movie using the kernel's pit.
>   

Playing a movie is a bit subjective.  I presume you're talking about the 
standard HAL as presumably the ACPI HAL is using the pm timer?

So the two cases I'm hearing where timer accuracy should improve is 
standard HAL on Windows and clock=pit on Linux?  I'd still like to see 
what the actual difference in timer accuracy is.  I have no doubt that 
moving the pit into the kernel is more efficient.  Moving everything 
into the kernel is more efficient because light weight exits are cheaper 
than heavy weight exits.

The thing I'm trying to get at is a quantitative statement about why 
moving the pit into the kernel is the right thing.  I'll try to give the 
patches a try myself in the next couple of days.  I don't think it's 
obvious that it's the right thing to do without some sort of benchmark 
supporting it.

Regards,

Anthony LIguori


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