This is a new set of patches to address some slab reclaim issues I observed when trying to convert btrfs over to a purely slab meta data system. The problem is our slab reclaim amounts are based on how page cache scanning goes, which is not related to how much slab is in use on the system. This means we waste a lot of cycles trying to reclaim really small numbers of objects and thus induce a huge latency hit when we hit memory pressure. The second patch is to fix another problem I noticed, namely that we will constantly evict active pages instead of trying to evict shorter lived pages and objects. You can run the following test to see the behavior https://github.com/josefbacik/debug-scripts/blob/master/cache-pressure/cache-pressure.sh Reading two files that fit nicely into RAM, and then add slab pressure from fs_mark and you'll see that we will evict both files 3 or 4 times during the run. With my patches we don't evict any of the file pages at all, only the slab pressure. CHANGES SINCE V2: After discussions with Minchan and Dave I went back to the drawing board. Dave's suggestion of shrinker specific callbacks to allow shrinkers to reclaim at their own rate was intriguing so I did this first. Unfortunately this fell apart as I couldn't figure out a scheme to make the reclaim stuff work without needlessly inducing latency during 'stable' periods. Without a view of the whole system it's hard to know when to trigger scanning of active objects without wasting CPU cycles that aren't actually needed. We will end up starting our background thread and scanning objects when in reality our workload would fit perfectly fine in memory. Minchan's main complaint of my stuff was that it was too aggressive, and any attempt to short circuit the aggression would be unfair to other shrinkers. So instead use his idea of using sc->priority to determine our scan count. But instead of using the traditional ratio, just use scan = nr_objects >> sc->priority. This gets us what we do with the LRU, which is scan = pages >> sc->priority, and gives us an appropriate amount of aggressiveness for slab reclaim. I've dropped the other two patches around stopping slab reclaim, and have changed the slab pressure to be based on sc->priority, which is consistent with every other LRU on the system, and gives me the same results in my testcases. Thanks, Josef -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
