Re: [PATCH 00/11] Use global pages with PTI

Ingo Molnar
2018-03-30T12:09:26Z

* Dave Hansen <dave.hansen@linux.intel.com> wrote:

> On 03/27/2018 01:07 PM, Ingo Molnar wrote:
> > * Thomas Gleixner <tglx@linutronix.de> wrote:
> >>> systems. Atoms are going to be the easiest thing to get my hands on,
> >>> but I tend to shy away from them for performance work.
> >> What I have in mind is that I wonder whether the whole circus is worth it
> >> when there is no performance advantage on PCID systems.
>
> I was waiting on trying to find a relatively recent Atom system (they
> actually come in reasonably sized servers [1]), but I'm hitting a snag
> there, so I figured I'd just share a kernel compile using Ingo's
> perf-based methodology on a Skylake desktop system with PCIDs.
>
> Here's the kernel compile:
>
> No Global pages (baseline): 186.951 seconds time elapsed ( +- 0.35% )
> 28 Global pages (this set): 185.756 seconds time elapsed ( +- 0.09% )
> -1.195 seconds (-0.64%)
>
> Lower is better here, obviously.
>
> I also re-checked everything using will-it-scale's llseek1 test[2] which
> is basically a microbenchmark of a halfway reasonable syscall. Higher
> here is better.
>
> No Global pages (baseline): 15783951 lseeks/sec
> 28 Global pages (this set): 16054688 lseeks/sec
> +270737 lseeks/sec (+1.71%)
>
> So, both the kernel compile and the microbenchmark got measurably faster.

Ok, cool, this is much better!

Mind re-sending the patch-set against latest -tip so it can be merged?

At this point !PCID Intel hardware is not a primary concern, if something bad
happens on them with global pages we can quirk global pages off on them in some
way, or so.

Thanks,

Ingo