Re: [PATCH v5] mm: Proactive compaction

[Nitin Gupta <ngupta@nitingupta.dev>]

On Thu, May 28, 2020 at 4:32 PM Khalid Aziz <khalid.aziz@oracle.com> wrote:
>
> This looks good to me. I like the idea overall of controlling
> aggressiveness of compaction with a single tunable for the whole
> system. I wonder how an end user could arrive at what a reasonable
> value would be for this based upon their workload. More comments below.
>

Tunables like the one this patch introduces, and similar ones like 'swappiness'
will always require some experimentations from the user.


> On Mon, 2020-05-18 at 11:14 -0700, Nitin Gupta wrote:
> > For some applications, we need to allocate almost all memory as
> > hugepages. However, on a running system, higher-order allocations can
> > fail if the memory is fragmented. Linux kernel currently does on-
> > demand
> > compaction as we request more hugepages, but this style of compaction
> > incurs very high latency. Experiments with one-time full memory
> > compaction (followed by hugepage allocations) show that kernel is
> > able
> > to restore a highly fragmented memory state to a fairly compacted
> > memory
> > state within <1 sec for a 32G system. Such data suggests that a more
> > proactive compaction can help us allocate a large fraction of memory
> > as
> > hugepages keeping allocation latencies low.
> >
> > For a more proactive compaction, the approach taken here is to define
> > a new tunable called 'proactiveness' which dictates bounds for
> > external
> > fragmentation wrt HUGETLB_PAGE_ORDER order which kcompactd tries to
> > maintain.
> >
> > The tunable is exposed through sysctl:
> >   /proc/sys/vm/compaction_proactiveness
> >
> > It takes value in range [0, 100], with a default of 20.
>
> Looking at the code, setting this to 100 would mean system would
> continuously strive to drive level of fragmentation down to 0 which can
> not be reasonable and would bog the system down. A cap lower than 100
> might be a good idea to keep kcompactd from dragging system down.
>

Yes, I understand that a value of 100 would be a continuous compaction
storm but I still don't want to artificially cap the tunable. The interpretation
of this tunable can change in future, and a range of [0, 100] seems
more intuitive than, say [0, 90]. Still, I think a word of caution should
be added to its documentation (admin-guide/sysctl/vm.rst).


> >

> > Total 2M hugepages allocated = 383859 (749G worth of hugepages out of
> > 762G total free => 98% of free memory could be allocated as
> > hugepages)
> >
> > - With 5.6.0-rc3 + this patch, with proactiveness=20
> >
> > echo 20 | sudo tee /sys/kernel/mm/compaction/node-*/proactiveness
>
> Should be "echo 20 | sudo tee /proc/sys/vm/compaction_proactiveness"
>

oops... I forgot to update the patch description. This is from the v4 patch
which used sysfs but v5 switched to using sysctl.


> >

> > diff --git a/Documentation/admin-guide/sysctl/vm.rst
> > b/Documentation/admin-guide/sysctl/vm.rst
> > index 0329a4d3fa9e..e5d88cabe980 100644
> > --- a/Documentation/admin-guide/sysctl/vm.rst
> > +++ b/Documentation/admin-guide/sysctl/vm.rst
> > @@ -119,6 +119,19 @@ all zones are compacted such that free memory is
> > available in contiguous
> >  blocks where possible. This can be important for example in the
> > allocation of
> >  huge pages although processes will also directly compact memory as
> > required.
> >
> > +compaction_proactiveness
> > +========================
> > +
> > +This tunable takes a value in the range [0, 100] with a default
> > value of
> > +20. This tunable determines how aggressively compaction is done in
> > the
> > +background. Setting it to 0 disables proactive compaction.
> > +
> > +Note that compaction has a non-trivial system-wide impact as pages
> > +belonging to different processes are moved around, which could also
> > lead
> > +to latency spikes in unsuspecting applications. The kernel employs
> > +various heuristics to avoid wasting CPU cycles if it detects that
> > +proactive compaction is not being effective.
> > +
>
> Value of 100 would cause kcompactd to try to bring fragmentation down
> to 0. If hugepages are being consumed and released continuously by the
> workload, it is possible that kcompactd keeps making progress (and
> hence passes the test "proactive_defer = score < prev_score ?")
> continuously but can not reach a fragmentation score of 0 and hence
> gets stuck in compact_zone() for a long time. Page migration for
> compaction is not inexpensive. Maybe either cap the value to something
> less than 100 or set a floor for wmark_low above 0.
>
> Some more guidance regarding the value for this tunable might be
> helpful here, something along the lines of what does a value of 100
> mean in terms of how kcompactd will behave. It can then give end user a
> better idea of what they are getting at what cost. You touch upon the
> cost above. Just add some more details so an end user can get a better
> idea of size of the cost for higher values of this tunable.
>

I like the idea of capping wmark_low to say, 5 to prevent admins from
overloading the system. Similarly, wmark_high should be capped at
say, 95 to allow tunable values below 10 to have any effect: currently
such low tunable values would give wmark_high=100 which would
cause proactive compaction to never get triggered.

Finally, I see your concern about lack of guidance on extreme values
of the tunable. I will address this in the next (v6) iteration.

Thanks,
Nitin