Jens Axboe <jens.axboe@xxxxxxxxxx> writes:
> On Wed, Jun 11 2008, Dmitri Monakhov wrote:
>> Jens Axboe <jens.axboe@xxxxxxxxxx> writes:
>>
>> > On Wed, Jun 11 2008, Dmitri Monakhov wrote:
>> >> I've found what any non continious sequence violation result in significant
>> >> pefrormance drawback. I've two types of requests:
>> >> 1)Ideally sequential writes:
>> >> for(i=0;i<num;i++) {
>> >> write(fd, chunk, page_size*32);
>> >> }
>> >> fsync(fd);
>> >>
>> >> 2) Sequential writes with dgap for each 32'th page
>> >> for(i=0;i<num;i++) {
>> >> write(fd, chunk, page_size*31);
>> >> lseek(fd, page_size, SEEK_CUR);
>> >> }
>> >> fsync(fd);
>> >>
>> >> I've found what second IO pattern is about twice times slower whan the
>> >> first one regardless to ioscheduler or HW disk. It is not clear to me
>> >> why this happen. Is it linux speciffic or general hardware behaviour
>> >> speciffic. I've naively expected what disk hardware cat merge several
>> >> 31-paged requests in to continious one by filling holes by some sort
>> >> of dummy activity.
>> >
>> > Performance should be about the same. The first is always going to be a
>> > little faster, on some hardware probably quite a bit. Are you using
>> > write back caching on the drive? I ran a quick test here, and the second
>> > test is about ~5% slower on this drive.
>> Hmmm... it is definitly not happen in may case.
>> I've tested following sata drive w and w/o write cache
>> AHCI
>> ata7.00: ATA-7: ST3250410AS, 3.AAC, max UDMA/133
>> ata7.00: 488397168 sectors, multi 0: LBA48 NCQ (depth 31/32)
>> In all cases it is about two times slower.
>>
>> # time /tmp/wr_test /dev/sda3 32 0 800
>> real 0m1.183s
>> user 0m0.002s
>> sys 0m0.079s
>>
>> # time /tmp/wr_test /dev/sda3 31 1 800
>> real 0m3.240s
>> user 0m0.000s
>> sys 0m0.078s
>
> Ahh, direct to device. Try making a 4kb fs on sda3, mount it, umount
> it, then rerun the test.
Noup.. no changes at all.
# cat /sys/block/sda/queue/scheduler
[noop] anticipatory deadline cfq
# blockdev --getbsz /dev/sda
4096
# hdparm -W 0 /dev/sda
/dev/sda:
setting drive write-caching to 0 (off)
# time ./wr_test /dev/sda 32 0 800
real 0m1.185s
user 0m0.000s
sys 0m0.106s
# time ./wr_test /dev/sda 31 1 800
real 0m3.391s
user 0m0.002s
sys 0m0.112s
I'll try to play with request queue parameters
>
> --
> Jens Axboe
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