Hi all,
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, clearly showing a non negligible hard-disk slow
down when the step value is set greater than 8. These result in fact far below
the hard-disk real speed (30~70MB/sec), as correctly measured instead in
correspondence of low STEP values (<8). In particular, with step of 512 or
above, the overall performance scored by the disk results below 2MB/sec.
At first I thought to a side-effect of the queue plug/unplug mechanism: the
scattered accesses involve the unplug timeout to each bio. So, I added the
BIO_RW_SYNC flag that - AFAIK - should force the queue
unplugging. Unfortunately nothing changes.
Now, as it is quite possible that I'm missing something, the question is: is
there an effective way of doing scattered disk accesses using bios? In other
words, how can I fix the following program in order to get disk full speed for
steps > 8?
TIA!
Damon
PS: please find below several results corresponding to various steps/scheduler
combinations, along with some configuration specs.
# hdparm -i /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
# uname -a
Linux 2.6.17.1 #2 SMP PREEMPT i686 Intel(R) Xeon(TM) CPU 2.80GHz GNU/Linux
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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