Resident memory not released

[Maxim Dounin]

Hello!

On Mon, Jul 29, 2019 at 11:25:08AM -0400, fredr wrote:

> Hi all,
> 
> We are using the kubernetes nginx-ingress for websocket connections in front
> of one of our applications. We have added automatic scaling based on the
> resident memory, as that seems to be a good scaling metric when dealing with
> persistent connections. But we noticed that the memory seems to never be
> released, and thus it only scales up and never down.
> 
> I've done some testing locally using this dockerfile and nginx.conf with
> vanilla nginx:
> https://gist.github.com/fredr/d58f8221b813e4fdcf7bbfc08df30afa and it seems
> to have the same behaviour.
> 
> Starting the docker image, nginx has allocated 211Mb in the resident memory
> column in htop. 
> Then I connected 20K websocket connections, memory raised to about 540Mb,
> and then I disconnecting all the connections, the used memory stays at
> 540Mb.
> 
> If I reconnect all the 20K websocket connections, it seems to reuse the
> already allocated memory. 
> So I wounder, is this by design? is it somehow configurable, so that nginx
> would release this memory? or is it the wrong value to base scaling on?

Whether or not allocated (and then freed) memory will be returned 
to the OS depends mostly on your system allocator and its 
settings.

-- 
Maxim Dounin
http://mdounin.ru/