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

[Michal Hocko]

On Tue 03-03-20 19:49:53, Huang, Ying wrote:
> Michal Hocko <mhocko@xxxxxxxxxx> writes:
> 
> > On Tue 03-03-20 16:47:54, Huang, Ying wrote:
> >> Michal Hocko <mhocko@xxxxxxxxxx> writes:
> >> 
> >> > On Tue 03-03-20 09:51:56, Huang, Ying wrote:
> >> >> Mel Gorman <mgorman@xxxxxxx> 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?
> >> >
> >> > This is already happening because if the target node is short on memory
> >> > it will start to reclaim and if MADV_FREE pages are at the tail of
> >> > inactive file LRU list then they will be dropped. Please note how that
> >> > follows proper aging and doesn't introduce any special casing. Really
> >> > MADV_FREE is an inactive cache for anonymous memory and we treat it like
> >> > inactive page cache. This is not carved in stone of course but it really
> >> > requires very good justification to change.
> >> 
> >> If my understanding were correct, the newly migrated clean MADV_FREE
> >> pages will be put at the head of inactive file LRU list instead of the
> >> tail.  So it's possible that some useful file cache pages will be
> >> reclaimed.
> >
> > This is the case also when you migrate other pages, right? We simply
> > cannot preserve the aging.
> 
> So you consider the priority of the clean MADV_FREE pages is same as
> that of page cache pages?

This is how MADV_FREE has been implemented, yes. See f7ad2a6cb9f7 ("mm:
move MADV_FREE pages into LRU_INACTIVE_FILE list") for the
justification.

> Because the penalty difference is so large, I
> think it may be a good idea to always put clean MADV_FREE pages at the
> tail of the inactive file LRU list?

You are again making assumptions without giving any actual real
examples. Reconstructing MADV_FREE pages cost can differ a lot. This
really depends on the specific usecase. Moving pages to the tail of LRU
would make them the primary candidate for the reclaim with a strange
LIFO semantic. Adding them to the head might be not the universal win
but it will at least provide a reasonable FIFO semantic. I also find
it much more easier to reason about MADV_FREE as an inactive cache.
-- 
Michal Hocko
SUSE Labs