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SubjectRe: [RFC PATCH 0/4] Restore change_pte optimization to its former glory
On Mon, Feb 18, 2019 at 12:45:05PM -0500, Andrea Arcangeli wrote:
> 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.

So using that and the checksum test removed there is an improvement,
roughly 7%~8% on time spends in the kernel:

before mean {real: 675.460632, user: 857.771423, sys: 215.929657, npages: 4773.066895}
before stdev {real: 37.035435, user: 4.395942, sys: 3.976172, npages: 675.352783}
after mean {real: 672.515503, user: 855.817322, sys: 200.902710, npages: 4899.000000}
after stdev {real: 37.340954, user: 4.051633, sys: 3.894153, npages: 742.413452}

I am guessing for kernel build this get lost in the noise and that
KSM changes do not have that much of an impact. So i will reposting
the mmu notifier changes shortly in hope to get them in 5.1 and i
will post the KVM part separatly shortly there after.

If there is any more testing you wish me to do let me know.

Cheers,
Jérôme

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