RE: [PATCH 1/2] x86/fpu: Measure the Latency of XSAVE and XRSTOR

[David Laight]

From: Dave Hansen
> Sent: 25 July 2022 18:44
> 
> On 7/24/22 13:54, David Laight wrote:
> > I've done some experiments that measure short instruction latencies.
> > Basically I found:
> 
> Short?  The instructions in question can write up to about 12k of data.
>  That's not "short" by any means.
> 
> I'm also not sure precision here is all that important.  The main things
> we want to know here when and where the init and modified optimizations
> are coming into play.  In other words, how often is there actual data
> that *needs* to be saved and restored and can't be optimized away.
> 
> So, sure, if we were measuring a dozen cycles here, you could make an
> argument that this _might_ be problematic.
> 
> But, in this case, we really just want to be able to tell when
> XSAVE/XRSTOR are getting more or less expensive and also get out a
> minimal amount of data (RFBM/XINUSE) to make a guess why that might be.
> 
> Is it *REALLY* worth throwing serializing instructions in and moving
> clock sources to do that?  Is the added precision worth it?

I suspect that if you run your test in a loop the cpu will
be running at 800MHz for the first iteration but will soon
be running at 3GHz.
That is a 4-fold change in execution time if you use the TSC.

IIRC RDTSC is a serialising instruction (of some form), XSAVE
and XRSTOR may also be - so you probably are waiting for the
instruction to finish.
But you do actually need to ensure that is happening.

	David

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