Re: Extremely low disk throughput under high compute load

On Sun, Apr 1, 2018 at 9:18 AM, Stefan Esser <se_at_freebsd.org> wrote:

> My i7-2600K based system with 24 GB RAM was in the midst of a buildworld
> -j8
> (starting from a clean state) which caused a load average of 12 for more
> than
> 1 hour, when I decided to move a directory structure holding some 10 GB to
> its
> own ZFS file system. File sizes varied, but were mostly in the range 0f
> 500KB.
>
> I had just thrown away /usr/obj, but /usr/src was cached in ARC and thus
> there
> was nearly no disk activity caused by the buildworld.
>
> The copying proceeded at a rate of at most 10 MB/s, but most of the time
> less
> than 100 KB/s were transferred. The "cp" process had a PRIO of 20 and thus
> a
> much better priority than the compute bound compiler processes, but it got
> just 0.2% to 0.5% of 1 CPU core. Apparently, the copy process was scheduled
> at such a low rate, that it only managed to issue a few controller writes
> per
> second.
>
> The system is healthy and does not show any problems or anomalies under
> normal use (e.g., file copies are fast, without the high compute load).
>
> This was with SCHED_ULE on a -CURRENT without WITNESS or malloc debugging.
>
> Is this a regression in -CURRENT?
>

Does 'sync' push a lot of I/O to the disk?

Is the effective throughput of CP tiny or large? It's tiny, if I read
right, and the I/O is slow (as opposed to it all buffering in memory and
being slow to drain own), right?

Warner