On Mon, Feb 18, 2019 at 11:04:13AM -0500, Jerome Glisse wrote:
> So i run 2 exact same VMs side by side (copy of same COW image) and
> built the same kernel tree inside each (that is the only important
> workload that exist ;)) but the change_pte did not have any impact:
>
> before mean {real: 1358.250977, user: 16650.880859, sys: 839.199524,
> npages: 76855.390625}
> before stdev {real: 6.744010, user: 108.863762, sys: 6.840437,
> npages: 1868.071899}
> after mean {real: 1357.833740, user: 16685.849609, sys: 839.646973,
> npages: 76210.601562}
> after stdev {real: 5.124797, user: 78.469360, sys: 7.009164,
> npages: 2468.017578}
> without mean {real: 1358.501343, user: 16674.478516, sys: 837.791992,
> npages: 76225.203125}
> without stdev {real: 5.541104, user: 97.998367, sys: 6.715869,
> npages: 1682.392578}
>
> Above is time taken by make inside each VM for all yes config. npages
> is the number of page shared reported on the host at the end of the
> build.
Did you set /sys/kernel/mm/ksm/sleep_millisecs to 0?
It would also help to remove the checksum check from mm/ksm.c:
- if (rmap_item->oldchecksum != checksum) {
- rmap_item->oldchecksum = checksum;
- return;
- }
One way or another, /sys/kernel/mm/ksm/pages_shared and/or
pages_sharing need to change significantly to be sure we're exercising
the COW/merging code that uses change_pte. KSM is smart enough to
merge only not frequently changing pages, and with the default KSM
code this probably works too well for a kernel build.
> Should we still restore change_pte() ? It does not hurt, but it does
> not seems to help in anyway. Maybe you have a better benchmark i could
> run ?
We could also try a microbenchmark based on
ltp/testcases/kernel/mem/ksm/ksm02.c that already should trigger a
merge flood and a COW flood during its internal processing.
Thanks,
Andrea
