On 08/30/2018 03:20 AM, Michal Hocko wrote: > 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. OK, I will need to spend more time to think about a better way of doing that. Cheers, Longman
