docker container keep increasing memory usage when writing into a file in a mounted directory

Question

I'm facing an issue that memory used by a container keep increasing when an app inside the container writing log into a file in a mounted directory.

I'd expect memory usage doesn't increase by this. Does anyone have idea why it increases ? Thank you !!

Here is what I did:

1. Write an app which just writes "hello world" into "/home/mylog/test.txt".

    func main(){ file, _ := os.OpenFile("/home/mylog/test.txt", os.O_WRONLY|os.O_CREATE, 0666) defer file.Close() for { fmt.Fprintln(file, "hello world") } } 

2. Build a docker image

docker build -t mylog .

Dockerfile

FROM golang
    RUN mkdir -p /home/mylog
    COPY main.go /go
    WORKDIR /go
    CMD ["go","run","main.go"]

3. Run a container with -v option mouting the current dir.

docker run -d -v $PWD:/home/mylog mylog

4. Check memory usage

docker stats

5. It's using 527MiB.

CONTAINER CPU% MEMUSAGE / LIMIT MEM% NET I/O BLOCK I/O PIDS 
     100.41% 527MiB / 15.5GiB 3.32% 648B /0B 72.3MB / 0B 15

6. After a few seconds, it is 844.8 MiB

CONTAINER CPU% MEMUSAGE / LIMIT MEM% NET I/O BLOCK I/O PIDS
     100.15% 844.8MiB / 15.5GiB 5.32% 648B /0B 72.3MB / 0B 15

7. It keeps increasing and the host downs in the end.

Answer 1

Call this from time to time.

file.Sync() 

https://golang.org/pkg/os/#File.Sync

If you do not call this it writes to memory and waits for file.Close() in order to commit the changes to the file. And in this case Close in not called because it is in a defer (this means it is called when the function returns, and here it will never return since it is a never-ending for).

LE: Also try using:

file.WriteString("hello world")

instead of

fmt.Fprintln(file, "hello world")