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

Dave Hansen
2018-03-29T00:18:00Z

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.

1.
https://ark.intel.com/products/97933/Intel-Atom-Processor-C3955-16M-Cache-up-to-2_40-GHz
2.