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

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Message-ID: <x49eikmph5t.fsf@segfault.boston.devel.redhat.com>
Date:	Mon, 15 Feb 2010 12:01:18 -0500
From:	Jeff Moyer <jmoyer@...hat.com>
To:	Richard Kennedy <richard@....demon.co.uk>
Cc:	Vivek Goyal <vgoyal@...hat.com>,
	Jens Axboe <jens.axboe@...cle.com>,
	Corrado Zoccolo <czoccolo@...il.com>,
	Gui Jianfeng <guijianfeng@...fujitsu.com>,
	lkml <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2]  cfq: reorder cfq_queue removing padding on 64bit & allowing more objects/slab in it's kmem_cache

Richard Kennedy <richard@....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@...hat.com>
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