CODEWITHSUNDEEP
linuxgolinux-kernelschedulercfs
xakepp35
xakepp35

Reputation: 3232

How to get delta percentage from /proc/schedstat

I am trying to get node CFS scheduler throttling in percent. For that i am reading 2 values 2 times (ignoring timeslices) from /proc/schedstat it has following format:

$ cat /proc/schedstat
version 15
timestamp 4297299139
cpu0 0 0 0 0 0 0 1145287047860 105917480368 8608857
                 CpuTime       RunqTime

so i read from file, sleep for some time, read again, calculate time passed and value delta between, and calc percent then using following code:

cputTime := float64(delta.CpuTime) / delta.TimeDelta / 10000000
runqTime := float64(delta.RunqTime) / delta.TimeDelta / 10000000
percent := runqTime

the trick is that percent could be like 2000%

i assumed that runqtime is incremental, and is expressed in nanoseconds, so i divided it by 10^7 (to get it to 0-100% range), and timedelta is difference between measurements in seconds. what is wrong with it? how to do that properly?

Upvotes: 0

Views: 972

Answers (1)

LeGEC
LeGEC

Reputation: 51850

I, for one, do not know how to interpret the output of /proc/schedstat.

You do quote an answer to a unix.stackexchange question, with a link to a mail in LKML that mentions a possible patch to the documentation.

However, "schedstat" is a term which is suspiciously missing from my local man proc page, and from the copies of man proc I could find on the internet. Actually, when searching for schedstat on Google, the results I get either do not mention the word "schedstat" (for example : I get links to copies of the man page, which mentions "sched" and "stat"), or non authoritative comments (fun fact : some of them quote that answer on stackexchange as a reference ...)

So at the moment : if I had to really understand what's in the output, I think I would try to read the code for my version of the kernel.

As far as "how do you compute delta ?", I understand what you intend to do, I had in mind something more like "what code have you written to do it ?".

By running cat /proc/schedstat; sleep 1 in a loop on my machine, I see that the "timestamp" entry is incremented by ~250 units on each iteration (so I honestly can't say what's the underlying unit for that field ...).
To compute delta.TimeDelta : do you use that field ? or do you take two instances of time.Now() ?

The other deltas are less ambiguous, I do imagine you took the difference between the counters you see :)
Do note that, on my mainly idle machine, I sometimes see increments higher than 10^9 over a second on these counters. So again : I do not know how to interpret these numbers.

Upvotes: 0

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