Re: btrfs metadata reclaim behavior/performance characteristics

[Nikolay Borisov <nborisov@suse.com>]

> From: Liu Bo <bo.li.liu@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.liu@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.