On Wed, Jun 10, 2009 at 02:37:46PM +0800, Wu Fengguang wrote:
Bug-Entry : http://bugzilla.kernel.org/show_bug.cgi?id=13463
Subject : Poor SSD performance
Submitter : Jake <ellowitz@xxxxxxxxxxxx>
Date : 2009-06-05 17:37 (3 days old)
Hi Jake,
Could you collect some blktrace data for the dd commands on new/old
kernels?
dd if=/dev/sda of=/dev/null bs=1M count=1024 iflag=direct
dd if=/dev/sda of=/dev/null bs=1M count=1024
I managed to get a SanDisk SSD for testing, and observes that
- one must increase read_ahead_kb to at least max_sectors_kb or better
"bs=1M" to make a fair comparison
- with increased readahead size, the dd reported throughputs are
75MB/s vs 77MB/s, while the blktrace reported throughputs are
75MB/s vs 75MB/s (buffered IO vs direct IO).
Here are details.
The dd throughputs are equal for rotational hard disks, but differs
for this SanDisk SSD (with default RA parameters):
% dd if=/dev/sda of=/dev/null iflag=direct bs=1M count=1024
1073741824 bytes (1.1 GB) copied, 13.905 s, 77.2 MB/s
1073741824 bytes (1.1 GB) copied, 13.9029 s, 77.2 MB/s
% dd if=/dev/sda of=/dev/null bs=1M count=1024
1073741824 bytes (1.1 GB) copied, 14.7294 s, 72.9 MB/s
1073741824 bytes (1.1 GB) copied, 14.8647 s, 72.2 MB/s
Here is the blktrace summary:
dd dd-direct
--------------------------------------------------------------------------------------
CPU0 (sda): | CPU0 (sda):
Reads Queued: 9,888, 39,552KiB | Reads Queued: 84, 43,008KiB
Read Dispatches: 302, 38,588KiB | Read Dispatches: 84, 43,008KiB
Reads Requeued: 0 | Reads Requeued: 0
Reads Completed: 337, 44,600KiB | Reads Completed: 83, 42,496KiB
Read Merges: 9,574, 38,296KiB | Read Merges: 0, 0KiB
Read depth: 2 | Read depth: 2
IO unplugs: 313 | IO unplugs: 42
CPU1 (sda): | CPU1 (sda):
Reads Queued: 11,840, 47,360KiB | Reads Queued: 96, 49,152KiB
Read Dispatches: 372, 48,196KiB | Read Dispatches: 96, 49,152KiB
Reads Requeued: 0 | Reads Requeued: 0
Reads Completed: 337, 42,312KiB | Reads Completed: 96, 49,152KiB
Read Merges: 11,479, 45,916KiB | Read Merges: 0, 0KiB
Read depth: 2 | Read depth: 2
IO unplugs: 372 | IO unplugs: 48
|
Total (sda): | Total (sda):
Reads Queued: 21,728, 86,912KiB | Reads Queued: 180, 92,160KiB
Read Dispatches: 674, 86,784KiB | Read Dispatches: 180, 92,160KiB
Reads Requeued: 0 | Reads Requeued: 0
Reads Completed: 674, 86,912KiB | Reads Completed: 179, 91,648KiB
Read Merges: 21,053, 84,212KiB | Read Merges: 0, 0KiB
IO unplugs: 685 | IO unplugs: 90
|
Throughput (R/W): 69,977KiB/s / 0KiB/s | Throughput (R/W): 75,368KiB/s / 0KiB/s
Events (sda): 46,804 entries | Events (sda): 1,158 entries
Another obvious difference is IO size.
