Re: [RFC PATCH 0/4] Restore change_pte optimization to its former glory

[Jerome Glisse <jglisse@redhat.com>]

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