Strange behavior of 'count'

[kehao95]

I have totally no idea what was going on here. So I will just describe how I tried this and what did I see.
I was trying to calculate the cpu_load pre-CPU using the metrics exported by the node_expoter.
So I calculated as this:

{pre_cpu_load by instance}
= A/B
=  {load1 by instance}/{number of CPU by instance}
=  {load1 by instance}/{count of {all cpu} by instance}
# So the query is: 
node_load1 / (count by(instance)(count by(instance, cpu)(node_cpu)))

What did I do?
In the prometheus web interface, exec
node_load1 / (count by(instance)(count by(instance, cpu)(node_cpu)))
'no data'
for debug I specific the instance id and tear this apart:
Here is some experiments:

label	query	expect	result
A	node_load1{instance="xxxxx"}	0.3	0.3
A'	node_load1	[0.3,0.3,0.3]	[0.3,0.3,0.3]
A+1	~	1.3	1.3
B	count by(instance)(count by(instance, cpu)(node_cpu{instance="xxxx"}))	4	4
B'	count by(instance)(count by(instance, cpu)(node_cpu))	[2,2,4]	[2,2,4]
B+1	~	5	5
A/B	node_load1{instance="xxxxx"}/(count by(instance)(count by(instance, cpu)(node_cpu{instance="xxxx"})))	0.075	no data
A'/B'	node_load1 / (count by(instance)(count by(instance, cpu)(node_cpu)))	[2,2,2,4]	no data

Environment

• System information:
  Linux 4.4.35-33.55.amzn1.x86_64 x86_64
• Prometheus version:

prometheus, version 1.5.2 (branch: master, revision: bd1182d29f462c39544f94cc822830e1c64cf55b)
  build user:       root@a8af9200f95d
  build date:       20170210-14:41:22
  go version:       go1.7.5

[kehao95]

One more interesting things here may be helpful.

There is a result of '4' in Query B but the graph did show it correctly. The line seemed to drop off the graph.

[brian-brazil]

For usage questions, https://groups.google.com/forum/#!aboutgroup/prometheus-users is the best place to get answers.

[kehao95]

Well, I think this may be a bug. So I report it. Could you plz take another look?
In brief, I came across a situation where I got a query 'A' with result of a. And query 'B' with result of b.
But when I query 'A/B' I got no data

[beorn7]

It is not a bug.

You need to understand label matching, see https://prometheus.io/docs/querying/operators/#vector-matching .

If you discuss this questions on the mailing list, other users can benefit from the answers.

[kehao95]

Oh sorry, thank you for your reply.

[lock]

This thread has been automatically locked since there has not been any recent activity after it was closed. Please open a new issue for related bugs.