Re: [RFC] Scheduler issue 1, RT tasks ...

Andrew Morton
2001-12-29T21:04:00Z

Dieter Nützel wrote:
>
> I ask because my MP3/Ogg-Vorbis hiccup during dbench isn't solved anyway.
> Running 2.4.17 + preempt + lock-break + 10_vm-21 (AA).
> Some wisdom?

Please test this elevator patch. I'll be putting it out more formally
in a day or two. Much more testing is needed yet, but for me, the
time to read a 16 megabyte file whilst running dbench 160 falls from
three minutes thirty seconds to seven seconds. (This is a VM thing,
not an elevator thing).

--- linux-2.4.18-pre1/drivers/block/elevator.c Thu Jul 19 20:59:41 2001
+++ linux-akpm/drivers/block/elevator.c Sat Dec 29 00:52:05 2001
@@ -82,6 +82,7 @@ int elevator_linus_merge(request_queue_t
{
struct list_head *entry = &q->queue_head;
unsigned int count = bh->b_size >> 9, ret = ELEVATOR_NO_MERGE;
+ const int max_bomb_segments = q->elevator.max_bomb_segments;

while ((entry = entry->prev) != head) {
struct request *__rq = blkdev_entry_to_request(entry);
@@ -116,6 +117,56 @@ int elevator_linus_merge(request_queue_t
}
}

+ /*
+ * If we failed to merge a read anywhere in the request
+ * queue, we really don't want to place it at the end
+ * of the list, behind lots of writes. So place it near
+ * the front.
+ *
+ * We don't want to place it in front of _all_ writes: that
+ * would create lots of seeking, and isn't tunable.
+ * We try to avoid promoting this read in front of existing
+ * reads.
+ *
+ * max_bomb_sectors becomes the maximum number of write
+ * requests which we allow to remain in place in front of
+ * a newly introduced read. We weight things a little bit,
+ * so large writes are more expensive than small ones, but it's
+ * requests which count, not sectors.
+ */
+ if (max_bomb_segments && rw == READ && ret == ELEVATOR_NO_MERGE) {
+ int cur_latency = 0;
+ struct request * const cur_request = *req;
+
+ entry = head->next;
+ while (entry != &q->queue_head) {
+ struct request *__rq;
+
+ if (entry == &q->queue_head)
+ BUG();
+ if (entry == q->queue_head.next &&
+ q->head_active && !q->plugged)
+ BUG();
+ __rq = blkdev_entry_to_request(entry);
+
+ if (__rq == cur_request) {
+ /*
+ * This is where the old algorithm placed it.
+ * There's no point pushing it further back,
+ * so leave it here, in sorted order.
+ */
+ break;
+ }
+ if (__rq->cmd == WRITE) {
+ cur_latency += 1 + __rq->nr_sectors / 64;
+ if (cur_latency >= max_bomb_segments) {
+ *req = __rq;
+ break;
+ }
+ }
+ entry = entry->next;
+ }
+ }
return ret;
}

@@ -188,7 +239,7 @@ int blkelvget_ioctl(elevator_t * elevato
output.queue_ID = elevator->queue_ID;
output.read_latency = elevator->read_latency;
output.write_latency = elevator->write_latency;
- output.max_bomb_segments = 0;
+ output.max_bomb_segments = elevator->max_bomb_segments;

if (copy_to_user(arg, &output, sizeof(blkelv_ioctl_arg_t)))
return -EFAULT;
@@ -207,9 +258,12 @@ int blkelvset_ioctl(elevator_t * elevato
return -EINVAL;
if (input.write_latency < 0)
return -EINVAL;
+ if (input.max_bomb_segments < 0)
+ return -EINVAL;

elevator->read_latency = input.read_latency;
elevator->write_latency = input.write_latency;
+ elevator->max_bomb_segments = input.max_bomb_segments;
return 0;
}

