On Fri 24-11-23 18:54:54, Dmytro Maluka wrote: [...] > But looking at the code in __alloc_pages() and around, I see you are > right: we don't try draining other CPUs' PCP lists *before* resorting to > direct reclaim, compaction etc. > > BTW, why not? Shouldn't draining PCP lists be cheaper than pageout() in > any case? My guess would be that draining remote pcp caches is quite expensive on its own. This requires IPIs, preempting whatever is running there and wait for the all the cpus with pcp caches to be done. On the other hand reclaiming a mostly clean page cache could be much less expensive. Also consider that refilling those pcp caches is not free either (you might hit zone lock contetion and who knows what else). Last but not least also consider that many systems could be just on the edge of low/min watermark with a lot of cached data. If we drained all pcp caches whenever we reclaim this could just make the cache pointless. All that being said, I do not remember any actual numbers or research about this. -- Michal Hocko SUSE Labs
