Hello guys,
experimenting a little the 2.6 block device layer I detected under some
circumstances a net slowness in the disk throughput. Strangely enough, in fact,
my IDE disk reported a significant performance drop off in correspondence of
certain access patterns.
Following further investigations I was able to simulate this ill behavior in
the following piece of code, where for step values greater than 8, the disk
starts perform badly. The performance score result in fact far below its real
speed (~70MB/sec), as correctly measured instead in correspondence of low STEP
values (<8). In particular, clamping STEP=512 or beyond, the overall
performance scored by the disk is below 2MB/sec. Clearly I can't believe this
is due to the seek latency, right?
At first I thought to a side-effect of the queue plug/unplug mechanism: the
scattered accesses impose on each bio the unplug timeout. So, I added the
BIO_RW_SYNC flag that - AFAIK - forces the queue unplugging, hoping to solve
the performance slow down. Unfortunately nothing changes.
Now, as it is quite possible that I'm missing something, therefore the question
is: is there an effective way of doing scattered disk accesses? In other words,
how can I fix the following program in order to get disk full speed for steps >
8?
Tanks in advance!
Damon
Moreover, please find below several steps/scheduler combination performance, as
reported by the following hard-disk:
/dev/hda:
ATA device, with non-removable media
Model Number: Maxtor 6Y080P0
Firmware Revision: YAR41BW0
Standards:
Supported: 7 6 5 4
Likely used: 7
Configuration:
Logical max current
cylinders 16383 16383
heads 16 16
sectors/track 63 63
--
CHS current addressable sectors: 16514064
LBA user addressable sectors: 160086528
device size with M = 1024*1024: 78167 MBytes
device size with M = 1000*1000: 81964 MBytes (81 GB)
Capabilities:
LBA, IORDY(can be disabled)
Queue depth: 1
Standby timer values: spec'd by Standard, no device specific minimum
R/W multiple sector transfer: Max = 16 Current = 16
Advanced power management level: unknown setting (0x0000)
Recommended acoustic management value: 192, current value: 254
DMA: mdma0 mdma1 mdma2 udma0 udma1 udma2 udma3 udma4 *udma5 udma6
Cycle time: min=120ns recommended=120ns
PIO: pio0 pio1 pio2 pio3 pio4
Cycle time: no flow control=120ns IORDY flow control=120ns
ANTICIPATORY SCHEDULER
STEP (hs) CYCLES WRITTEN (MB) ELAPSED (s) SPEED (MB/s)
1 61954 242 3 75.432
2 59394 232 3 71.3032
3 16473 64 3 21.843
4 52482 205 3 62.3135
5 14448 56 3 18.1951
6 13617 53 3 17.1732
7 12849 50 3 16.1695
8 47874 187 3 56.2823
9 2569 10 3 3.468
10 2608 10 3 3.716
11 2416 9 3 2.3085
12 2576 10 3 3.468
13 2480 9 3 3.222
14 2424 9 3 2.3084
15 2616 10 3 3.738
16 2288 8 3 2.2619
32 2376 9 3 2.2849
64 2400 9 3 2.3059
128 2408 9 3 2.3098
256 1384 5 3 1.2104
512 1048 4 3 1.761
DEADLINE SCHEDULER
STEP (hs) CYCLES WRITTEN (MB) ELAPSED (s) SPEED (MB/s)
1 61955 242 3 75.736
2 59907 234 3 72.1307
3 16473 64 3 21.843
4 52994 207 3 63.1816
5 14330 55 3 18.1526
6 13569 53 3 17.1476
7 12817 50 3 16.1618
8 47618 186 3 56.1991
9 2625 10 3 3.734
10 2472 9 3 3.185
11 2512 9 3 3.371
12 2624 10 3 3.764
13 2392 9 3 2.3051
14 2472 9 3 2.3214
15 2664 10 3 3.863
16 2512 9 3 3.305
32 2448 9 3 3.10
64 2520 9 3 3.375
128 2417 9 3 2.3017
256 1305 5 3 1.1776
512 1160 4 3 1.1258
CFQ SCHEDULER
STEP (hs) CYCLES WRITTEN (MB) ELAPSED (s) SPEED (MB/s)
1 62850 245 3 76.1395
2 60416 236 3 73.940
3 15970 62 3 20.1902
4 53225 207 3 63.2719
5 14945 58 3 19.865
6 14250 55 3 18.1160
7 13682 53 3 17.1986
8 47870 186 3 56.2472
