Re: [PATCH 4/8] x86/asm: add clear_page_nt()

[Ankur Arora <ankur.a.arora@oracle.com>]

On 2020-10-14 12:56 p.m., Borislav Petkov wrote:
> On Wed, Oct 14, 2020 at 01:32:55AM -0700, Ankur Arora wrote:
>> This can potentially improve page-clearing bandwidth (see below for
>> performance numbers for two microarchitectures where it helps and one
>> where it doesn't) and can help indirectly by consuming less cache
>> resources.
>>
>> Any performance benefits are expected for extents larger than LLC-sized
>> or more -- when we are DRAM-BW constrained rather than cache-BW
>> constrained.
> 
> "potentially", "expected", I don't like those formulations.
That's fair. The reason for those weasel words is mostly because it
is microarchitecture specific.
For example on Intel where I did compare across generations: I see good
performance on Broadwellx, not good on Skylakex and then good again on
some pre-production CPUs.

> Do you have
> some actual benchmark data where this shows any improvement and not
> microbenchmarks only, to warrant the additional complexity?
Yes, guest creation under QEMU (pinned guests) shows similar improvements.
I've posted performance numbers in patches 7, 8 with a simple page-fault
test derived from that.

I can add numbers from QEMU as well.

Thanks,
Ankur

>