Why multiple threads using too much memory when holding Mutex - rust

Question

Below code uses ~150MB in single thread but uses several GBs in 100 threads:
use std::sync::{Arc, Mutex};
use std::thread;
fn main() {
let f = Arc::new(Mutex::new(Foo::new("hello")));
let mut threads = vec![];
for i in 0..100 {
let f = f.clone();
let t = thread::spawn(move || loop {
let mut locked = f.lock().unwrap();
*locked = Foo::new("hello");
drop(locked);
println!("{} reloaded", i);
thread::yield_now();
});
threads.push(t);
}
threads.into_iter().for_each(|h| h.join().unwrap());
}
pub struct Foo {
_data: Vec<String>,
}
impl Foo {
fn new(s: &str) -> Foo {
Foo {
_data: vec![s.to_owned(); 1024 * 1024],
}
}
}
While holding the LockGuard, a thread should have exclusive access. So, new Foo should be allocated and old value should be dropped at that point. So, it doesn't make any sense to me this much memory is being used when called from multiple threads.
Can anyone please explain why this code is using this much memory?
Similar code in Java keeps memory ~200mb even with 1000 threads.
import java.util.ArrayList;
import java.util.List;
public class Foo {
private List<String> data;
public static void main(String[] args) {
Foo f = new Foo();
for (int i = 0; i < 1000; i++) {
int n = i;
new Thread(() -> {
while (true) {
f.update();
System.gc();
System.out.println(n + " updated");
}
}).start();
}
}
public synchronized void update() {
data = new ArrayList<>(1024 * 1024);
for (int i = 0; i < 1024 * 1024; i++) {
data.add(new String("hello"));
}
}
}

Answer 1

So the problem was in the big numbers of glibc's malloc arenas,
every arena has cache of preallocated memory. The simple way to check it is running binary with MALLOC_ARENA_MAX=2, but final solution depend on usage pattern, there are a lot variables to tune glibc's allocator: http://man7.org/linux/man-pages/man3/mallopt.3.html .
Java virtual machine is also actually affected by malloc's allocator. From my experience some time it is suitable to configure number of arenas to prevent huge memory usage of jvm inside docker.