[nodejs] os.cpus() and times

On Dec 7, 2014, at 11:06 PM, // ravi wrote:


I am stumped by what must clearly be a misreading on my part. Node 0.10 docs say that each element of os.cpus() contains information about each CPU/core. This information includes "the number of milliseconds the CPU/core spent in: user, nice, sys, idle, and irq” (the docs do not say milliseconds of what, but I assume it means milliseconds since the system was last started). Note that this used to be, in v0.8 (and I assume before), reported in CPU ticks, but seems to have switched to milliseconds for v0.10.

Now for the puzzle: say I do this:

console.log(os.cpus()[0].times.idle);
setTimeout(function() { console.log(os.cpus()[0].times.idle); }, 2000);

The output I see is:

39035126800
39035144500

Assuming that ordering of CPUs/cores is consistent in the array returned by os.cpus(), it’s not clear how to square the above with the docs. The difference between the times reported is 17700ms i.e., CPU 0 was idle for 17 seconds in the 2 seconds between the two calls!

On Dec 7, 2014, at 11:37 PM, // ravi wrote:

One thing to consider is if os.cpus() returns CPU usage information from /proc/stats. If so, two things could affect the above (more likely the second):

(1) the first line of /proc/stats in GNU/Linux reports the totals across CPUs.
(2) these numbers seem to be in “jiffies” or 1/100th of a second (USER_HZ).

Time to start digging into the ‘os’ module sources, I guess, though I think Ockham’s Razor continues to point at me.

On Dec 7, 2014 10:07 PM, "// ravi" wrote:


Hello all,

I posted this question on node-forward/help on Gitter, so apologies if you
are seeing this again.

I am stumped by what must clearly be a misreading on my part. Node 0.10
docs say that each element of os.cpus() contains information about each
CPU/core. This information includes "the number of milliseconds the
CPU/core spent in: user, nice, sys, idle, and irq” (the docs do not say
milliseconds of what, but I assume it means milliseconds since the system
was last started). Note that this used to be, in v0.8 (and I assume
before), reported in CPU ticks, but seems to have switched to milliseconds
for v0.10.

Now for the puzzle: say I do this:

console.log(os.cpus()[0].times.idle);
setTimeout(function() { console.log(os.cpus()[0].times.idle); }, 2000);

The output I see is:

39035126800
39035144500

Assuming that ordering of CPUs/cores is consistent in the array returned
by os.cpus(), it’s not clear how to square the above with the docs. The
difference between the times reported is 17700ms i.e., CPU 0 was idle for
17 seconds in the 2 seconds between the two calls!

I guess I am doing something trivially wrong in the above, but what is it?

Any help appreciated!

—ravi

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On Dec 8, 2014, at 12:43 AM, Christopher Rust wrote:

I can't comment on how cpu idle time is calculated or if the cpus() array is consistently ordered but I can point out that setTimeout is not expected to be perfectly accurate.

<…>
Still, 17 seconds seems like a lot. Are you saying that's 17 seconds real time as well?

On Dec 8, 2014, at 10:17 AM, // ravi wrote:

Closer inspection reveals what might be the issue: if you take the idle time reported to be not in ms, but 100*ms (where 100 = GNU/Linux clock ticks per sec), then it starts to seem right. Throwing away the *100, the idle times become:

393117729
393117901
393118080
393118246

Or approx 2s, 1s, 2s each.