Subject: Re: [PATCH 00/16] Remove hash page table slot tracking from linux PTE

From: Ram Pai <[email-masked]>
Date: Sat, 28 Oct 2017 15:35:32 -0700

On Fri, Oct 27, 2017 at 10:57:13AM +0530, Aneesh Kumar K.V wrote:
> 
> 
> On 10/27/2017 10:04 AM, Paul Mackerras wrote:
> >On Fri, Oct 27, 2017 at 09:38:17AM +0530, Aneesh Kumar K.V wrote:
> >>Hi,
> >>
> >>With hash translation mode we always tracked the hash pte slot details in linux page table.
> >>This occupied space in the linux page table and also limitted our ability to support
> >>linux features that require additional PTE bits. This series attempt to lift this
> >>limitation by not tracking slot number in linux page table. We still track slot details
> >>w.r.t Transparent Hugepage entries because an invalidate there requires us to go through
> >>all the 256 hash pte slots. So tracking whether hash page table entry is valid helps us in
> >>avoiding a lot of hcalls there. With THP entries we don't keep slot details in the primary
> >>linux page table entry but in the second half of page table. Hence tracking slot details
> >>for THP doesn't take up space in PTE.
> >>
> >>Even though we don't track slot, for removing/updating hash page table entry, PAPR hcalls expect
> >>hash page table slot details. On pseries we find slot using H_READ hcall using H_READ_4 flags.
> >>This implies an additional 2 hcalls in the updatepp and remove paths. The patch series also
> >>attempt to limit the impact of this by adding new hcalls that does remove/update of hash page table
> >>entry using hash value instead of hash page table slot.
> >>
> >>Below is the performance numbers observed when running a workload that does the below sequence
> >>
> >>for(5000) {
> >>mmap(128M)
> >>touch every page of 2048 page
> >>munmap()
> >>}
> >>

I like the idea of not tracking the slots at all. It is something the
guest should not be knowing or tracking.

> >>The test is run with address randomization off, swap disabled in both host and guest.
> >>
> >>
> >>|------------+----------+---------------+--------------------------+-----------------------|
> >>| iterations | platform | without patch | With series and no hcall | With series and hcall |
> >>|------------+----------+---------------+--------------------------+-----------------------|
> >>|          1 | powernv  |               |                50.818343 |                       |
> >>|          2 | powernv  |               |                50.744123 |                       |
> >>|          3 | powernv  |               |                50.721603 |                       |
> >>|          4 | powernv  |               |                50.739922 |                       |
> >>|          5 | powernv  |               |                50.638555 |                       |
> >>|          1 | powernv  |     51.388249 |                          |                       |
> >>|          2 | powernv  |     51.789701 |                          |                       |
> >>|          3 | powernv  |     52.240394 |                          |                       |
> >>|          4 | powernv  |     51.432255 |                          |                       |
> >>|          5 | powernv  |     51.392947 |                          |                       |
> >>|------------+----------+---------------+--------------------------+-----------------------|
> >>|          1 | pseries  |               |                          |            123.154394 |
> >>|          2 | pseries  |               |                          |            122.253956 |
> >>|          3 | pseries  |               |                          |            117.666344 |
> >>|          4 | pseries  |               |                          |            117.681479 |
> >>|          5 | pseries  |               |                          |            117.735808 |
> >>|          1 | pseries  |               |               119.424940 |                       |
> >>|          2 | pseries  |               |               117.663078 |                       |
> >>|          3 | pseries  |               |               118.345584 |                       |
> >>|          4 | pseries  |               |               119.620934 |                       |
> >>|          5 | pseries  |               |               119.463185 |                       |
> >>|          1 | pseries  |    122.810867 |                          |                       |
> >>|          2 | pseries  |    115.760801 |                          |                       |
> >>|          3 | pseries  |    115.257030 |                          |                       |
> >>|          4 | pseries  |    116.617884 |                          |                       |
> >>|          5 | pseries  |    117.247036 |                          |                       |
> >>|------------+----------+---------------+--------------------------+-----------------------|
> >>
> >

What does 'With series and no hcall' mean?  does it mean -- no calls to  new hcalls,
     instead use H_READ_4 followed by old HCALLs?

And I am assuming the code is not using any of my slot-move-to-secondary-pte changes.

RP