linux-kernel - Re: [PATCH][SCSI]: Save some bytes in struct scsi

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Date:	Mon, 11 Dec 2006 20:52:22 -0700
From:	Matthew Wilcox <matthew@....cx>
To:	Arnaldo Carvalho de Melo <acme@...driva.com>
Cc:	"James E.J. Bottomley" <James.Bottomley@...elEye.com>,
	Andrew Morton <akpm@...l.org>, linux-scsi@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH][SCSI]: Save some bytes in struct scsi_target

On Tue, Dec 12, 2006 at 01:17:18AM -0200, Arnaldo Carvalho de Melo wrote:
> }; /* size: 368, cachelines: 12 */
> }; /* size: 364, cachelines: 12 */

Saving space is always good ;-)

> -	unsigned int		create:1; /* signal that it needs to be added */
> +	char			scsi_level;
> +	unsigned char		create:1; /* signal that it needs to be added */
>  	unsigned int		pdt_1f_for_no_lun;	/* PDT = 0x1f */
>  						/* means no lun present */
>  
> -	char			scsi_level;

However, pdt_1f_for_no_lun is really only one bit, saving another 4 bytes.

>  	struct execute_work	ew;
>  	enum scsi_target_state	state;

enums are a bit of a pain.  Even though scsi_target_state uses only two
values, it's represented as an int.  Unless you're on arm-eabi, when
it'll use less.  And even then, it won't use less than a byte, as it has
to be addressable.  I wonder if we can turn scsi_target_state into a
bit.  That'll save another 8 bytes total.
-
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