Re: [PATCH v2] btrfs: remove unnecessary EXTENT_UPTODATE state in buffered I/O path

[David Sterba <dsterba@suse.cz>]

On Fri, Aug 19, 2022 at 10:44:08AM +0800, ethanlien wrote:
> From: Ethan Lien <ethanlien@synology.com>
> 
> After we copied data to page cache in buffered I/O, we
> 1. Insert a EXTENT_UPTODATE state into inode's io_tree, by
>    endio_readpage_release_extent(), set_extent_delalloc() or
>    set_extent_defrag().
> 2. Set page uptodate before we unlock the page.
> 
> But the only place we check io_tree's EXTENT_UPTODATE state is in
> btrfs_do_readpage(). We know we enter btrfs_do_readpage() only when we
> have a non-uptodate page, so it is unnecessary to set EXTENT_UPTODATE.
> 
> For example, when performing a buffered random read:
> 
> 	fio --rw=randread --ioengine=libaio --direct=0 --numjobs=4 \
> 		--filesize=32G --size=4G --bs=4k --name=job \
> 		--filename=/mnt/file --name=job
> 
> Then check how many extent_state in io_tree:
> 
> 	cat /proc/slabinfo | grep btrfs_extent_state | awk '{print $2}'
> 
> w/o this patch, we got 640567 btrfs_extent_state.
> w/  this patch, we got    204 btrfs_extent_state.
> 
> Maintaining such a big tree brings overhead since every I/O needs to insert
> EXTENT_LOCKED, insert EXTENT_UPTODATE, then remove EXTENT_LOCKED. And in
> every insert or remove, we need to lock io_tree, do tree search, alloc or
> dealloc extent states. By removing unnecessary EXTENT_UPTODATE, we keep
> io_tree in a minimal size and reduce overhead when performing buffered I/O.
> 
> Signed-off-by: Ethan Lien <ethanlien@synology.com>
> Reviewed-by: Robbie Ko <robbieko@synology.com>
> ---
> 
> V2: Remove set_extent_uptodate() from btrfs_get_extent(), and when we found
>     a inline extent, set page uptodate in btrfs_do_readpage().

Added to misc-next, thanks.