Re: [RFC 0/3] mm: Discard lazily freed pages when migrating

[Mel Gorman <mgorman@suse.de>]

On Tue, Mar 03, 2020 at 09:51:56AM +0800, Huang, Ying wrote:
> Mel Gorman <mgorman@suse.de> writes:
> > On Mon, Mar 02, 2020 at 07:23:12PM +0800, Huang, Ying wrote:
> >> If some applications cannot tolerate the latency incurred by the memory
> >> allocation and zeroing.  Then we cannot discard instead of migrate
> >> always.  While in some situations, less memory pressure can help.  So
> >> it's better to let the administrator and the application choose the
> >> right behavior in the specific situation?
> >> 
> >
> > Is there an application you have in mind that benefits from discarding
> > MADV_FREE pages instead of migrating them?
> >
> > Allowing the administrator or application to tune this would be very
> > problematic. An application would require an update to the system call
> > to take advantage of it and then detect if the running kernel supports
> > it. An administrator would have to detect that MADV_FREE pages are being
> > prematurely discarded leading to a slowdown and that is hard to detect.
> > It could be inferred from monitoring compaction stats and checking
> > if compaction activity is correlated with higher minor faults in the
> > target application. Proving the correlation would require using the perf
> > software event PERF_COUNT_SW_PAGE_FAULTS_MIN and matching the addresses
> > to MADV_FREE regions that were freed prematurely. That is not an obvious
> > debugging step to take when an application detects latency spikes.
> >
> > Now, you could add a counter specifically for MADV_FREE pages freed for
> > reasons other than memory pressure and hope the administrator knows about
> > the counter and what it means. That type of knowledge could take a long
> > time to spread so it's really very important that there is evidence of
> > an application that suffers due to the current MADV_FREE and migration
> > behaviour.
> 
> OK.  I understand that this patchset isn't a universal win, so we need
> some way to justify it.  I will try to find some application for that.
> 
> Another thought, as proposed by David Hildenbrand, it's may be a
> universal win to discard clean MADV_FREE pages when migrating if there are
> already memory pressure on the target node.  For example, if the free
> memory on the target node is lower than high watermark?
> 

That is an extremely specific corner case that is not likely to occur.
NUMA balancing is not going to migrate a MADV_FREE page under these
circumstances as a write cancels MADV_FREE is read attempt will probably
fail to allocate a destination page in alloc_misplaced_dst_page so the
data gets lost instead of remaining remote. sys_movepages is a possibility
but the circumstances of an application delibertly trying to migrate to
a loaded node is low. Compaction never migrates cross-node so the state
of a remote node under pressure do not matter.

Once again, there needs to be a reasonable use case to be able to
meaningfully balance between the benefits and risks of changing the
MADV_FREE semantics.

-- 
Mel Gorman
SUSE Labs