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Full Discussion: Understanding & Monitoring CPU performance (Load vs SAR)
Operating Systems Solaris Understanding & Monitoring CPU performance (Load vs SAR) Post 302974876 by javanoob on Sunday 5th of June 2016 10:32:13 AM
Old 06-05-2016
Quote:
Originally Posted by jlliagre
Yes, Linux is well known to include uninterruptible I/O in its load average calculation.

q1) The so called 1 min load average will tend to reach 1 but if the initial load was negligible, you'll need to wait for several minutes for it to get close to 1. It will be about 0.6 instead of 1 after one minute. Reciprocally, if the initial load was higher than one, you'll need to wait long enough (and likely more than 1 minute) to get close enough to it.

q2) The load average is derived from the run queue size which is sampled at 10 ms interval. The CPU load is computed from micro-state accounting with "exact" precision (i.e. several degrees of magnitude better, in the nanosecond range). A dtrace script should allow to figure out what is the cause of the discrepancy but in any case, the CPU utilization values are accurate, the average load is a rough approximation.
Hi jlliagre,

Thanks for your reply.

For q1) Yeap, when i run a sar -q 1 60 (for 1 minute average), the run queue's average is about 1+ , but my load avg for 1 minute (using uptime) shows only about 0.13.
Reading Brendan Gregg 's load average video - seems to talk about exponential decay of the load calculation (but i am no maths expert).
Thus, i will leave it as it is -> that having a load of 1 for a minute, will require more then 1 minute to be reflected in the " 1 minute load average " .

For q2) I am still confuse about the difference between CPU load and CPU utilization.

(on a 1 cpu - no multicore, or hyperthread computer)
If i have a continuous load of 1 for 1 minute, does that means my CPU utilization is near 100% / 0% idle for that 1 minute ?

q3) You mentioned that CPU load is sampled at 10ms interval.
How about the sampling interval for CPU utilization/time ?

In a nutshell, if i have 6 core cpu (12 thread total), and i have a average load of 3 most of the time;
Can i expect my CPU utilization to be around 3/12 * 100 = 25% (when the load is 3) ?

p.s. 1 last question -> does sar -q include thread currently running in cpu or only those runnable/ready in run queue ?

Regards,
Noob
Last edited by javanoob; 06-05-2016 at 12:11 PM..
 

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LEARN ABOUT DEBIAN

xsysinfo

XSYSINFO(1x)							 Debian GNU/Linux						      XSYSINFO(1x)

NAME

       xsysinfo - Display Linux kernel parameters in graphical form

SYNOPSIS

       xsysinfo [-help] [-update n] [-[no]title] [-[no]labels] [-[no]loadavg] [-[no]load] [-[no]mem] [-[no]swap] [-[no]smp]

DESCRIPTION

       Xsysinfo  is  an  X application to display some Linux kernel parameters in graphical form. It is like a mix of top, free and xload with the
       difference that the values are shown in form of a horizontal bar. The displayed values are: CPU load average, CPU  load,  memory  and  swap
       sizes (details see below).

OPTIONS

       -update n
	      Set update rate to n milli-seconds

       -title Show title string

       -notitle
	      Don't show title string

       -labels
	      Show gauge labels

       -nolabels
	      Don't show gauge labels

       -loadavg
	      Show CPU load average value

       -noloadavg
	      Don't show CPU load average value

       -load  Show CPU load value

       -noload
	      Don't show CPU load value

       -smp   Show separate SMP loads

       -nosmp Don't show separate SMP loads.

       -mem   Show memory info

       -nomem Don't show memory info

       -swap  Show swap info

       -noswap
	      Don't show swap info

       -help  Display options

DISPLAY

       Xsysinfo display the following values:

       CPU load average
	      CPU load average between 0.000-8.000. The gauge's bar is subdivided into segments, where one segment represents a load value of 1.0.
	      The bar's full length is automatically scaled, depending on the displayed value.

       CPU load
	      percentage CPU load time to CPU idle time subdivided in three segments: user load, system load and nice load. On an SMP  system  the
	      -smp option replaces the single total load meter with a separate meter for each processor.

       Memory The  memory  gauge's  bar is subdivided into two segments with the amount of physical memory, which is used by processes on the left
	      and physical memory used for the page and buffer cache on the right. The length of the whole bar, which is the sum of these two val-
	      ues, shows the amount of physical memory currently used by the system.

       Swap   The percentage of swap space used by the system to total amount of swap space.

AUTHORS

       Xsyinfo	is written by Gabor Herr <herr@iti.informatik.th-darmstadt.de> and currently maintained by Ronald Wahl <ronald.wahl@informatik.tu-
       chemnitz.de>.

       This manual page was created by Roland Rosenfeld <roland@spinnaker.de> for the Debian GNU/Linux system (but may be used by others).

Debian Project							   December 2005						      XSYSINFO(1x)
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