[PATCH] cfq: reorder cfq_queue removing padding on 64bit & allowing more objects/slab in it's kmem_cache

[PATCH] cfq: reorder cfq_queue removing padding on 64bit & allowing more objects/slab in it's kmem_cache

[Richard Kennedy]

This removes 8 bytes of padding from struct cfq_queue on 64 bit builds,
shrinking it's size to 256 bytes, so fitting into 1 fewer cachelines and
allowing 1 more object/slab in it's kmem_cache.
 

Signed-off-by: Richard Kennedy <richard@rsk.demon.co.uk>
----
patch against 2.6.33-rc8
tested on x86_64 AMDX2

regards
Richard


diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
index 023f4e6..db4c127 100644
--- a/block/cfq-iosched.c
+++ b/block/cfq-iosched.c
@@ -116,10 +116,10 @@ struct cfq_queue {
 	unsigned long dispatch_start;
 	unsigned int allocated_slice;
 	/* time when first request from queue completed and slice started. */
+	unsigned int slice_dispatch;
 	unsigned long slice_start;
 	unsigned long slice_end;
 	long slice_resid;
-	unsigned int slice_dispatch;
 
 	/* pending metadata requests */
 	int meta_pending;
@@ -130,13 +130,13 @@ struct cfq_queue {
 	unsigned short ioprio, org_ioprio;
 	unsigned short ioprio_class, org_ioprio_class;
 
+	pid_t pid;
+
 	unsigned int seek_samples;
 	u64 seek_total;
 	sector_t seek_mean;
 	sector_t last_request_pos;
 
-	pid_t pid;
-
 	struct cfq_rb_root *service_tree;
 	struct cfq_queue *new_cfqq;
 	struct cfq_group *cfqg;

Re: [PATCH] cfq: reorder cfq_queue removing padding on 64bit & allowing more objects/slab in it's kmem_cache

[Vivek Goyal]

On Mon, Feb 15, 2010 at 02:10:36PM +0000, Richard Kennedy wrote:
> This removes 8 bytes of padding from struct cfq_queue on 64 bit builds,
> shrinking it's size to 256 bytes, so fitting into 1 fewer cachelines and
> allowing 1 more object/slab in it's kmem_cache.
>  
> 
> Signed-off-by: Richard Kennedy <richard@rsk.demon.co.uk>
> ----
> patch against 2.6.33-rc8
> tested on x86_64 AMDX2
> 
> regards
> Richard
> 
> 
> diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
> index 023f4e6..db4c127 100644
> --- a/block/cfq-iosched.c
> +++ b/block/cfq-iosched.c
> @@ -116,10 +116,10 @@ struct cfq_queue {
>  	unsigned long dispatch_start;
>  	unsigned int allocated_slice;
>  	/* time when first request from queue completed and slice started. */
> +	unsigned int slice_dispatch;

Can you move it above the comment. The comment describes "slice_start" and
now comment and variable have been separated.

Vivek

>  	unsigned long slice_start;
>  	unsigned long slice_end;
>  	long slice_resid;
> -	unsigned int slice_dispatch;
>  
>  	/* pending metadata requests */
>  	int meta_pending;
> @@ -130,13 +130,13 @@ struct cfq_queue {
>  	unsigned short ioprio, org_ioprio;
>  	unsigned short ioprio_class, org_ioprio_class;
>  
> +	pid_t pid;
> +
>  	unsigned int seek_samples;
>  	u64 seek_total;
>  	sector_t seek_mean;
>  	sector_t last_request_pos;
>  
> -	pid_t pid;
> -
>  	struct cfq_rb_root *service_tree;
>  	struct cfq_queue *new_cfqq;
>  	struct cfq_group *cfqg;
> 

Re: [PATCH] cfq: reorder cfq_queue removing padding on 64bit & allowing more objects/slab in it's kmem_cache

[Richard Kennedy]

