Re: [PATCH 08/11] vfs: inode cache conversion to hash-bl

[Kent Overstreet]

On Wed, Dec 06, 2023 at 05:05:37PM +1100, Dave Chinner wrote:
> From: Dave Chinner <dchinner@xxxxxxxxxx>
> 
> Scalability of the global inode_hash_lock really sucks for
> filesystems that use the vfs inode cache (i.e. everything but XFS).

Ages ago, we talked about (and I attempted, but ended up swearing at
inode lifetime rules) - conversion to rhashtable instead, which I still
believe would be preferable since that code is fully lockless (and
resizeable, of course). But it turned out to be a much bigger project...

But IIRC the bulk of the work was going to be "clean up inode
refcounting/lifetime rules into something sane/modern" - maybe we could
leave some breadcrumbs/comments in fs/inode.c for what that would take,
if/when someone else is sufficiently motivated?

> threads		vanilla	 patched	vanilla	patched
> 2		 7.923	  7.358		 8.003	 7.276
> 4		 8.152	  7.530		 9.097	 8.506
> 8		13.090	  7.871		11.752	10.015
> 16		24.602	  9.540		24.614	13.989
> 32		49.536	 19.314		49.179	25.982

nice

> The big wins here are at >= 8 threads, with both filesytsems now
> being limited by internal filesystem algorithms, not the VFS inode
> cache scalability.
> 
> Ext4 contention moves to the buffer cache on directory block
> lookups:
> 
> -   66.45%     0.44%  [kernel]              [k] __ext4_read_dirblock
>    - 66.01% __ext4_read_dirblock
>       - 66.01% ext4_bread
>          - ext4_getblk
>             - 64.77% bdev_getblk
>                - 64.69% __find_get_block
>                   - 63.01% _raw_spin_lock
>                      - 62.96% do_raw_spin_lock
>                           59.21% __pv_queued_spin_lock_slowpath
> 
> bcachefs contention moves to internal btree traversal locks.
> 
>  - 95.37% __lookup_slow
>     - 93.95% bch2_lookup
>        - 82.57% bch2_vfs_inode_get
> 	  - 65.44% bch2_inode_find_by_inum_trans
> 	     - 65.41% bch2_inode_peek_nowarn
> 		- 64.60% bch2_btree_iter_peek_slot
> 		   - 64.55% bch2_btree_path_traverse_one
> 		      - bch2_btree_path_traverse_cached
> 			 - 63.02% bch2_btree_path_traverse_cached_slowpath
> 			    - 56.60% mutex_lock

dlist-lock ought to be perfect for solving this one

Reviewed-by: Kent Overstreet <kent.overstreet@xxxxxxxxx>