On Thu 06-10-22 12:25:00, Ojaswin Mujoo wrote: > On Thu, Sep 29, 2022 at 02:53:11PM +0200, Jan Kara wrote: > > On Tue 27-09-22 14:46:48, Ojaswin Mujoo wrote: > > > Earlier, inode PAs were stored in a linked list. This caused a need to > > > periodically trim the list down inorder to avoid growing it to a very > > > large size, as this would severly affect performance during list > > > iteration. > > > > > > Recent patches changed this list to an rbtree, and since the tree scales > > > up much better, we no longer need to have the trim functionality, hence > > > remove it. > > > > > > Signed-off-by: Ojaswin Mujoo <ojaswin@xxxxxxxxxxxxx> > > > Reviewed-by: Ritesh Harjani (IBM) <ritesh.list@xxxxxxxxx> > > > > I'm kind of wondering: Now there won't be performance issues with much > > more inode PAs but probably we don't want to let them grow completely out > > of control? E.g. I can imagine that if we'd have 1 billion of inode PAs > > attached to an inode, things would get wonky both in terms of memory > > consumption and also in terms of CPU time spent for the cases where we > > still do iterate all of the PAs... Is there anything which keeps inode PAs > > reasonably bounded? > > > > Honza > > > Hi Jan, > > Sorry for the delay in response, I was on leave for the last few days. > > So as per my understanding, after this patch, the only path where we > would need to traverse all the PAs is the ext4_discard_preallocations() > call where we discard all the PAs of an inode one by one (eg when > closing the file etc). Such a discard is a colder path as we don't > usually expect to do it as often as say allocating blocks to an inode. > > Originally, the limit was added in this patch [1] because of the time > lost in O(N) traversal in the allocation path (ext4_mb_use_preallocated > and ext4_mb_normalize_request). Since the rbtree addressed this > scalability issue we had decided to remove the trim logic in this > patchset. > > [1] > https://lore.kernel.org/all/d7a98178-056b-6db5-6bce-4ead23f4a257@xxxxxxxxx/ I agree the O(N) traversal is not in any performance sensitive path. > That being said, I do agree that there should be some way to limit the > PAs from taking up an unreasonable amount of buddy space, memory and CPU > cycles in use cases like database files and disk files of long running > VMs. Previously the limit was 512 PAs per inode and trim was happening > in an LRU fashion, which is not very straightforward to implement in > trees. > > Another approach is rather than having a hard limit, we can throttle the > PAs based on some parameter like total active PAs in FS or FSUtil% of > the PAs but we might need to take care of fairness so one inode is not > holding all the PAs while others get throttled. > > Anyways, I think the trimming part would need some brainstorming to get > right so just wondering if we could keep that as part of a separate > patchset and remove the trimming logic for now since rbtree has > addressed the scalability concerns in allocation path. I agree the fact it took until 2020 for someone to notice inode PAs can be cumulating enough for full scan to matter on block allocation means that this is not a pressing issue. So I'm OK postponing it for now since I also don't have a great idea how to best trim excessive preallocations. Honza -- Jan Kara <jack@xxxxxxxx> SUSE Labs, CR
