On Wed 29-08-18 15:58:52, Waiman Long wrote: > On 08/29/2018 03:51 AM, Michal Hocko wrote: > > On Tue 28-08-18 13:19:40, Waiman Long wrote: > >> For negative dentries that are accessed once and never used again, they > >> should be removed first before other dentries when shrinker is running. > >> This is done by putting negative dentries at the head of the LRU list > >> instead at the tail. > >> > >> A new DCACHE_NEW_NEGATIVE flag is now added to a negative dentry when it > >> is initially created. When such a dentry is added to the LRU, it will be > >> added to the head so that it will be the first to go when a shrinker is > >> running if it is never accessed again (DCACHE_REFERENCED bit not set). > >> The flag is cleared after the LRU list addition. > > Placing object to the head of the LRU list can be really tricky as Dave > > pointed out. I am not familiar with the dentry cache reclaim so my > > comparison below might not apply. Let me try anyway. > > > > Negative dentries sound very similar to MADV_FREE pages from the reclaim > > POV. They are primary candidate for reclaim, yet you want to preserve > > aging to other easily reclaimable objects (including other MADV_FREE > > pages). What we do for those pages is to move them from the anonymous > > LRU list to the inactive file LRU list. Now you obviously do not have > > anon/file LRUs but something similar to active/inactive LRU lists might > > be a reasonably good match. Have easily reclaimable dentries on the > > inactive list including negative dentries. If negative entries are > > heavily used then they can promote to the active list because there is > > no reason to reclaim them soon. > > > > Just my 2c > > As mentioned in my reply to Dave, I did considered using a 2 LRU list > solution. However, that will add more complexity to the dcache LRU > management code than my current approach and probably more potential for > slowdown. I completely agree with Dave here. This is not easy but trying to sneak in something that works for an _artificial_ workload is simply a no go. So if it takes to come with a more complex solution to cover more general workloads then be it. Someone has to bite a bullet and explore that direction. It won't be a simple project but well, if negative dentries really matter then it is worth making the reclaim design robust and comprehensible rather than adhoc and unpredictable. -- Michal Hocko SUSE Labs
