On Tue, Oct 09 2007 at 1:50 +0200, Matthew Wilcox <matthew@xxxxxx> wrote:
> Thanks to acme's pahole utility, I found some places where we can save
> a lot of bytes in scsi_cmnd, just by rearranging struct elements and
> reducing the size of some elements. We go from 272 to 260 bytes on x86
> and from 368 to 344 bytes on x86-64.
>
> - eh_eflags had a 4-byte hole after it on 64-bit. In fact, this has
> value 0 or 1, so reduce it to an unsigned char, and put it with the
> other chars in scsi_cmnd.
> - sc_data_direction has a value from 0-3, so make it an unsigned char
> rather than an enum. Saves at least 4 bytes.
> - Putting 'tag' with the other char elements saves another 4 bytes
> - Moving 'result' up from the end saves another 4 bytes on 64-bit
>
> Signed-off-by: Matthew Wilcox <willy@xxxxxxxxxxxxxxx>
>
> diff --git a/include/scsi/scsi_cmnd.h b/include/scsi/scsi_cmnd.h
> index 65ab514..4a8c15f 100644
> --- a/include/scsi/scsi_cmnd.h
> +++ b/include/scsi/scsi_cmnd.h
> @@ -15,12 +15,10 @@ struct scsi_device;
> /* embedded in scsi_cmnd */
> struct scsi_pointer {
> char *ptr; /* data pointer */
> - int this_residual; /* left in this buffer */
> struct scatterlist *buffer; /* which buffer */
> + dma_addr_t dma_handle;
> + int this_residual; /* left in this buffer */
> int buffers_residual; /* how many buffers left */
> -
> - dma_addr_t dma_handle;
> -
> volatile int Status;
> volatile int Message;
> volatile int have_data_in;
> @@ -32,7 +30,6 @@ struct scsi_cmnd {
> struct scsi_device *device;
> struct list_head list; /* scsi_cmnd participates in queue lists */
> struct list_head eh_entry; /* entry for the host eh_cmd_q */
> - int eh_eflags; /* Used by error handlr */
>
> /*
> * A SCSI Command is assigned a nonzero serial_number before passed
> @@ -54,9 +51,12 @@ struct scsi_cmnd {
> int retries;
> int allowed;
> int timeout_per_command;
> + int result; /* Status code from lower level driver */
>
> unsigned char cmd_len;
> - enum dma_data_direction sc_data_direction;
> + unsigned char eh_eflags; /* Used by error handler */
> + unsigned char sc_data_direction;
> + unsigned char tag; /* SCSI-II queued command tag */
>
> /* These elements define the operation we are about to perform */
> #define MAX_COMMAND_SIZE 16
> @@ -109,10 +109,6 @@ struct scsi_cmnd {
> * to release this memory. (The memory
> * obtained by scsi_malloc is guaranteed
> * to be at an address < 16Mb). */
> -
> - int result; /* Status code from lower level driver */
> -
> - unsigned char tag; /* SCSI-II queued command tag */
> };
>
> extern struct scsi_cmnd *scsi_get_command(struct scsi_device *, gfp_t);
>
Please don't do this patch for now. This is because of the
soon to come scsi_data_buffer patch that rearages most of the members above
and puts them in a substructure. Maybe after the scsi_data_buffer patch you can
try to do this again.
Thanks
Boaz
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