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According to http://net-snmp.sourceforge.net/docs/mibs/ucdavis.html#scalar_notcurrent ssCpuUser, ssCpuSystem, ssCpuIdle, etc are deprecated in favor of the raw variants (ssCpuRawUser, etc).

The former values (which don't cover things like nice, wait, kernel, interrupt, etc) returned a percentage value:

The percentage of CPU time spent processing user-level code, calculated over the last minute.

This object has been deprecated in favour of 'ssCpuRawUser(50)', which can be used to calculate the same metric, but over any desired time period.

The raw values return the "raw" number of ticks the CPU spent:

The number of 'ticks' (typically 1/100s) spent processing user-level code.

On a multi-processor system, the 'ssCpuRaw*' counters are cumulative over all CPUs, so their sum will typically be N*100 (for N processors).

My question is: how do you turn the number of ticks into percentage?

That is, how do you know how many ticks per second (it's typically — which implies not always — 1/100s, which either means 1 every 100 seconds or that a tick represents 1/100th of a second).

I imagine you also need to know how many CPUs there are or you need to fetch all the CPU values to add them all together. I can't seem to find a MIB that gives you an integer value for # of CPUs which makes the former route awkward. The latter route seems unreliable because some of the numbers overlap (sometimes). For example, ssCpuRawWait has the following warning:

This object will not be implemented on hosts where the underlying operating system does not measure this particular CPU metric. This time may also be included within the 'ssCpuRawSystem(52)' counter.

Some help would be appreciated. Everywhere seems to just say that % is deprecated because it can be derived, but I haven't found anywhere that shows the official standard way to perform this derivation.

The second component is that these "ticks" seem to be cumulative instead of over some time period. How do I sample values over some time period?

The ultimate information I want is: % of user, system, idle, nice (and ideally steal, though there doesn't seem to be a standard MIB for this) "currently" (over the last 1-60s would probably be sufficient, with a preference for smaller time spans).

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2 Answers

up vote 2 down vote accepted

Since these are absolute counters, you would have to regularly retrieve these metrics and then do the calculation yourself. So, if you want the number over the next minute, you would have to get the numbers, wait a minute, and get the numbers again. SNMP would not update those numbers too often so you may not be able to get these every second anyway.

Once you have the raw user, nice, system, idle, interrupts counters you can get the total number of ticks by summing these up. Even the MIB description says that adding them up is expected.

$ snmptranslate -Td .1.3.6.1.4.1.2021.11.52
UCD-SNMP-MIB::ssCpuRawSystem
...
    This object may sometimes be implemented as the
    combination of the 'ssCpuRawWait(54)' and
    'ssCpuRawKernel(55)' counters, so care must be
    taken when summing the overall raw counters."

Then, regardless of how long it has been since you took the measurements, the total number of ticks over that period is total1 - total0. And the idle percentage would be (idle1-idle0)/(total1-total0).

You are asking "how do you know how many ticks per second it is typically" but as you can see, you don't need to know that.

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Cool, I expected this might be the case. My problem with summing the values is that a lot of the values say that it may include ticks for other times (as in your example, ssCpuRawSystem may be wait+kernel). Is there a definitive way to know whether x includes y to prevent double-counting? –  bjeanes Oct 24 '12 at 19:29
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In addition to what has been already written by chutz, the reference to the duration of a tick can be found in man 2 times:

The number of clock ticks per second can be obtained using:

     sysconf(_SC_CLK_TCK);

which is a system function to be called in C but also can be obtained by simply running getconf CLK_TCK in your shell. This number is a compile-time constant and could be changed by anyone touching the source files, but this would be a rather rare event - common Linux distros all come with the value 100.

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I assume this value would have to be acquired from the hardware I'm asking about over SNMP, not locally. I'll see if I can do this through the limited shell I'm given — unless it is exposed over SNMP...? –  bjeanes Oct 24 '12 at 19:28
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@bjeanes I don't believe it is. BTW, I looked up what method of calculation OpenNMS is using in its NetSNMP-MIB-fed graphs - it resembles the one described by chutz: it lets RRD calculate the units/timeperiod average for ssCpuRawUser, ssCpuRawNice, and ssCpuRawSystem, divides the respective value by the sum of ssCpuRawUser + ssCpuRawNice + ssCpuRawSystem + ssCpuRawIdle and multiplies by 100 to get the percentage. Another graph shows a simplistic approach by just dividing ssCpuRaw* values by 100 (blindly assuming an _SC_CLK_TCK of 100) –  the-wabbit Oct 24 '12 at 20:45
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