[RndTbl] Load average under RHEL 6.x systems?

Gilbert E. Detillieux gedetil at cs.umanitoba.ca
Fri May 11 11:33:02 CDT 2012

On 2012-04-11 17:00, I wrote:
> After upgrading many of our systems, both workstations and servers, from
> CentOS 5.x to Scientific Linux 6.x, I'm seeing higher load averages on
> idle systems than I used to. Under EL5, loads would drop to zero and
> pretty much stay there most of the time for idles systems. Under EL6,
> the load might drop down to 0.1, but doesn't stay there for very long,
> and even on seemingly idle systems, I see loads at or near 1 (sometimes
> even higher than 1 on some of our servers). It's also intermittent, with
> load averages dropping and climbing on fairly short intervals (of a few
> minutes or so).

Problem solved (at long last)!...

It turns out the problem was with "hald" polling the CD/DVD-ROM drive 
every two seconds.  I had previously dismissed that as the potential 
problem, given that this seemed to be no different than the way hald 
worked under EL5 systems.

> Running top, iotop, ftop, iftop, etc. doesn't really point to any major
> culprits. I've even run PowerTop, and implemented some of its suggested
> improvements, but that didn't make a difference on load.

My bad...  PowerTop had indeed recommended I disable polling in hald, 
but I wasn't sure I wanted to disable that feature, particularly on the 
workstations (not really needed on the servers, though).  Also, as I 
said above, I didn't think this was any different than in EL5, but 
apparently it is.

Also, hald-addon-storage (the sub-process that does the polling) wasn't 
sticking around long enough to show a big CPU load in "top", 
particularly with the default 3 second update delay, but when I dropped 
the delay to 1/2 a second, I was seeing it show up briefly every once in 
a while.  (I was also seeing the irqbalance process show up as well, and 
mistakenly thought it might be the culprit.  This seemed to make sense 
at the time, since I was seeing higher loads on our 16-core servers than 
the dual-core workstations, but that was a red herring.)

> Just wondering if anyone else has seen similar behaviour with hosts
> running Red Hat and/or Fedora distributions? Would moving to the
> "tickless" kernel have anything to do with it? (I.e. does it somehow
> affect the way load averages are calculated?)

Still not sure if the new kernel makes a difference or not, but there 
must be something different about the way hald-addon-storage interacts 
with it to do the polling in EL6, compared to EL5.  (Or have they just 
made the polling more aggressive, by reducing the interval?)

> Or is it some system service that can be shut down? (If it is, it's not
> creating an obvious load on its own, that top or ftop would show, but it
> may be affecting something in the kernel...)

As you can see by the attached graph of the load average, disabling 
polling on the CD-ROM drive yesterday afternoon seems to have made all 
the difference.  Here's the command PowerTop recommended:

hal-disable-polling --device /dev/cdrom

(Device name may vary.)  The beauty of this, compared to disabling 
polling for all storage devices, is that you can disable it on a device 
basis, and keep polling enabled, e.g. for USB devices that might get 

Gilbert E. Detillieux		E-mail: <gedetil at muug.mb.ca>
Manitoba UNIX User Group	Web:	http://www.muug.mb.ca/
PO Box 130 St-Boniface		Phone:  (204)474-8161
Winnipeg MB CANADA  R2H 3B4	Fax:    (204)474-7609
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