On Mon, 2010-02-15 at 09:16 -0500, Vivek Goyal wrote:
> On Mon, Feb 15, 2010 at 02:10:36PM +0000, Richard Kennedy wrote:
> > This removes 8 bytes of padding from struct cfq_queue on 64 bit builds,
> > shrinking it's size to 256 bytes, so fitting into 1 fewer cachelines and
> > allowing 1 more object/slab in it's kmem_cache.
> >  
> > 
> > Signed-off-by: Richard Kennedy <richard@rsk.demon.co.uk>
> > ----
> > patch against 2.6.33-rc8
> > tested on x86_64 AMDX2
> > 
> > regards
> > Richard
> > 
> > 
> > diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
> > index 023f4e6..db4c127 100644
> > --- a/block/cfq-iosched.c
> > +++ b/block/cfq-iosched.c
> > @@ -116,10 +116,10 @@ struct cfq_queue {
> >  	unsigned long dispatch_start;
> >  	unsigned int allocated_slice;
> >  	/* time when first request from queue completed and slice started. */
> > +	unsigned int slice_dispatch;
> 
> Can you move it above the comment. The comment describes "slice_start" and
> now comment and variable have been separated.
> 
> Vivek
Sure, no problem. I'll send out a new version.
Richard

Re: [PATCH v2] cfq: reorder cfq_queue removing padding on 64bit & allowing more objects/slab in it's kmem_cache

[Richard Kennedy]

This removes 8 bytes of padding from struct cfq_queue on 64 bit builds,
shrinking it's size to 256 bytes, so fitting into 1 fewer cachelines and
allowing 1 more object/slab in it's kmem_cache.
 

Signed-off-by: Richard Kennedy <richard@rsk.demon.co.uk>
----
patch against 2.6.33-rc8
tested on x86_64 AMDX2


diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
index 023f4e6..e3dedfd 100644
--- a/block/cfq-iosched.c
+++ b/block/cfq-iosched.c
@@ -115,11 +115,11 @@ struct cfq_queue {
 	/* time when queue got scheduled in to dispatch first request. */
 	unsigned long dispatch_start;
 	unsigned int allocated_slice;
+	unsigned int slice_dispatch;
 	/* time when first request from queue completed and slice started. */
 	unsigned long slice_start;
 	unsigned long slice_end;
 	long slice_resid;
-	unsigned int slice_dispatch;
 
 	/* pending metadata requests */
 	int meta_pending;
@@ -130,13 +130,13 @@ struct cfq_queue {
 	unsigned short ioprio, org_ioprio;
 	unsigned short ioprio_class, org_ioprio_class;
 
+	pid_t pid;
+
 	unsigned int seek_samples;
 	u64 seek_total;
 	sector_t seek_mean;
 	sector_t last_request_pos;
 
-	pid_t pid;
-
 	struct cfq_rb_root *service_tree;
 	struct cfq_queue *new_cfqq;
 	struct cfq_group *cfqg;

Re: [PATCH v2] cfq: reorder cfq_queue removing padding on 64bit & allowing more objects/slab in it's kmem_cache

[Jeff Moyer]

Richard Kennedy <richard@rsk.demon.co.uk> writes:

> This removes 8 bytes of padding from struct cfq_queue on 64 bit builds,
> shrinking it's size to 256 bytes, so fitting into 1 fewer cachelines and
> allowing 1 more object/slab in it's kmem_cache.

OK, I ran pahole to verify your findings:

$ pahole -C cfq_queue build/master/block/cfq-iosched.o 
struct cfq_queue {
...
         unsigned int               allocated_slice;      /*   136     4  */

         /* XXX 4 bytes hole, try to pack */

         long unsigned int          slice_start;          /*   144     8  */

...
        pid_t                      pid;                  /*   216     4   */

        /* XXX 4 bytes hole, try to pack */

        struct cfq_rb_root *       service_tree;         /*   224     8   */

...
        /* size: 264, cachelines: 5, members: 34 */
        /* sum members: 256, holes: 2, sum holes: 8 */
        /* last cacheline: 8 bytes */

After applying the patch, it indeed does save a cacheline.

Reviewed-by: Jeff Moyer <jmoyer@redhat.com>

Re: [PATCH v2] cfq: reorder cfq_queue removing padding on 64bit & allowing more objects/slab in it's kmem_cache

[Jens Axboe]

On Mon, Feb 15 2010, Richard Kennedy wrote:
> This removes 8 bytes of padding from struct cfq_queue on 64 bit builds,
> shrinking it's size to 256 bytes, so fitting into 1 fewer cachelines and
> allowing 1 more object/slab in it's kmem_cache.

Thanks, applied!

-- 
Jens Axboe