--- linux-2.4.18-pre1/include/linux/elevator.h Thu Feb 15 16:58:34 2001
+++ linux-akpm/include/linux/elevator.h Sat Dec 29 12:57:33 2001
@@ -3,10 +3,11 @@

typedef void (elevator_fn) (struct request *, elevator_t *,
struct list_head *,
- struct list_head *, int);
+ struct list_head *);

-typedef int (elevator_merge_fn) (request_queue_t *, struct request **, struct list_head *,
- struct buffer_head *, int, int);
+typedef int (elevator_merge_fn)(request_queue_t *, struct request **,
+ struct list_head *, struct buffer_head *bh,
+ int rw, int max_sectors);

typedef void (elevator_merge_cleanup_fn) (request_queue_t *, struct request *, int);

@@ -16,6 +17,7 @@ struct elevator_s
{
int read_latency;
int write_latency;
+ int max_bomb_segments;

elevator_merge_fn *elevator_merge_fn;
elevator_merge_cleanup_fn *elevator_merge_cleanup_fn;
@@ -24,13 +26,13 @@ struct elevator_s
unsigned int queue_ID;
};

-int elevator_noop_merge(request_queue_t *, struct request **, struct list_head *, struct buffer_head *, int, int);
-void elevator_noop_merge_cleanup(request_queue_t *, struct request *, int);
-void elevator_noop_merge_req(struct request *, struct request *);
-
-int elevator_linus_merge(request_queue_t *, struct request **, struct list_head *, struct buffer_head *, int, int);
-void elevator_linus_merge_cleanup(request_queue_t *, struct request *, int);
-void elevator_linus_merge_req(struct request *, struct request *);
+elevator_merge_fn elevator_noop_merge;
+elevator_merge_cleanup_fn elevator_noop_merge_cleanup;
+elevator_merge_req_fn elevator_noop_merge_req;
+
+elevator_merge_fn elevator_linus_merge;
+elevator_merge_cleanup_fn elevator_linus_merge_cleanup;
+elevator_merge_req_fn elevator_linus_merge_req;

typedef struct blkelv_ioctl_arg_s {
int queue_ID;
@@ -54,22 +56,6 @@ extern void elevator_init(elevator_t *,
#define ELEVATOR_FRONT_MERGE 1
#define ELEVATOR_BACK_MERGE 2

-/*
- * This is used in the elevator algorithm. We don't prioritise reads
- * over writes any more --- although reads are more time-critical than
- * writes, by treating them equally we increase filesystem throughput.
- * This turns out to give better overall performance. -- sct
- */
-#define IN_ORDER(s1,s2) \
- ((((s1)->rq_dev == (s2)->rq_dev && \
- (s1)->sector < (s2)->sector)) || \
- (s1)->rq_dev < (s2)->rq_dev)
-
-#define BHRQ_IN_ORDER(bh, rq) \
- ((((bh)->b_rdev == (rq)->rq_dev && \
- (bh)->b_rsector < (rq)->sector)) || \
- (bh)->b_rdev < (rq)->rq_dev)
-
static inline int elevator_request_latency(elevator_t * elevator, int rw)
{
int latency;
@@ -85,7 +71,7 @@ static inline int elevator_request_laten
((elevator_t) { \
0, /* read_latency */ \
0, /* write_latency */ \
- \
+ 0, /* max_bomb_segments */ \
elevator_noop_merge, /* elevator_merge_fn */ \
elevator_noop_merge_cleanup, /* elevator_merge_cleanup_fn */ \
elevator_noop_merge_req, /* elevator_merge_req_fn */ \
@@ -95,7 +81,7 @@ static inline int elevator_request_laten
((elevator_t) { \
8192, /* read passovers */ \
16384, /* write passovers */ \
- \
+ 6, /* max_bomb_segments */ \
elevator_linus_merge, /* elevator_merge_fn */ \
elevator_linus_merge_cleanup, /* elevator_merge_cleanup_fn */ \
elevator_linus_merge_req, /* elevator_merge_req_fn */ \