[zfs-discuss] Increase number of I/O workers?

Durval Menezes durval.menezes at gmail.com
Thu Feb 11 10:19:13 EST 2016

Hello Jonny,

On Thu, Feb 11, 2016 at 12:48 PM, João Carlos Mendes Luís <
zfs-discuss at list.zfsonlinux.org> wrote:

>    I am benchmarking ZoL for personal use, and got a case where I think
> performance is limited by number of I/O threads (zd_rd_int, z_wr_int,
> etc.), probably because I am using gzip compression.

Humrmmr... I've seen no such issues with either LZ4 or LZJB. I would
suggest you repeat your testing with LZ4.

> The problem is that these threads are currently limited by number of CPU
> cores, but I still have lots of idle CPU available. Even scrubbing appears
> to be limited by this, drastically reducing overall speed where block size
> is smaller (more compression achieved).

How many cores you have, what benchmark did you run, and how many cores got
100% use?

>    AFAIK, it's not currently possible to increase the number of these
> threads. Am I wrong? Already changed zio_taskq_batch_pct to 200, but
> nothing changed.

I never had to change any parameters to saturate my systems with either LZ4
or LZJB. But perhaps, as mine saturated with I/O,  your case is different
due to GZIP use (which I don't do exactly because it uses too much CPU).

Also, AFAIK ZFS doesn't use threads but full-blown kernel tasks to do its
thing. I could be wrong in that, but some fussing around /proc here seems
to indicate each of these tasks has exactly one thread (so no
multi-threading at all).

>    Is there a reason to keep these limited?

To avoid starving the CPU not only for user process, but also for other
stuff going in the kernel at the same time?

   Please, do not suggest to disable compression, as this is one of the
> reasons I'm trying ZFS. Also, I'd be happy to help increase ZFS performance
> for everybody use, not only datacenter folks.

I'd retest with LZ4; I don't think you have much sense of reaching any kind
of good performance with GZIP (in ZFS or otherwise) for any kind of
disk-intensive load.  If you do retest it, please let us know the results.

Cheers, and good luck,

>     Thanks for any comment,
>                 Jonny
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> zfs-discuss at list.zfsonlinux.org
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