Hi, On 31 August 2017 at 10:13, Elhamer Oussama AbdelKHalek <abdelkhalekdev@xxxxxxxxx> wrote: > > On Wed, Aug 30, 2017 at 8:21 PM, Sitsofe Wheeler <sitsofe@xxxxxxxxx> wrote: >> >> On 30 August 2017 at 14:16, Elhamer Oussama AbdelKHalek >> <abdelkhalekdev@xxxxxxxxx> wrote: >> > I got the same result at my end. The thing is that at first i didn't > know you could dump the generated offsets using --debug=io , so i've > printed the values of '*b' for 8 different seeds (--size=2m --bs=128k > --rw=randwrite --ioengine=libaio) explicitly ( printf the values in > the source code / recompile fio), the result contains collisions! > > If i am understanding correctly, when a generated random value is > already in the axmap table, another one will be generated! > Here a dump fo both b* and off values (16 offsets) > https://gist.github.com/SamTheDev/29ede9f3ef2466c5b5d2e026f4bd2aec > > Where exactly the *b offset is updated (according to axmap table) in > the code? and does that mean that changing only the *b value for a > custom random generator won't do the trick? See https://github.com/axboe/fio/blob/fio-3.0/io_u.c#L129 where b can be "corrected" by axmap_next_free(). >> I should have asked - what performance were you expecting to see? >> > Sorry, i forgot to mention that the drive was fully written and > partially trimmed before running the randwrite job. > On a full nvme drive, i ran a randtrim job, after a reasonable delay i > ran the randwrite job using the same rand-seed. My drive bandwidth can > go up to 1.4 Gb/s, when using the default TAUSWORTHE random generator > the bw was around 150 Mb/s, which sounds reasonable, but when using > the Fisher-yates generator i was getting top performance (around 1.3 > Gb/s!) > the job files look something like that: > [global] > ioengine=libaio > iodepth=256 > size=1117G > direct=1 > do_verify=0 > continue_on_error=all > filename=/dev/nvme0n1 > randseed=23534 > sync=1 > [write-job] > rw=randwrite / randtrim > bs=128k I don't have a good explanation for this other than to check that the contents of the blocks being written looks suitably random and to plot the sequence numbers your Fisher/Yate algorithm generates. Another thing to try is using fio's record/replay facilities (http://fio.readthedocs.io/en/latest/fio_doc.html#i-o-replay ) to recheck the order but with different data. -- Sitsofe | http://sucs.org/~sits/ -- To unsubscribe from this list: send the line "unsubscribe fio" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
