Odtnhj

I have a 4 cores CPU, I create a Kubernetes Pod with CPU resource limit 100m, which mean it will occupy 1/10 of a core power.

I wondering in this case, 100m is not even a full core, if my app is a multithread app, will my app's threads run in parallel? Or all the threads will run in the part of core (100 milli core) only?

Can anyone further explain the mechanism behind?

Take a look to this documentation related to resources in Kubernetes:

You can use resources as described in the article:

To specify a CPU request for a Container, include the
resources:requests field in the Container resource manifest. To
specify a CPU limit, include resources:limits.

In this exercise, you create a Pod that has one Container. The
Container has a request of 0.5 CPU and a limit of 1 CPU. Here is the
configuration file for the Pod:

apiVersion: v1
kind: Pod
metadata:
 name: cpu-demo
 namespace: cpu-example
spec:
 containers:
 - name: cpu-demo-ctr
 image: vish/stress
 resources:
 limits:
 cpu: "1"
 requests:
 cpu: "0.5"
 args:
 - -cpus
 - "2"

Additional to your question:
Yes it is not gonna run in parallels (multi core threads). But you can show few core for your application in pod and then use multi threads to execute it.

The args section of the configuration file provides arguments for
the Container when it starts. The -cpus "2" argument tells the
Container to attempt to use 2 CPUs.

The closest answer I found so far is this one:

For a single-threaded program, a cpu usage of 0.1 means that if you
could freeze the machine at a random moment in time, and look at what
each core is doing, there is a 1 in 10 chance that your single thread
is running at that instant. The number of cores on the machine does
not affect the meaning of 0.1. For a container with multiple threads,
the container's usage is the sum of its thread's usage (per previous
definition.) There is no guarantee about which core you run on, and
you might run on a different core at different points in your
container's lifetime. A cpu limit of 0.1 means that your usage is not
allowed to exceed 0.1 for a significant period of time. A cpu request
of 0.1 means that the system will try to ensure that you are able to
have a cpu usage of at least 0.1, if your thread is not blocking
often.

I think above sound quite logical. Based on my question, 100m core of CPUs power will spread across all the CPU cores, which mean multithreading should work in Kubernetes.