[PATCH] Btrfs: remove unused wait queue in struct extent_buffer

[PATCH] Btrfs: remove unused wait queue in struct extent_buffer

[Filipe David Borba Manana]

The lock_wq wait queue is not used anywhere, therefore just remove it.
On a x86_64 system, this reduced sizeof(struct extent_buffer) from 320
bytes down to 296 bytes, which means a 4Kb page can now be used for
13 extent buffers instead of 12.

Signed-off-by: Filipe David Borba Manana <fdmanana@gmail.com>
---
 fs/btrfs/extent_io.h | 1 -
 1 file changed, 1 deletion(-)

diff --git a/fs/btrfs/extent_io.h b/fs/btrfs/extent_io.h
index 8b63f2d..dbbea4f 100644
--- a/fs/btrfs/extent_io.h
+++ b/fs/btrfs/extent_io.h
@@ -158,7 +158,6 @@ struct extent_buffer {
 	 * to unlock
 	 */
 	wait_queue_head_t read_lock_wq;
-	wait_queue_head_t lock_wq;
 	struct page *pages[INLINE_EXTENT_BUFFER_PAGES];
 #ifdef CONFIG_BTRFS_DEBUG
 	struct list_head leak_list;
-- 
1.9.1

Re: [PATCH] Btrfs: remove unused wait queue in struct extent_buffer

[Satoru Takeuchi]

Hi Filipe,

(2014/06/16 21:14), Filipe David Borba Manana wrote:
> The lock_wq wait queue is not used anywhere, therefore just remove it.
> On a x86_64 system, this reduced sizeof(struct extent_buffer) from 320
> bytes down to 296 bytes, which means a 4Kb page can now be used for
> 13 extent buffers instead of 12.
> 
> Signed-off-by: Filipe David Borba Manana <fdmanana@gmail.com>

It looks good to me. This field was once removed by bd681513.
However, re-introduced in error by 727011e07c
(both are Chris's patch).

I consider it's better to fix the following comment too.

===
$ cat -n fs/btrfs/extent_io.h | grep -B 3 write_lock_wq
   152          /* readers use lock_wq while they wait for the write
   153           * lock holders to unlock
   154           */
   155          wait_queue_head_t write_lock_wq;
===

We should "s/ lock_wq / write_lock_wq /" here.

Thanks,
Satoru

> ---
>   fs/btrfs/extent_io.h | 1 -
>   1 file changed, 1 deletion(-)
> 
> diff --git a/fs/btrfs/extent_io.h b/fs/btrfs/extent_io.h
> index 8b63f2d..dbbea4f 100644
> --- a/fs/btrfs/extent_io.h
> +++ b/fs/btrfs/extent_io.h
> @@ -158,7 +158,6 @@ struct extent_buffer {
>   	 * to unlock
>   	 */
>   	wait_queue_head_t read_lock_wq;
> -	wait_queue_head_t lock_wq;
>   	struct page *pages[INLINE_EXTENT_BUFFER_PAGES];
>   #ifdef CONFIG_BTRFS_DEBUG
>   	struct list_head leak_list;
>