One is read_ahead_kb=128K, another is max_sectors_kb=512K:
dd:
8,0 0 13497 0.804939305 0 C R 782592 + 256 [0]
8,0 0 13498 0.806713692 0 C R 782848 + 256 [0]
8,0 1 16275 0.808488708 0 C R 783104 + 256 [0]
8,0 0 13567 0.810261350 0 C R 783360 + 256 [0]
8,0 0 13636 0.812036226 0 C R 783616 + 256 [0]
8,0 1 16344 0.813806353 0 C R 783872 + 256 [0]
8,0 1 16413 0.815578436 0 C R 784128 + 256 [0]
8,0 0 13705 0.817347935 0 C R 784384 + 256 [0]
dd-direct:
8,0 0 428 0.998831975 0 C R 357376 + 1024 [0]
8,0 1 514 1.005683404 0 C R 358400 + 1024 [0]
8,0 1 515 1.012402554 0 C R 359424 + 1024 [0]
8,0 0 440 1.019303850 0 C R 360448 + 1024 [0]
8,0 1 526 1.026024048 0 C R 361472 + 1024 [0]
8,0 1 538 1.032875967 0 C R 362496 + 1024 [0]
8,0 0 441 1.039595815 0 C R 363520 + 1024 [0]
The non-direct dd throughput can improve with 512K and 1M readahead size,
but still a bit slower than the direct dd case:
1073741824 bytes (1.1 GB) copied, 14.1619 s, 75.8 MB/s
1073741824 bytes (1.1 GB) copied, 14.1517 s, 75.9 MB/s
dd-512k dd-direct2
-------------------------------------------------------------------------------------
Total (sda): | Total (sda):
Reads Queued: 23,808, 95,232KiB | Reads Queued: 178, 91,136KiB
Read Dispatches: 215, 95,232KiB | Read Dispatches: 178, 91,136KiB
Reads Requeued: 0 | Reads Requeued: 0
Reads Completed: 215, 95,232KiB | Reads Completed: 177, 90,624KiB
Read Merges: 23,593, 94,372KiB | Read Merges: 0, 0KiB
IO unplugs: 236 | IO unplugs: 89
|
Throughput (R/W): 75,222KiB/s / 0KiB/s | Throughput (R/W): 75,520KiB/s / 0KiB/s
Events (sda): 48,687 entries | Events (sda): 1,145 entries
Interestingly, the throughput reported by blktrace is almost the same,
whereas the dd report favors the dd-direct case.
More parameters.
[ 10.137350] scsi 3:0:0:0: Direct-Access ATA SanDisk SSD SATA 1.13 PQ: 0 ANSI: 5
[ 10.147137] sd 3:0:0:0: [sda] 61500000 512-byte hardware sectors: (31.4 GB/29.3 GiB)
[ 10.155060] sd 3:0:0:0: [sda] Write Protect is off
[ 10.159922] sd 3:0:0:0: [sda] Mode Sense: 00 3a 00 00
[ 10.165179] sd 3:0:0:0: [sda] Write cache: disabled, read cache: enabled, doesn't support DPO or FUA
[ 10.174994] sda:
/dev/sda:
Model=SanDisk SSD SATA 5000 2.5 , FwRev=1.13 , SerialNo= 81402200246
Config={ Fixed }
RawCHS=16383/16/63, TrkSize=0, SectSize=0, ECCbytes=4
BuffType=unknown, BuffSize=0kB, MaxMultSect=1, MultSect=?1?
CurCHS=16383/16/63, CurSects=16514064, LBA=yes, LBAsects=61500000
IORDY=yes, tPIO={min:120,w/IORDY:120}, tDMA={min:120,rec:120}
PIO modes: pio0 pio1 pio2 pio3 pio4
DMA modes: mdma0 mdma1 mdma2
UDMA modes: udma0 udma1 udma2 udma3 udma4 *udma5
AdvancedPM=yes: disabled (255) WriteCache=disabled
Drive conforms to: unknown: ATA/ATAPI-2,3,4,5,6,7
* signifies the current active mode
/sys/block/sda/queue/nr_requests:128
/sys/block/sda/queue/read_ahead_kb:128
/sys/block/sda/queue/max_hw_sectors_kb:32767
/sys/block/sda/queue/max_sectors_kb:512
/sys/block/sda/queue/scheduler:noop [cfq]
/sys/block/sda/queue/hw_sector_size:512
/sys/block/sda/queue/rotational:1
/sys/block/sda/queue/nomerges:0
/sys/block/sda/queue/rq_affinity:0
/sys/block/sda/queue/iostats:1
/sys/block/sda/queue/iosched/quantum:4
/sys/block/sda/queue/iosched/fifo_expire_sync:124
/sys/block/sda/queue/iosched/fifo_expire_async:248
/sys/block/sda/queue/iosched/back_seek_max:16384
/sys/block/sda/queue/iosched/back_seek_penalty:2
/sys/block/sda/queue/iosched/slice_sync:100
/sys/block/sda/queue/iosched/slice_async:40
/sys/block/sda/queue/iosched/slice_async_rq:2
/sys/block/sda/queue/iosched/slice_idle:8
Thanks,
Fengguang
