[PATCH] Improve CD/DVD packet driver write performance

This patch improves write performance for the CD/DVD packet writing driver.
 The logic for switching between reading and writing has been changed so
that streaming writes are no longer interrupted by read requests.

Signed-off-by: Peter Osterlund <petero2@telia.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 20 insertions, 16 deletions

diff --git a/drivers/block/pktcdvd.c b/drivers/block/pktcdvd.c
index bc56770bcc90..7f3d78de265c 100644
--- a/drivers/block/pktcdvd.c
+++ b/drivers/block/pktcdvd.c
@@ -467,14 +467,12 @@ static int pkt_set_speed(struct pktcdvd_device *pd, unsigned write_speed, unsign
  * Queue a bio for processing by the low-level CD device. Must be called
  * from process context.
  */
-static void pkt_queue_bio(struct pktcdvd_device *pd, struct bio *bio, int high_prio_read)
+static void pkt_queue_bio(struct pktcdvd_device *pd, struct bio *bio)
 {
         spin_lock(&pd->iosched.lock);
         if (bio_data_dir(bio) == READ) {
                 pkt_add_list_last(bio, &pd->iosched.read_queue,
                                   &pd->iosched.read_queue_tail);
-                if (high_prio_read)
-                        pd->iosched.high_prio_read = 1;
         } else {
                 pkt_add_list_last(bio, &pd->iosched.write_queue,
                                   &pd->iosched.write_queue_tail);
@@ -490,15 +488,16 @@ static void pkt_queue_bio(struct pktcdvd_device *pd, struct bio *bio, int high_p
  * requirements for CDRW drives:
  * - A cache flush command must be inserted before a read request if the
  *   previous request was a write.
- * - Switching between reading and writing is slow, so don't it more often
+ * - Switching between reading and writing is slow, so don't do it more often
  *   than necessary.
+ * - Optimize for throughput at the expense of latency. This means that streaming
+ *   writes will never be interrupted by a read, but if the drive has to seek
+ *   before the next write, switch to reading instead if there are any pending
+ *   read requests.
  * - Set the read speed according to current usage pattern. When only reading
  *   from the device, it's best to use the highest possible read speed, but
  *   when switching often between reading and writing, it's better to have the
  *   same read and write speeds.
- * - Reads originating from user space should have higher priority than reads
- *   originating from pkt_gather_data, because some process is usually waiting
- *   on reads of the first kind.
  */
 static void pkt_iosched_process_queue(struct pktcdvd_device *pd)
 {
@@ -512,21 +511,24 @@ static void pkt_iosched_process_queue(struct pktcdvd_device *pd)
 
         for (;;) {
                 struct bio *bio;
-                int reads_queued, writes_queued, high_prio_read;
+                int reads_queued, writes_queued;
 
                 spin_lock(&pd->iosched.lock);
                 reads_queued = (pd->iosched.read_queue != NULL);
                 writes_queued = (pd->iosched.write_queue != NULL);
-                if (!reads_queued)
-                        pd->iosched.high_prio_read = 0;
-                high_prio_read = pd->iosched.high_prio_read;
                 spin_unlock(&pd->iosched.lock);
 
                 if (!reads_queued && !writes_queued)
                         break;
 
                 if (pd->iosched.writing) {
-                        if (high_prio_read || (!writes_queued && reads_queued)) {
+                        int need_write_seek = 1;
+                        spin_lock(&pd->iosched.lock);
+                        bio = pd->iosched.write_queue;
+                        spin_unlock(&pd->iosched.lock);
+                        if (bio && (bio->bi_sector == pd->iosched.last_write))
+                                need_write_seek = 0;
+                        if (need_write_seek && reads_queued) {
                                 if (atomic_read(&pd->cdrw.pending_bios) > 0) {
                                         VPRINTK("pktcdvd: write, waiting\n");
                                         break;
@@ -559,8 +561,10 @@ static void pkt_iosched_process_queue(struct pktcdvd_device *pd)
 
                 if (bio_data_dir(bio) == READ)
                         pd->iosched.successive_reads += bio->bi_size >> 10;
-                else
+                else {
                         pd->iosched.successive_reads = 0;
+                        pd->iosched.last_write = bio->bi_sector + bio_sectors(bio);
+                }
                 if (pd->iosched.successive_reads >= HI_SPEED_SWITCH) {
                         if (pd->read_speed == pd->write_speed) {
                                 pd->read_speed = MAX_SPEED;
@@ -765,7 +769,7 @@ static void pkt_gather_data(struct pktcdvd_device *pd, struct packet_data *pkt)
 
                 atomic_inc(&pkt->io_wait);
                 bio->bi_rw = READ;
-                pkt_queue_bio(pd, bio, 0);
+                pkt_queue_bio(pd, bio);
                 frames_read++;
         }
 
@@ -1062,7 +1066,7 @@ static void pkt_start_write(struct pktcdvd_device *pd, struct packet_data *pkt)
 
         atomic_set(&pkt->io_wait, 1);
         pkt->w_bio->bi_rw = WRITE;
-        pkt_queue_bio(pd, pkt->w_bio, 0);
+        pkt_queue_bio(pd, pkt->w_bio);
 }
 
 static void pkt_finish_packet(struct packet_data *pkt, int uptodate)
@@ -2120,7 +2124,7 @@ static int pkt_make_request(request_queue_t *q, struct bio *bio)
                 cloned_bio->bi_private = psd;
                 cloned_bio->bi_end_io = pkt_end_io_read_cloned;
                 pd->stats.secs_r += bio->bi_size >> 9;
-                pkt_queue_bio(pd, cloned_bio, 1);
+                pkt_queue_bio(pd, cloned_bio);
                 return 0;
         }