Subject: Re: [PATCH 4/7] powerpc: Free up four 64K PTE bits in 64K backed HPTE pages

From: "Aneesh Kumar K.V" <[email-masked]>
Date: Mon, 23 Oct 2017 14:22:44 +0530

Benjamin Herrenschmidt <[email-masked]> writes:

> On Fri, 2017-09-08 at 15:44 -0700, Ram Pai wrote:
>> The second part of the PTE will hold
>> (H_PAGE_F_SECOND|H_PAGE_F_GIX) at bit 60,61,62,63.
>> NOTE: None of the bits in the secondary PTE were not used
>> by 64k-HPTE backed PTE.
>
> Have you measured the performance impact of this ? The second part of
> the PTE being in a different cache line there could be one...
>

I am also looking at a patch series removing the slot tracking
completely. With randomize address turned off and no swap in guest/host
and making sure we touched most of guest ram, I don't find much impact
in performance when we don't track the slot at all. I will post the
patch series with numbers in a day or two. But my test was

while (5000) {
      mmap(128M)
      touch every page of 2048 pages
      munmap()
}

I could also be the best case in my run because i might have always
found the hash pte slot in the primary. In one measurement with swap on
and address randmization enabled, i did find a 50% impact. But then i
was not able to recreate that again. So could be something i did wrong
in the test setup.

Ram,

Will you be able to get a test run with the above loop?

-aneesh