Every Thread::spawn() adds ~68 of memory?!

I'm new to Rust and I'm trying to build a fast and low-memory multi-threaded CLI application, but I was shocked to see the memory consumption afterwards. I figured out the code to blame, but it doesn't make any sense to me: Thread::spawn()

use std::thread;
use std::time::Duration;

fn main() {
    // Every thread adds: ~68 MB
    thread::spawn(|| {

    // Baseline: ~13 MB
    loop {

However I'm unable to find any reports about this on the internet, so I'm asking myself whether or not this is "expected" behavior, although I can't image.

A ps will show the following ~80 MB:

raymond  16417  0.0  0.0  80772  1092 pts/3    S+   08:32   0:00 target/release/threads

Is there something that I'm doing wrong, do I miss some vital information to understand why this happens, or do I misinterpreted the memory numbers?

Any help will be appreciated!

64MB per thread is a lot, AFAIK creating a thread only reserves the virtual memory for the stack. And the default thread stack size tends to be something from 2MB to 8MB, depending on the architecture and distribution (ulimit -s).

That said, I do see the same behavior with your example (amd64 vm);

  • no threads
user      3137  1.0  0.0  13168  1044 pts/0    S+   07:03   0:00 target/release/
  • 1 thread
user      3016  1.5  0.0  80776  1048 pts/0    Sl+  07:02   0:00 target/release/
  • 2 threads
user      3084  1.5  0.0 148372  1048 pts/0    Sl+  07:02   0:00 target/release/
  • 3 threads
user      3162  1.5  0.0 215972  1080 pts/0    Sl+  07:07   0:00 target/release/

(…seems to continue indefinitely, I tried up to 16. For a comparable test with C and pthreads the increase is approximately 8MB per thread)

It doesn't reflect in the RSS which is the amount of actual memory consumed, but it's still curious.

I now understand that I have to look at RSS for the actual memory that is consumed by my CLI application (not the snippet above), which means ~2.3 MB. That is a total relief and totally acceptable!

Yes, it's not a problem on 64 bit where the address space is pretty much unexhaustible.

Edit: for completeness I investigated where the virtual memory usage comes from. It has nothing to do with rust, nor rust's runtime, but with glibc's allocator creating per-thread arenas. I tried setting export MALLOC_ARENA_MAX=1 and with 16 threads:

user     12040  0.7  0.0  46164  1048 pts/0    Sl+  07:56   0:00 target/release/

This amounts to a default stack size of 2MB per thread (+14MB baseline), which is less than the C pthreads default.

1 Like

And even that will be allocated by the OS only to the amount that is used.