linux-kernel - Re: Lower bound 0.05 on 15-Minute load?

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Date:	Thu, 02 Jul 2015 11:26:48 +0200
From:	Martin Steigerwald <martin@...htvoll.de>
To:	Ulrich Windl <Ulrich.Windl@...uni-regensburg.de>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: Lower bound 0.05 on 15-Minute load?

On Thursday 02 July 2015 10:50:13 Ulrich Windl wrote:
> Hi!

Hi Ulrich,

> I'm not subscribed, so plese CC: me for your replies.
> 
> When graphing the CPU load, I noticed that the 15-minute average never
> drops below 0.05, while the 5-minute load and the 1-minute load does
> (Kernel 3.0.101-0.47.52-xen of SLES11 on x86_64).

Load average is *NOT* the CPU load although this is a very common 
misconception.

Load average indicates the amount of processes that are waiting to be 
scheduled / running (which is CPU saturation) *and* those that are waiting 
uninterruptable. You can have a high load average without much CPU 
utilizitation, for example by running 20 find processes on a /home on NFS.

A high load can be CPU-bound but it doesn't need to be.

So a high load only can indicate that things are running more slowly, but 
not why, or well the why can be at least two things and does not need to be 
CPU.

Also the load is normalized to CPU cores.

> Ist that a known bug? Interactive call of "uptime" seems to confirm my
> suspect: windl> uptime
>  10:41am  up 23 days 18:49,  1 user,  load average: 0.08, 0.05, 0.05
> windl> uptime
>  10:48am  up 23 days 18:56,  1 user,  load average: 0.00, 0.04, 0.05
> windl> cat /proc/loadavg
> 0.00 0.04 0.05 1/108 9704
> 
> I'll attach a sample graph.

Why should it be? As you can see in the graph you have higher spikes with 1-
minute average. As its just a average about one minute it more easily drops 
below 0,05. But the 5 minute and 15 minute avergage need more time to drop 
lower, so for it to become lower, you need longer times without spikes in 
load average.

So its natural you get "flatter" curves with higher average. Average easily 
hide things like spikes.

Thanks,
-- 
Martin
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