On Fri 20-05-22 10:45:43, Vaibhav Jain wrote: > > Thanks for looking into this patch Michal, > > Michal Hocko <mhocko@xxxxxxxx> writes: > > > On Thu 19-05-22 04:08:15, Vaibhav Jain wrote: > >> [1] Provides a way for user-space to trigger proactive reclaim by introducing > >> a write-only memcg file 'memory.reclaim'. However reclaim stats like number > >> of pages scanned and reclaimed is still not directly available to the > >> user-space. > >> > >> This patch proposes to extend [1] to make the memcg file 'memory.reclaim' > >> readable which returns the number of pages scanned / reclaimed during the > >> reclaim process from 'struct vmpressure' associated with each memcg. This should > >> let user-space asses how successful proactive reclaim triggered from memcg > >> 'memory.reclaim' was ? > >> > >> With the patch following command flow is expected: > >> > >> # echo "1M" > memory.reclaim > >> > >> # cat memory.reclaim > >> scanned 76 > >> reclaimed 32 > > > > Why cannot you use memory.stat? Sure it would require to iterate over > > the reclaimed hierarchy but the information about scanned and reclaimed > > pages as well as other potentially useful stats is there. > > Agree that "memory.stat" is more suitable for scanned/reclaimed stats as > it already is exposing bunch of other stats. > > The discussion on this patch however seems to have split into two parts: > > 1. Is it a good idea to expose nr_scanned/nr_reclaimed to users-space > and if yes how ? > > IMHO, I think it will be better to expose this info via 'memory.stat' as it > can be useful insight into the reclaim efficiency and vmpressure. We already do that with some more metrics pgrefill 9801926 pgscan 27329762 pgsteal 22715987 pgactivate 250691267 pgdeactivate 9521843 pglazyfree 0 pglazyfreed 0 > 2. Will it be useful to provide feedback to userspace when it writes to > 'memory.reclaim' on how much memory has been reclaimed ? > > IMHO, this will be a useful feeback to userspace to better adjust future > proactive reclaim requests via 'memory.reclaim' How precise this information should be? A very simplistic approach would be cp memory.stat stats.before echo $WHATEVER > memory.reclaim cp memory.stat stats.after This will obviously contain also activity outside of the explicitly triggered reclaim (racing background/direct reclaim) but isn't that what actually matters? Are there any cases where the only metric you care about is the triggered reclaim in isolation? -- Michal Hocko SUSE Labs
