mongodb writing use cpu fully but not use memory at all - mongodb

Question

I don't know why but very interesting stats shows.
First of all, I installed MongoDB on ec2, c5.4xlarge.
And I make some test write on the DB.
The funny thing is,, when I wrote 10 documents concurrently, MongoDB uses almost 100% CPU per core, but no memory increases at all!!
If I knew correctly, mongodb writes at first memory, so what I expect is increasing memory usage.
In order to reduce CPU usage per core, what I did is as follows:
disable atime
make MongoDB data folder running on xfs file system.
store MongoDB data on EBS with provisioning for increasing IOPS
upgrade glibc new latest version
What I used MongoDB version is 4.4 and used engine is WiredTiger.
Please help me if you have experience of running MongoDB on AWS for writing many many data continuously.
Below is my mongodb.conf
# Where and how to store data.
storage:
dbPath: "/home/ubuntu/mongodb/data"
journal:
enabled: true
wiredTiger:
engineConfig:
cacheSizeGB: 24
# where to write logging data.
systemLog:
destination: file
logAppend: true
path: "/home/ubuntu/mongodb/log/mongod.log"
# network interfaces
net:
port: 27017
bindIp: 0.0.0.0
# how the process runs
processManagement:
timeZoneInfo: /usr/share/zoneinfo
replication:
replSetName: "rs-exchange"

Answer 1

There is no requirement that a database uses either a specific amount of memory or that the amount of memory it uses is constantly growing.
For example, if I perform a replace operation in an infinite loop, naturally a lot of CPU would be consumed but the amount of memory used may well stay flat.