Messages in this thread | | | Date | Sun, 29 Jan 2012 15:52:06 +0000 | From | Pádraig Brady <> | Subject | Re: Bad SSD performance with recent kernels |
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On 01/29/2012 01:13 PM, Eric Dumazet wrote: > Le dimanche 29 janvier 2012 à 19:16 +0800, Wu Fengguang a écrit : > > >> Note that as long as buffered read(2) is used, it makes almost no >> difference (well, at least for now) to do "dd bs=128k" or "dd bs=2MB": >> the 128kb readahead size will be used underneath to submit read IO. >> > > Hmm... > > # echo 3 >/proc/sys/vm/drop_caches ;dd if=/dev/sda of=/dev/null bs=128k count=32768 > 32768+0 enregistrements lus > 32768+0 enregistrements écrits > 4294967296 octets (4,3 GB) copiés, 20,7718 s, 207 MB/s > > > # echo 3 >/proc/sys/vm/drop_caches ;dd if=/dev/sda of=/dev/null bs=2M count=2048 > 2048+0 enregistrements lus > 2048+0 enregistrements écrits > 4294967296 octets (4,3 GB) copiés, 27,7824 s, 155 MB/s
Same here on 2.6.40.4-5.fc15.x86_64 Note the SSD is rated for 500MB/s but is on a SATA II port, and so limited by that. So the 128k result below is close to the limit on this system.
Hmm, I previously tested this SSD with kernel-2.6.38.6-26.rc1.fc15.src.rpm and got 270MB/s. Testing now gives variable and lower results:
# echo 3 >/proc/sys/vm/drop_caches; hdparm -tT /dev/sdb /dev/sdb: Timing cached reads: 8388 MB in 2.00 seconds = 4200.73 MB/sec Timing buffered disk reads: 550 MB in 3.00 seconds = 183.19 MB/sec # echo 3 >/proc/sys/vm/drop_caches; hdparm -tT /dev/sdb /dev/sdb: Timing cached reads: 8260 MB in 2.00 seconds = 4134.30 MB/sec Timing buffered disk reads: 680 MB in 3.00 seconds = 226.63 MB/sec # echo 3 >/proc/sys/vm/drop_caches; hdparm -tT /dev/sdb /dev/sdb: Timing cached reads: 8426 MB in 2.00 seconds = 4217.87 MB/sec Timing buffered disk reads: 588 MB in 3.00 seconds = 195.96 MB/sec
Anyway testing different block sizes with dd:
# echo 3 >/proc/sys/vm/drop_caches; timeout -sINT 5 dd if=/dev/sdb of=/dev/null bs=2M 966787072 bytes (967 MB) copied, 5.00525 s, 193 MB/s # echo 3 >/proc/sys/vm/drop_caches; timeout -sINT 5 dd if=/dev/sdb of=/dev/null bs=128k 1246494720 bytes (1.2 GB) copied, 4.99563 s, 250 MB/s
On a probably unrelated note, I've always noticed dd getting slower, independent of disks, when the buffer size increases beyond 2M.
for i in $(seq 0 15); do size=$((16*1024**3)) #ensure this is big enough bs=$((1024*2**$i)) printf "%8s=" $bs dd bs=$bs if=/dev/zero of=/dev/null count=$(($size/$bs)) 2>&1 | sed -n 's/.* \([0-9.]* [GM]B\/s\)/\1/p' done
1024=1.4 GB/s 2048=2.6 GB/s 4096=4.5 GB/s 8192=6.7 GB/s 16384=8.8 GB/s 32768=9.4 GB/s 65536=10.8 GB/s 131072=11.5 GB/s 262144=11.5 GB/s 524288=11.3 GB/s 1048576=11.3 GB/s 2097152=10.6 GB/s 4194304=6.5 GB/s 8388608=5.9 GB/s 16777216=6.6 GB/s 33554432=6.6 GB/s
cheers, Pádraig. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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