9 2529 9 3 3.170
10 2576 10 3 3.477
11 2472 9 3 3.44
12 2672 10 3 3.933
13 2481 9 3 3.256
14 2592 10 3 3.627
15 2512 9 3 3.386
16 2688 10 3 3.1008
32 2384 9 3 2.2996
64 2320 9 3 2.2734
128 2720 10 3 3.1130
256 1265 4 3 1.1664
512 1088 4 3 1.768
NOOP SCHEDULER
STEP (hs) CYCLES WRITTEN (MB) ELAPSED (s) SPEED (MB/s)
1 20987 81 3 27.413
2 19974 78 3 25.2373
3 16434 64 3 21.712
4 18541 72 3 23.2482
5 14217 55 3 18.1067
6 13625 53 3 17.1729
7 12489 48 3 16.337
8 48898 191 3 57.3135
9 2560 10 3 3.499
10 2568 10 3 3.332
11 2472 9 3 3.161
12 2568 10 3 3.371
13 2352 9 3 2.2875
14 2584 10 3 3.487
15 2320 9 3 2.2740
16 2544 9 3 3.481
32 2344 9 3 2.2832
64 2416 9 3 2.3069
128 2328 9 3 2.2649
256 1360 5 3 1.2010
512 1440 5 3 1.2190
--- empty 2006-09-05 00:16:24.000000000 +0200
+++ test.c 2006-09-05 00:16:49.000000000 +0200
@@ -0,0 +1,145 @@
+#include <linux/module.h>
+#include <linux/timer.h>
+#include <linux/bio.h>
+
+#define START(t) ({ \
+ struct timeval __tv; \
+ do_gettimeofday(&__tv); \
+ (t) = timeval_to_ns(&__tv); \
+ })
+
+#define STOP(t) ({ \
+ struct timeval __tv; \
+ do_gettimeofday(&__tv); \
+ (t) = timeval_to_ns(&__tv) - (t); \
+ })
+
+DECLARE_WAIT_QUEUE_HEAD(wait);
+atomic_t errors, busy;
+int halt;
+
+void stop_write(unsigned long arg)
+{
+ halt = 1;
+}
+
+int endio(struct bio *bio, unsigned int bytes_done, int error)
+{
+ if (bio->bi_size) {
+ return 1;
+ }
+
+ if (error || !test_bit(BIO_UPTODATE, &bio->bi_flags)) {
+ atomic_inc(&errors);
+ }
+
+ if (atomic_dec_and_test(&busy)) {
+ wake_up(&wait);
+ }
+
+ return 0;
+}
+
+int do_write(struct block_device *bdev,
+ struct page *zero, unsigned long expires, int step)
+{
+ DEFINE_TIMER(timer, stop_write, expires, (unsigned long) NULL);
+ int i;
+
+ add_timer(&timer);
+
+ for (halt = i = 0; !halt; i++) {
+ struct bio *bio = bio_alloc(GFP_NOIO, 1);
+ if (bio) {
+ atomic_inc(&busy);
+
+ bio->bi_bdev = bdev;
+ bio->bi_sector = step * i;
+ bio_add_page(bio, zero, PAGE_SIZE, 0);
+ bio->bi_end_io = endio;
+ submit_bio((1 << BIO_RW) | (1 << BIO_RW_SYNC), bio);
+ } else {
+ atomic_inc(&errors);
+ }
+ }
+
+ wait_event(wait, !atomic_read(&busy));
+
+ return i;
+}
+
+int write(struct block_device *bdev, int secs, int step)
+{
+ struct page *zero;
+
+ s64 time;
+ unsigned long space;
+ int cycles;
+
+ zero = alloc_page(GFP_KERNEL);
+ if (!zero) {
+ return -ENOMEM;
+ }
+
+ memset(kmap(zero), 0, PAGE_SIZE);
+ kunmap(zero);
+
+ atomic_set(&errors, 0);
+ atomic_set(&busy, 0);
+
+ START(time);
+
+ cycles = do_write(bdev, zero, jiffies + secs * HZ, step);
+
+ STOP(time);
+
+ put_page(zero);
+
+ (void) do_div(time, 1000000);
+
+ space = ((unsigned long) cycles * 1000 * (PAGE_SIZE >> 10)) >> 10;
+
+ printk("%d\t\t%d\t\t%lu\t\t%lu\t\t%lu.%-3lu\n",
+ step, cycles, space / 1000,
+ (unsigned long ) time / 1000,
+ space / (unsigned long) time,
+ space % (unsigned long) time);
+
+ return 0;
+}
+
+static int __init init(void)
+{
+ struct block_device *bdev;
+ int i, err;
+
+ bdev = open_bdev_excl("/dev/hda", 0, THIS_MODULE);
+ if (IS_ERR(bdev)) {
+ printk("device won't open!\n");
+ return PTR_ERR(bdev);
+ }
+
+ printk("STEP (hs)\tCYCLES\t\tWRITTEN (MB)\tELAPSED (s)\tSPEED (MB/s)\n");
+
+ for (i = 1; i < 16; i++) {
+ err = write(bdev, 3, i);
+ if (err < 0) {
+ printk("%d\t-\t\t-\t\t-\t\t-\n", i);
+ }
+ }
+
+ for (; i < 1024; i <<= 1) {
+ err = write(bdev, 3, i);
+ if (err < 0) {
+ printk("%d\t-\t\t-\t\t-\t\t-\n", i);
+ }
+ }
+
+ close_bdev_excl(bdev);
+
+ return -EIO;
+}
+
+module_init(init);
+
+MODULE_LICENSE("GPL v2");
--
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