unexpected idprio 31 behavior on 9.2-BETA2 and 9.2-RC1

[Eric van Gyzen]

On 08/05/2013 16:15, Dave Mischler wrote:
> I have an i5-2500 machine 8GB RAM now running 9.2-RC1 amd64 with the
> GENERIC kernel. Today, while still running 9.2-BETA2, I updated my
> source tree and started building world with idprio 31 and I looked back
> a while later and all the CPU cores and disk were essentially idle, and
> hardly any progress had been made on the build. I stopped and restarted
> the build without the idle priority setting and it ran fine. Anybody
> else seen any of this? Anybody know about any fairly recent changes that
> might account for it?
>
> I did a "rm -rf /usr/src /usr/obj" and loaded a new source tree before
> going to RC1.  I still see odd behavior at RC1.  Sometimes it works just
> like it should (i.e. compute bound processes use most/all of the
> available CPU time), but a lot of the time both the CPU and disk are
> idle (e.g. CPU 97.8% idle, disk 1% busy per systat).  I don't think I
> ever saw this behavior before while running "make buildworld -j4".  Can
> anyone else confirm/rebut my findings?  Thanks.

I can confirm your findings, on 9.1-RELEASE-p5 amd64 GENERIC.

I ran

    $ idprio 31 make buildworld buildkernel > /tmp/build.log 2>&1 <
/dev/null &

on an otherwise idle amd64 system with 4 CPUs.  The first command in the
build.log file:

    rm -rf /usr/obj/home/freebsd/tmp

took over three minutes.  It should have taken about three /seconds/.

"uptime" reported a load average of around 1.00.
"top" showed no threads (user or kernel) using CPU.
"iostat" showed an average of less than 20 tps on ada0.
"rm" was usually in the RUN state.

/home/freebsd (src) is UFS+SUJ.
/usr/obj is UFS+SU.
/tmp/build.log is tmpfs.

Both UFS file systems are on ada0:

ada0 at ata2 bus 0 scbus2 target 0 lun 0
ada0: <WDC WD2502ABYS-18B7A0 02.03B05> ATA-8 SATA 2.x device
ata2: <ATA channel> at channel 0 on atapci0
atapci0: <Intel 5 Series/3400 Series PCH SATA300 controller>

CPU: Intel(R) Xeon(R) CPU           X3430  @ 2.40GHz (2394.04-MHz
K8-class CPU)
real memory  = 8589934592 (8192 MB)
FreeBSD/SMP: Multiprocessor System Detected: 4 CPUs
FreeBSD/SMP: 1 package(s) x 4 core(s)

FreeBSD 9.1-RELEASE-p5 #0 r+94f2ad5: Tue Aug  6 09:40:22 CDT 2013    
root at srv5:/usr/obj/home/freebsd/sys/GENERIC  amd64

/boot/loader.conf contains:

    console="comconsole vidconsole"
    comconsole_speed="115200"
    comconsole_port="0x2f8"

/etc/sysctl.conf is empty.

I'll update to releng/9.2 and try again.

Eric