> From: Liu Bo <bo.li....@oracle.com>
>
> Subject: [PATCH] Btrfs: skip commit transaction if we don't have enough
> pinned bytes
>
> We commit transaction in order to reclaim space from pinned bytes because it
> could process delayed refs, and in may_commit_transaction(), we check first
> if pinned bytes are enough for the required space, we then check if that plus
> bytes reserved for delayed insert are enough for the required space.
>
> This changes the code to the above logic.
>
> Signed-off-by: Liu Bo <bo.li....@oracle.com>
> ---
> fs/btrfs/extent-tree.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
> index e390451c72e6..bded1ddd1bb6 100644
> --- a/fs/btrfs/extent-tree.c
> +++ b/fs/btrfs/extent-tree.c
> @@ -4837,7 +4837,7 @@ static int may_commit_transaction(struct btrfs_fs_info
> *fs_info,
>
> spin_lock(&delayed_rsv->lock);
> if (percpu_counter_compare(&space_info->total_bytes_pinned,
> - bytes - delayed_rsv->size) >= 0) {
> + bytes -
> delayed_rsv->size) < 0) {
>
> spin_unlock(&delayed_rsv->lock);
> return -ENOSPC;
> }
>
Your patch does make a very big difference. Here are a couple of runs of
slow-rm:
root@ubuntu-virtual:~# ./slow-rm.sh
Created 837 files before returning error, time taken 3
Created 920 files before returning error, time taken 3
Created 949 files before returning error, time taken 3
Created 930 files before returning error, time taken 3
Created 1101 files before returning error, time taken 4
Created 1082 files before returning error, time taken 4
Created 1608 files before returning error, time taken 5
Created 1735 files before returning error, time taken 5
rming took 1 seconds
root@ubuntu-virtual:~# ./slow-rm.sh
Created 801 files before returning error, time taken 3
Created 829 files before returning error, time taken 3
Created 983 files before returning error, time taken 3
Created 978 files before returning error, time taken 3
Created 1023 files before returning error, time taken 3
Created 1126 files before returning error, time taken 3
Created 1538 files before returning error, time taken 4
Created 1737 files before returning error, time taken 5
rming took 2 seconds
root@ubuntu-virtual:~# ./slow-rm.sh
Created 875 files before returning error, time taken 3
Created 891 files before returning error, time taken 3
Created 969 files before returning error, time taken 4
Created 1002 files before returning error, time taken 4
Created 1039 files before returning error, time taken 4
Created 1051 files before returning error, time taken 4
Created 1191 files before returning error, time taken 4
Created 2137 files before returning error, time taken 8
rming took 2 seconds
So rming is a lot faster, but we create less files all in all and get
ENOSPC earlier. This means that most of the time bytes_pinned is not
enough to satisfy the allocation hence we are hitting the second
percpu_counter comparison.
Also, the reason why the previous links showed 0 for bytes_pinned was
due to me having completely forgotten that bytes_pinned is a percpu
counter, hence my stap script wasn't actually reading it correctly.
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