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SubjectRe: RFC on io-stalls patch
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Ok, I grabbed Jens' patch from yesterday and made one small change to
limit the total amount of io past blk_oversized_batch. The new patch is
attached as axboe-2l.diff.

We should probably drop the BH_Sync stuff, I need more time to make sure
it is worth while. Also, the patch needs to be changed to calculate
rlim once instead of in every get_request call. But I wanted to get
some numbers out.

Starting with my simple script yesterday to combine rio write loads and
tiobench (8 tiotest threads):

2.4.22-axboe2 was Jens' patch (adds q->pending[rw]), 2.4.22-axboe-2l is
from the patch attached.

Sequential Reads
File Blk Num Avg Maximum Lat% Lat% CPU
Identifier Size Size Thr Rate (CPU%) Latency Latency >2s >10s Eff
---------------- ------ ----- --- ------ ------ --------- ----------- -------- -------- -----
2.4.21 1792 4096 8 8.15 3.344% 3.564 9557.91 0.00022 0.00000 244
2.4.22-pre5 1792 4096 8 5.15 2.168% 5.964 2781.71 0.00022 0.00000 237
2.4.22-axboe2 1792 4096 8 5.07 2.119% 6.105 1095.73 0.00000 0.00000 239
2.4.22-axboe-2l 1792 4096 8 5.14 2.211% 5.998 970.74 0.00000 0.00000 232
Random Reads
File Blk Num Avg Maximum Lat% Lat% CPU
Identifier Size Size Thr Rate (CPU%) Latency Latency >2s >10s Eff
---------------- ------ ----- --- ------ ------ --------- ----------- -------- -------- -----
2.4.21 1792 4096 8 0.83 0.318% 28.295 529.56 0.00000 0.00000 260
2.4.22-pre5 1792 4096 8 0.53 0.203% 45.613 770.82 0.00000 0.00000 260
2.4.22-axboe2 1792 4096 8 0.66 0.503% 41.791 637.23 0.00000 0.00000 130
2.4.22-axboe-2l 1792 4096 8 0.64 0.163% 46.923 742.38 0.00000 0.00000 391
Sequential Writes
File Blk Num Avg Maximum Lat% Lat% CPU
Identifier Size Size Thr Rate (CPU%) Latency Latency >2s >10s Eff
---------------- ------ ----- --- ------ ------ --------- ----------- -------- -------- -----
2.4.21 1792 4096 8 15.51 11.40% 1.243 11776.20 0.01460 0.00109 136
2.4.22-pre5 1792 4096 8 11.99 9.960% 1.695 4628.58 0.00371 0.00000 120
2.4.22-axboe2 1792 4096 8 11.75 9.324% 1.755 2406.28 0.00065 0.00000 126
2.4.22-axboe-2l 1792 4096 8 11.73 9.320% 1.752 2924.61 0.00109 0.00000 126
Random Writes
File Blk Num Avg Maximum Lat% Lat% CPU
Identifier Size Size Thr Rate (CPU%) Latency Latency >2s >10s Eff
---------------- ------ ----- --- ------ ------ --------- ----------- -------- -------- -----
2.4.21 1792 4096 8 0.41 0.318% 0.033 18.81 0.00000 0.00000 130
2.4.22-pre5 1792 4096 8 0.57 1.234% 0.041 37.63 0.00000 0.00000 46
2.4.22-axboe2 1792 4096 8 0.64 1.435% 0.040 29.32 0.00000 0.00000 45
2.4.22-axboe-2l 1792 4096 8 0.64 1.355% 0.033 20.79 0.00000 0.00000 47
So the max sequential read and write latencies were helped by
elevator-low-latency while the averages were not. A good std deviation
setup would help here, but tiobench's percentage setup shows some
numbers. In practice, those max latencies are visible for interactive
loads.

Why did max sequential write latencies go down with -axboe? my guess is
that we read the ext2 indirect blocks faster, but that's only a guess.

For a little variety, I measured some read latencies in the face of a
dbench 50 workload (after adding some latency stats to rio). These were
done on scsi because my IDE drive wasn't big enough. You're looking at
the last line from dbench (worthless) and the read throughput/latency
stats from rio (rio was killed right after dbench finished).

2.4.21 Throughput 100.617 MB/sec (NB=125.771 MB/sec 1006.17 MBit/sec)
2.4.22-pre5 Throughput 111.563 MB/sec (NB=139.454 MB/sec 1115.63 MBit/sec)
2.4.21-axboe-2l Throughput 126.298 MB/sec (NB=157.873 MB/sec 1262.98 MBit/sec)
2.4.21 reads: 34.160 MB, 0.512 MB/s
2.4.22-pre5 reads: 34.254 MB, 0.569 MB/s
2.4.21-axboe-2l reads: 47.733 MB, 0.896 MB/s
2.4.21 read latency min 0.00 avg 7.54 max 5742.93 > 2s 0.00% > 10s 0.00%
2.4.22-pre5 read latency min 0.00 avg 6.74 max 7225.02 > 2s 0.00% > 10s 0.00%
2.4.21-axboe-2l read latency min 0.00 avg 4.32 max 2128.92 > 2s 0.00% > 10s 0.00%
Both the dbench throughput numbers and the rio stats were fairly stable
across runs. As the number of dbench procs increases vanilla 2.4.21
does worse on latency.

