Re: [PATCH 20/21] btrfs: stop doing excessive space reservation for csum deletion

[Josef Bacik <josef@toxicpanda.com>]

On Fri, Sep 08, 2023 at 01:09:22PM +0100, fdmanana@kernel.org wrote:
> From: Filipe Manana <fdmanana@suse.com>
> 
> Currently when reserving space for deleting the csum items for a data
> extent, when adding or updating a delayed ref head, we determine how
> many leaves of csum items we can have and then pass that number to the
> helper btrfs_calc_delayed_ref_bytes(). This helper is used for calculating
> space for all tree modifications we need when running delayed references,
> however the amount of space it computes is excessive for deleting csum
> items because:
> 
> 1) It uses btrfs_calc_insert_metadata_size() which is excessive because
>    we only need to delete csum items from the csum tree, we don't need
>    to insert any items, so btrfs_calc_metadata_size() is all we need (as
>    it computes space needed to delete an item);
> 
> 2) If the free space tree is enabled, it doubles the amount of space,
>    which is pointless for csum deletion since we don't need to touch the
>    free space tree or any other tree other than the csum tree.
> 
> So improve on this by tracking how many csum deletions we have and using
> a new helper to calculate space for csum deletions (just a wrapper around
> btrfs_calc_metadata_size() with a comment). This reduces the amount of
> space we need to reserve for csum deletions by a factor of 4, and it helps
> reduce the number of times we have to block space reservations and have
> the reclaim task enter the space flushing algorihm (flush delayed items,
> flush delayed refs, etc) in order to satisfy tickets.
> 
> For example this results in a total time decrease when unlinking (or
> truncating) files with many extents, as we end up having to block on space
> metadata reservations less often. Example test:
> 
>   $ cat test.sh
>   #!/bin/bash
> 
>   DEV=/dev/nullb0
>   MNT=/mnt/test
> 
>   umount $DEV &> /dev/null
>   mkfs.btrfs -f $DEV
>   # Use compression to quickly create files with a lot of extents
>   # (each with a size of 128K).
>   mount -o compress=lzo $DEV $MNT
> 
>   # 100G gives at least 983040 extents with a size of 128K.
>   xfs_io -f -c "pwrite -S 0xab -b 1M 0 120G" $MNT/foobar
> 
>   # Flush all delalloc and clear all metadata from memory.
>   umount $MNT
>   mount -o compress=lzo $DEV $MNT
> 
>   start=$(date +%s%N)
>   rm -f $MNT/foobar
>   end=$(date +%s%N)
>   dur=$(( (end - start) / 1000000 ))
>   echo "rm took $dur milliseconds"
> 
>   umount $MNT
> 
> Before this change rm took: 7504 milliseconds
> After this change rm took:  6574 milliseconds  (-12.4%)
> 
> Signed-off-by: Filipe Manana <fdmanana@suse.com>

Reviewed-by: Josef Bacik <josef@toxicpanda.com>

Thanks,

Josef