Constant load of 1 on a recent 12-STABLE

[Gordon Bergling]

Hi Allan,

On Wed, Jun 03, 2020 at 05:33:37PM -0400, Allan Jude wrote:
> On 2020-06-03 16:29, Gordon Bergling wrote:
> > Hi Allan,
> > 
> > On Wed, Jun 03, 2020 at 03:13:47PM -0400, Allan Jude wrote:
> >> On 2020-06-03 06:16, Gordon Bergling via freebsd-hackers wrote:
> >>> since a while I am seeing a constant load of 1.00 on 12-STABLE,
> >>> but all CPUs are shown as 100% idle in top.
> >>>
> >>> Has anyone an idea what could caused this?
> >>>
> >>> The load seems to be somewhat real, since the buildtimes on this
> >>> machine for -CURRENT increased from about 2 hours to 3 hours.
> >>>
> >>> This a virtualized system running on Hyper-V, if that matters.
> >>>
> >>> Any hints are more then appreciated.
> >>>
> >> Try running 'top -SP' and see if that shows a specific CPU being busy,
> >> or a specific process using CPU time
> > 
> > Below is the output of 'top -SP'. The only relevant process / thread that is
> > relatively constant consumes CPU time seams to be 'zfskern'.
> > 
> > -----------------------------------------------------------------------------
> > last pid: 68549;  load averages:  1.10,  1.19,  1.16 up 0+14:59:45  22:17:24
> > 67 processes:  2 running, 64 sleeping, 1 waiting
> > CPU 0:  0.0% user,  0.0% nice,  0.0% system,  0.0% interrupt,  100% idle
> > CPU 1:  0.0% user,  0.0% nice,  0.0% system,  0.0% interrupt,  100% idle
> > CPU 2:  0.0% user,  0.0% nice,  0.4% system,  0.0% interrupt, 99.6% idle
> > CPU 3:  0.0% user,  0.0% nice,  0.0% system,  0.0% interrupt,  100% idle
> > Mem: 108M Active, 4160M Inact, 33M Laundry, 3196M Wired, 444M Free
> > ARC: 1858M Total, 855M MFU, 138M MRU, 96K Anon, 24M Header, 840M Other
> >      461M Compressed, 1039M Uncompressed, 2.25:1 Ratio
> > Swap: 2048M Total, 2048M Free
> > 
> >   PID USERNAME    THR PRI NICE   SIZE    RES STATE    C   TIME    WCPU COMMAND
> >    11 root          4 155 ki31     0B    64K RUN      0  47.3H 386.10% idle
> >     8 root         65  -8    -     0B  1040K t->zth   0 115:39  12.61% zfskern
> > -------------------------------------------------------------------------------
> > 
> > The only key performance indicator that is relatively high IMHO, for a 
> > non-busy system, are the context switches, that vmstat has reported.
> > 
> > -------------------------------------------------------------------------------
> > procs  memory       page                    disks     faults         cpu
> > r b w  avm   fre   flt  re  pi  po    fr   sr da0 da1   in    sy    cs us sy id
> > 0 0 0 514G  444M  7877   2   7   0  9595  171   0   0    0  4347 43322 17  2 81
> > 0 0 0 514G  444M     1   0   0   0     0   44   0   0    0   121 40876  0  0 100
> > 0 0 0 514G  444M     0   0   0   0     0   40   0   0    0   133 42520  0  0 100
> > 0 0 0 514G  444M     0   0   0   0     0   40   0   0    0   120 43830  0  0 100
> > 0 0 0 514G  444M     0   0   0   0     0   40   0   0    0   132 42917  0  0 100
> > --------------------------------------------------------------------------------
> > 
> > Any other ideas what could generate that load?
> 
> I agree that load average looks out of place here when you look at the %
> cpu idle, but I wonder if it is caused by a lot of short lived processes
> or threads.
> 
> How quickly is the 'last pid' number going up?
> 
> You might also look at `zpool iostat 1` or `gstat -p` to see how busy
> your disks are

In the IDLE state the last pid isn't changing within at least 60 seconds.
During 'buildworld' times it is off course much shorter, but a "-j 4" is
resulting in a load average from about 5.0, so that the underlying problem
still persists. 'zpool iostat 1' and 'gstat -p' doesn't show anything 
suspicious.

I had a private mail that made me aware of PR173541, where this problem is
documentated. I'll add my hardware information and performance measurements 
to it when I find some time.

I am currently thinking about how to measure the spawned threads/s. Did you
have an idea how to do it?

Best regards,

Gordon
-- 
Gordon Bergling
Mobile: +49 170 23 10 948
Web: https://www.gordons-perspective.com/
Mail: gbergling at gmail.com

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