-chris

===== drivers/block/ll_rw_blk.c 1.47 vs edited =====
--- 1.47/drivers/block/ll_rw_blk.c Fri Jul 11 04:30:54 2003
+++ edited/drivers/block/ll_rw_blk.c Wed Jul 16 12:27:24 2003
@@ -458,6 +458,7 @@

INIT_LIST_HEAD(&q->rq.free);
q->rq.count = 0;
+ q->rq.pending[READ] = q->rq.pending[WRITE] = 0;
q->nr_requests = 0;

si_meminfo(&si);
@@ -549,18 +550,35 @@
static struct request *get_request(request_queue_t *q, int rw)
{
struct request *rq = NULL;
- struct request_list *rl;
+ struct request_list *rl = &q->rq;

- rl = &q->rq;
- if (!list_empty(&rl->free) && !blk_oversized_queue(q)) {
+ if (blk_oversized_queue(q)) {
+ int rlim = q->nr_requests >> 5;
+
+ if (rlim < 4)
+ rlim = 4;
+
+ /*
+ * if its a write, or we have more than a handful of reads
+ * pending, bail out
+ */
+ if ((rw == WRITE) || (rw == READ && rl->pending[READ] > rlim))
+ return NULL;
+ if (blk_oversized_queue_reads(q))
+ return NULL;
+ }
+
+ if (!list_empty(&rl->free)) {
rq = blkdev_free_rq(&rl->free);
list_del(&rq->queue);
rl->count--;
+ rl->pending[rw]++;
rq->rq_status = RQ_ACTIVE;
rq->cmd = rw;
rq->special = NULL;
rq->q = q;
}
+
return rq;
}

@@ -866,13 +884,25 @@
* assume it has free buffers and check waiters
*/
if (q) {
+ struct request_list *rl = &q->rq;
int oversized_batch = 0;

if (q->can_throttle)
oversized_batch = blk_oversized_queue_batch(q);
- q->rq.count++;
- list_add(&req->queue, &q->rq.free);
- if (q->rq.count >= q->batch_requests && !oversized_batch) {
+ rl->count++;
+ /*
+ * paranoia check
+ */
+ if (req->cmd == READ || req->cmd == WRITE)
+ rl->pending[req->cmd]--;
+ if (rl->pending[READ] > q->nr_requests)
+ printk("reads: %u\n", rl->pending[READ]);
+ if (rl->pending[WRITE] > q->nr_requests)
+ printk("writes: %u\n", rl->pending[WRITE]);
+ if (rl->pending[READ] + rl->pending[WRITE] > q->nr_requests)
+ printk("too many: %u + %u > %u\n", rl->pending[READ], rl->pending[WRITE], q->nr_requests);
+ list_add(&req->queue, &rl->free);
+ if (rl->count >= q->batch_requests && !oversized_batch) {
smp_mb();
if (waitqueue_active(&q->wait_for_requests))
wake_up(&q->wait_for_requests);
@@ -947,7 +977,7 @@
static int __make_request(request_queue_t * q, int rw,
struct buffer_head * bh)
{
- unsigned int sector, count;
+ unsigned int sector, count, sync;
int max_segments = MAX_SEGMENTS;
struct request * req, *freereq = NULL;
int rw_ahead, max_sectors, el_ret;
@@ -958,6 +988,7 @@

count = bh->b_size >> 9;
sector = bh->b_rsector;
+ sync = test_and_clear_bit(BH_Sync, &bh->b_state);

rw_ahead = 0; /* normal case; gets changed below for READA */
switch (rw) {
@@ -1124,6 +1155,8 @@
blkdev_release_request(freereq);
if (should_wake)
get_request_wait_wakeup(q, rw);
+ if (sync)
+ __generic_unplug_device(q);
spin_unlock_irq(&io_request_lock);
return 0;
end_io:
===== fs/buffer.c 1.89 vs edited =====
--- 1.89/fs/buffer.c Tue Jun 24 17:11:29 2003
+++ edited/fs/buffer.c Tue Jul 15 15:31:07 2003
@@ -1142,6 +1142,7 @@
bh = getblk(dev, block, size);
if (buffer_uptodate(bh))
return bh;
+ set_bit(BH_Sync, &bh->b_state);
ll_rw_block(READ, 1, &bh);
wait_on_buffer(bh);
if (buffer_uptodate(bh))
===== include/linux/blkdev.h 1.24 vs edited =====
--- 1.24/include/linux/blkdev.h Fri Jul 4 13:18:12 2003
+++ edited/include/linux/blkdev.h Tue Jul 15 16:32:32 2003
@@ -66,6 +66,7 @@

struct request_list {
unsigned int count;
+ unsigned int pending[2];
struct list_head free;
};

@@ -292,6 +293,13 @@
{
if (q->can_throttle)
return atomic_read(&q->nr_sectors) > q->max_queue_sectors;
+ return q->rq.count == 0;
+}
+
+static inline int blk_oversized_queue_reads(request_queue_t * q)
+{
+ if (q->can_throttle)
+ return atomic_read(&q->nr_sectors) > q->max_queue_sectors + q->batch_sectors;
return q->rq.count == 0;
}

===== include/linux/fs.h 1.82 vs edited =====
--- 1.82/include/linux/fs.h Wed Jul 9 14:42:38 2003
+++ edited/include/linux/fs.h Tue Jul 15 15:31:07 2003
@@ -221,6 +221,7 @@
BH_Launder, /* 1 if we can throttle on this buffer */
BH_Attached, /* 1 if b_inode_buffers is linked into a list */
BH_JBD, /* 1 if it has an attached journal_head */
+ BH_Sync, /* 1 if the buffer is a sync read */

BH_PrivateStart,/* not a state bit, but the first bit available
* for private allocation by other entities
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