RE: Scheduler benchmarks

[David Laight]

From: Bernd Petrovitsch
> Sent: 19 August 2020 11:22
> 
> On 19/08/2020 10:16, Muni Sekhar wrote:
> > On Tue, Aug 18, 2020 at 11:45 PM peter enderborg
> > <peter.enderborg@xxxxxxxx> wrote:
> [...]
> >> On the 4.4 kernel you dont have
> >>
> >> +CONFIG_RETPOLINE=y
> >> +CONFIG_INTEL_RDT=y
> > Thanks! That is helpful. Yes, I see 4.4 kernel don't have the above
> > two config options.
> > What analysis can be done to narrow down the root cause?
> > Any example of reference could be helpful to understand.
> 
> I haven't checked the date of the older kernel but Spectre+
> Meltdown mitigation costs a lot (20%-30% speed IIRC, out
> of the top of my head).

It depends greatly on the benchmark.

Page table separation has a massive effect on anything that
does a lot of short system calls.
There may be some mitigation in very recent hardware.
And I can't remember if it gets disabled for 'root' processes
(which can read kernel memory anyway).

It is definitely worth building a kernel with the mitigations
disabled  - if only to check that they are responsible.

	David

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