On Wed, 15 Jul 2020, Chris Down wrote: > Hi David, > > I'm somewhat against adding more metrics which try to approximate availability > of memory when we already know it not to generally manifest very well in > practice, especially since this *is* calculable by userspace (albeit with some > knowledge of mm internals). Users and applications often vastly overestimate > the reliability of these metrics, especially since they heavily depend on > transient page states and whatever reclaim efficacy happens to be achieved at > the time there is demand. > Hi Chris, With the proposed anon_reclaimable, do you have any reliability concerns? This would be the amount of lazy freeable memory and memory that can be uncharged if compound pages from the deferred split queue are split under memory pressure. It seems to be a very precise value (as slab_reclaimable already in memory.stat is), so I'm not sure why there is a reliability concern. Maybe you can elaborate? Today, this information is indeed possible to calculate from userspace. The idea is to present this information that will be backwards compatible, however, as the kernel implementation changes. When lazy freeable memory was added, for instance, userspace likely would not have preemptively been doing an "active_file + inactive_file - file" calculation to factor that in as reclaimable anon :) > What do you intend to do with these metrics and how do you envisage other > users should use them? Is it not possible to rework the strategy to use > pressure information and/or workingset pressurisation instead? > Previously, users would interpret their anon usage as non reclaimable if swap is disabled and now that value can include a *lot* of easily reclaimable memory. Our users would also carefully monitor their current memcg usage and/or anon usage to detect abnormalities without concern for what is reclaimable, especially for things like deferred split queues that was purely a kernel implementation change. Memcg usage and anon usag then becomes wildly different between kernel versions and our users alert on that abnormality. The example I gave earlier in the thread showed how dramatically different memory.current is before and after the introduction of deferred split queues. Userspace sees ballooning memcg usage and alerts on it (suspects a memory leak, for example) when in reality this is purely reclaimable memory under pressure and is the result of a kernel implementation detail. We plan on factoring this information in when determining what the actual amount of memory that can and cannot be reclaimed from a memcg hierarchy at any given time.
