On Thu, Nov 10, 2022 at 6:42 AM Michal Koutný <mkoutny@xxxxxxxx> wrote: > > Hello Jialin. > > On Thu, Nov 10, 2022 at 02:53:16PM +0800, Lu Jialin <lujialin4@xxxxxxxxxx> wrote: > > When user use memory.reclaim to reclaim memory, after drain percpu lru > > caches, drain percpu charge caches for given memcg stock in the hope > > of introducing more evictable pages. > > Do you have any data on materialization of this hope? > > IIUC, the stock is useful for batched accounting to page_counter but it > doesn't represent real pages. I.e. your change may reduce the > page_counter value but it would not release any pages. Or have I missed > a way how it helps with the reclaim? +1 It looks like we just overcharge the memcg if the number of allocated pages are less than the charging batch size, so that upcoming allocations can go through a fast accounting path and consume from the precharged stock. I don't understand how draining this charge may help reclaim. OTOH, it will reduce the page counters, so if userspace is relying on memory.current to gauge how much reclaim they want to do, it will make it "appear" like the usage dropped. If userspace is using other signals (refaults, PSI, etc), then we would be more-or-less tricking it into thinking we reclaimed pages when we actually didn't. In that case we didn't really reclaim anything, we just dropped memory.current slightly, which wouldn't matter to the user in this case, as other signals won't change. The difference in perceived usage coming from draining the stock IIUC has an upper bound of 63 * PAGE_SIZE (< 256 KB with 4KB pages), I wonder if this is really significant anyway. > > Thanks, > Michal
