2005-12-15 15:51:30

by Benjamin LaHaise

Subject: [AIO] reorder kiocb structure elements to make sync iocb setup faster

The patch below reorders members of the kiocb structure to make sync kiocb
setup faster. By setting the elements sequentially, the write combining
buffers on the CPU are able to combine the writes into a single burst,
which results in fewer cache cycles being consumed, freeing them up for
other code. This results in a 10-20KB/s[*] increase on the bw_unix part
of LMbench on my test system.

-ben

* - The improvement varies based on what other patches are in the system,
as there are a number of bottlenecks, so this number is not absolutely
accurate.

Signed-off-by: Benjamin LaHaise <[email protected]>
diff --git a/include/linux/aio.h b/include/linux/aio.h
index 49fd376..00c8efa 100644
--- a/include/linux/aio.h
+++ b/include/linux/aio.h
@@ -94,26 +94,27 @@ struct kiocb {
ssize_t (*ki_retry)(struct kiocb *);
void (*ki_dtor)(struct kiocb *);

- struct list_head ki_list; /* the aio core uses this
- * for cancellation */
-
union {
void __user *user;
struct task_struct *tsk;
} ki_obj;
+
__u64 ki_user_data; /* user's data for completion */
+ wait_queue_t ki_wait;
loff_t ki_pos;
+
+ void *private;
/* State that we remember to be able to restart/retry */
unsigned short ki_opcode;
size_t ki_nbytes; /* copy of iocb->aio_nbytes */
char __user *ki_buf; /* remaining iocb->aio_buf */
size_t ki_left; /* remaining bytes */
- wait_queue_t ki_wait;
long ki_retried; /* just for testing */
long ki_kicked; /* just for testing */
long ki_queued; /* just for testing */

- void *private;
+ struct list_head ki_list; /* the aio core uses this
+ * for cancellation */
};

#define is_sync_kiocb(iocb) ((iocb)->ki_key == KIOCB_SYNC_KEY)
@@ -126,6 +127,7 @@ struct kiocb {
(x)->ki_filp = (filp); \
(x)->ki_ctx = NULL; \
(x)->ki_cancel = NULL; \
+ (x)->ki_retry = NULL; \
(x)->ki_dtor = NULL; \
(x)->ki_obj.tsk = tsk; \
(x)->ki_user_data = 0; \
--
"You know, I've seen some crystals do some pretty trippy shit, man."
Don't Email: <[email protected]>.

2005-12-15 16:43:30

by Zach Brown

Subject: Re: [AIO] reorder kiocb structure elements to make sync iocb setup faster

Benjamin LaHaise wrote:
> The patch below reorders members of the kiocb structure to make sync kiocb
> setup faster. By setting the elements sequentially, the write combining
> buffers on the CPU are able to combine the writes into a single burst,

Seems reasonable, but can we get comments in both the struct and where
it's initialized explaining this coordination?

- z