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SubjectRe: [Bug #13463] Poor SSD performance
Dear Fengguang,

Thanks so much for the attention you paid to this problem. I did not
want to respond until I got a chance to give the new kernel a shot to
see if the bug was still present. It appears not to be -- hdparm and dd
both register read speeds between 200 and 220 MB/s as opposed to the 70
to 80 MB/s I was getting with kernel 2.6.29. So, I guess this strange
bug has sort of resolved itself.

Best,
Jake



Wu Fengguang wrote:
> 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@uchicago.edu>
>>> 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
>


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