[PATCH v2] fs: shave 8 bytes off of struct inode

[Amir Goldstein <amir73il@gmail.com>]

Here is a link to Linus' reply to Jan's concern about making
i_blkbibts byte addressable:
https://marc.info/?l=linux-fsdevel&m=152882624707975&w=2

Here is a link to an lkp.org report about potential performance
improvement in some workload, which could(?) be related to packing
i_blkbits closer to i_bytes/i_lock:
https://marc.info/?l=linux-fsdevel&m=153077048108198&w=2

Changes since v1:
- Add links to relevant discussions

Signed-off-by: Amir Goldstein <amir73il@gmail.com>
---

Al,

Re-posting the patch per your request.

Regarding your question:
"I would like more details about the variation of timing -
 what's the dispersion from boot to boot, for starters?"

I haven't run those performance tests, just got them from lkp robot,
so perhaps Xiaolong can answer your question.
I too, find the reported improvement a bit too good to be reliably
true. I do see in the graphs at the bottom of the report that there
are many good vs. bad samples though.

Thanks,
Amir.

 include/linux/fs.h | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/include/linux/fs.h b/include/linux/fs.h
index 760d8da1b6c7..6d0489613dc1 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -274,6 +274,7 @@ struct writeback_control;
 
 /*
  * Write life time hint values.
+ * Stored in struct inode as u8.
  */
 enum rw_hint {
 	WRITE_LIFE_NOT_SET	= 0,
@@ -607,8 +608,8 @@ struct inode {
 	struct timespec		i_ctime;
 	spinlock_t		i_lock;	/* i_blocks, i_bytes, maybe i_size */
 	unsigned short          i_bytes;
-	unsigned int		i_blkbits;
-	enum rw_hint		i_write_hint;
+	u8			i_blkbits;
+	u8			i_write_hint;
 	blkcnt_t		i_blocks;
 
 #ifdef __NEED_I_SIZE_ORDERED
-- 
2.7.4