Re: [benchmark] 1% performance overhead of paravirt_ops on nativekernels

[Jeremy Fitzhardinge]

Ingo Molnar wrote:
> I did more 'perf stat mmap-perf 1' measurements (bound to a single 
> core, running single thread - to exclude cross-CPU noise), which in 
> essence measures CONFIG_PARAVIRT=y overhead on native kernels:
>   

Thanks for taking the time to make these measurements.  You'll agree 
they're much better numbers than the last time you ran these tests?

>  Performance counter stats for './mmap-perf':
>
>       [vanilla]  [PARAVIRT=y]
>
>     1230.805297   1242.828348  task clock ticks     (msecs)   + 0.97%
>      3602663413    3637329004  CPU cycles           (events)  + 0.96%
>      1927074043    1958330813  instructions         (events)  + 1.62%
>
> That's around 1% on really fast hardware (Core2 E6800 @ 2.93 GHz, 
> 4MB L2 cache), i.e. still significant overhead. Distros generally 
> enable CONFIG_PARAVIRT, even though a large majority of users never 
> actually runs them as Xen guests.
>   

Did you do only a single run, or is this the result of multiple runs?  
If so, what was your procedure? How did you control for page 
placement/cache effects/other boot-to-boot variations?

Your numbers are not dissimilar to my measurements, but I also saw up to 
1% performance improvement vs native from boot to boot (I saw up to 10% 
reduction of cache misses with pvops, possibly because of its 
de-inlining effects).

I also saw about 1% boot to boot variation with the non-pvops kernel.

While I think pvops does add *some* overhead, I think the absolute 
magnitude is swamped in the noise.  The best we can say is "somewhere 
under 1% on modern hardware".

> Are there plans to analyze and fix this overhead too, beyond the 
> paravirt-spinlocks overhead you analyzed? (Note that i had 
> CONFIG_PARAVIRT_SPINLOCKS disabled in this test.)
>
> I think only those users should get overhead who actually run such 
> kernels in a virtualized environment.
>
> I cannot cite a single other kernel feature that has so much 
> performance impact when runtime-disabled. For example, an often 
> cited bloat and overhead source is CONFIG_SECURITY=y.
>   

Your particular benchmark does many, many mmap/mprotect/munmap/mremap 
calls, and takes a lot of pagefaults.  That's going to hit the hot path 
with lots of pte updates and so on, but very few security hooks.  How 
does it compare with a more balanced workload?

> Its runtime overhead (same system, same workload) is:
>
>       [vanilla]  [SECURITY=y]
>
>     1219.652255   1230.805297  task clock ticks     (msecs)   + 0.91%
>      3574548461    3602663413  CPU cycles           (events)  + 0.78%
>      1915177924    1927074043  instructions         (events)  + 0.62%
>
> ( With the difference that the distros that enable CONFIG_SECURITY=y
>   tend to install and use at least one security module by default. )
>
> So everyone who runs a CONFIG_PARAVIRT=y distro kernel has 1% of 
> overhead in this mmap-test workload - even if no Xen is used on that 
> box, ever.
>   

So you're saying that:

   * CONFIG_SECURITY adding +0.91% to wallclock time is OK, but pvops
     adding +0.97% is not,
   * your test is sensitive enough to make 0.06% difference
     significant, and
   * this benchmark is representative enough of real workloads that its
     results are overall meaningful?


> Config attached.
>   

Is this derived from a RH distro config?

   J
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