Re: [PATCH 00/16] Remove hash page table slot tracking from linux PTE
flat view
From: Paul Mackerras <hidden>
Date: 2017-10-27 04:34:41
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? Can you give us the mean and standard deviation for each set of 5 please? Paul.