I’ve been playing around with Rustler NIFs to get familiar with them and understand how they interact with Elixir.

While running some simple toy NIFS and benchmarking them to be impressed with the performance differences, I was surprised with :process-memory-high-watermark warnings and opening the observer showed that memory had ballooned up to a couple of GBs.

The code isn’t creating any long term data and simply sums the numbers from A..B so I was surprised to see the jump in memory. The create is built release mode and benched with Benchee and it’s unclear what’s causing the memory jump.

Code is as follows

#[rustler::nif]
fn add(a: i64, b: i64) -> i64 {
    let mut r:i64 = 0;
    for x in a..b {
        r += x;
    }
    return r;
}

rustler::init!("Elixir.FastMath", [add]);

Any hints on where to look or insights into what’s causing the high memory usage?