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

[Aneesh Kumar K.V]

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()
>> }
>>
>> 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 |                          |                       |
>> |------------+----------+---------------+--------------------------+-----------------------|
>>
> 
> How do we interpret these numbers?  Are they times, or speed?  Is
> larger better or worse?

Sorry for not including the details. They are time in seconds. Test case 
is a modified mmap_bench included in powerpc/selftest.

> 
> Can you give us the mean and standard deviation for each set of 5
> please?
> 

powernv without patch
median= 51.432255
stdev = 0.370835

with patch
median = 50.739922
stdev = 0.06419662

pseries without patch
median = 116.617884
stdev = 3.04531023

with patch no hcall
median = 119.42494
stdev = 0.85874552

with patch and hcall
median = 117.735808
stdev = 2.7624151

-aneesh