Rust has many built-in concepts for memory safety, but it cannot prevent application level logic errors that take up system memory. An example would be a server application that stores something for each incoming request in a growing collection or list. If the program does not clean up the growing list then it will take up more and more server memory - thereby exposing a memory leak.

While working on my reverse proxy project I discovered such a leak in the HTTP library Hyper. In order to prevent and detect memory leaks in the future I set out my goal 7:

Add an integration test that ensures that the proxy server is not leaking memory (growing RAM usage without shrinking again). Use /proc information to compare memory usage of the current process before and after the test.

Finding memory leaks manually first

A very primitive way of inspecting the memory usage of a program is ps on Linux. First we start our Rustnish reverse proxy:

cargo run --release

Then get the memory information from ps for rustnish in a new terminal:

$ ps aux | grep '[r]ustnish'
klausi    3840  0.0  0.0  38504  7832 pts/0    Sl+  17:56   0:00 target/release/rustnish

The 6th column is the resident memory usage in kilobytes. Which means our server process is taking up ~8MB in RAM right now.

Now we want to see how our server is doing after it had to deal with a lot of requests. A tool for that is Apache Bench, which is used for load testing on servers. Installation on Ubuntu for example:

sudo apt-get install apache2-utils

Then fire 1 million requests at our reverse proxy, 4 requests concurrently:

ab -c 4 -n 1000000 http://localhost:9090/

Now running ps again:

$ ps aux | grep '[r]ustnish'
klausi    3840 47.8  3.6 304836 283588 pts/0   Sl+  18:15   2:04 target/release/rustnish

Wow, the 6th column is now showing 283,588KB which is ~278MB, something is definitely very wrong here!

Luckily I could track down the problem pretty quick to the Hyper library and after reporting it to the author he committed a fix. Thanks Sean McArthur!

Automating a memory leak test

Update 2018-01-07: It turns out that the automated test described here is unstable and has random test fails. Procinfo is probably not exact enough, so this does not work reliably.

Now that the memory leak is fixed we want to make sure it does not happen again. We can setup an integration test that runs on Travis CI whenever code is changed. The strategy for such a test is similar to what we did manually:

  1. Start the reverse proxy.
  2. Measure the memory footprint.
  3. Make a large amount of requests against the proxy, similar to what Apache Bench does.
  4. Measure the memory footprint again.
  5. Assert that memory usage is below a certain threshold.

The biggest problem is that Rust has no built-in function to get memory usage information of the current program (in PHP there is for example memory_get_usage()). The closest thing is the procinfo crate, which uses memory information from /proc on Linux. This is of course platform dependent and can for example not work on MacOS or Windows.

The full test can be found in

Getting the current memory usage (resident number of kilobytes in RAM):

extern crate procinfo;
let memory_before = procinfo::pid::statm_self().unwrap().resident;

Emulating Apache Bench and sending 30K requests, 4 at a time:

let mut core = Core::new().unwrap();
let client = Client::new(&core.handle());

let url: Uri = ("".to_string() + &port.to_string())

let nr_requests = 30_000;
let concurrency = 4;

let mut parallel = Vec::new();
for _i in 0..concurrency {
    let requests_til_done = loop_fn(0, |counter| {
        let mut request = Request::new(Method::Get, url.clone());
            .then(move |_| -> Result<_, hyper::Error> {
                if counter < (nr_requests / concurrency) {
                    Ok(Loop::Continue(counter + 1))
                } else {

let work = join_all(parallel);;

We are building 4 loop futures here with the loop_fn() construct, each iteration sending one request. The 4 futures are executed in parallel and we wait with a join_all() future until they all are finished.

Note that this is test code, that's why there are lots of unwrap() calls because we don't care about errors (I have written about unwrap() before).

As always the hardest part about assembling futures is to get the type spaghetti right. loop_fn() has 4 (!!!) generic type parameters, so writing and reasoning about it takes quite some time. As you can see there is a hyper::Error type in there because the compiler would greet me with this if I leave out all type decalrations:

error[E0282]: type annotations needed
  --> tests/
40 |     let mut parallel = Vec::new();
   |         ------------ consider giving `parallel` a type
48 |                     Ok(Loop::Continue(counter + 1))
   |                     ^^ cannot infer type for `E`

What E? I'm not even using any error type in the code here, how would I know? And the compiler is asking me to annotate the type of my list of futures, which is ... difficult. It is a vector of loop futures, each of them having 4 type parameters, each iteration having a Hyper client request future and the loop iteration future. Writing out that type spaghetti code is not something I can do, so instead I have annotated the inner return type of the loop iteration future. That is the other place where the compiler is complaining and where I can do something about it. The _ type placeholder lets me conveniently ignore types I don't care about and I just specify the only error type I think is relevant in my code: the hyper::Error that can be returned by a client request.

After solving that I can successfully run my test and demonstrate the memory leak on Hyper version 0.11.2 and that it is fixed in 0.11.4. Here is what the fail looks like:

$ cargo test --test memory_leaks
running 1 test
Listening on
test test_memory_after_30_000_requests ... FAILED


---- test_memory_after_30_000_requests stdout ----
    thread 'test_memory_after_30_000_requests' panicked at 'Memory usage at
    server start is 1551KB, memory usage after 30000 requests is 7098KB',
note: Run with `RUST_BACKTRACE=1` for a backtrace.


test result: FAILED. 0 passed; 1 failed; 0 ignored; 0 measured; 0 filtered out

error: test failed, to rerun pass '--test memory_leaks'

Very good, now the tests will fail if the server should expose a similar memory leak in the future! The downside of this test is that it is a bit slow to execute (because it makes so many requests) - it takes about 22 seconds on my desktop computer.

Limiting the test to run on Linux only

The problem with using procinfo is that it is only available on the Linux platform - so running this test on other operating systems will not work (the procinfo crate does not even compile on MacOS for example). I am using Travis CI to also run the tests on MacOS, so I need a platform specific exception to skip this test.

We can use conditional compilation for that.


# Memory Leak tests require procinfo, which only works on Linux.
procinfo = ">=0.4.2"

At the beginning of the test file:

// This test uses procinfo, so can only be run on Linux.
#![cfg(target_os = "linux")]

That means the test case is not even compiled on other platforms, so it does not show up as test for the test runner, so it never gets executed.


Testing for the memory footprint of a Rust server application is important to detect regressions when developing it.


  • As an integration test this approach treats the server application as black box. Whatever refactoring you do - the test case does not care and just covers the memory usage.
  • The test works as automated benchmarking tool. Instead of manually examining the runtime performance of you application this is directly built into automated test execution.

Some downsides:

  • Using the procinfo crate is not ideal because it is platform dependent (Linux). Please let me know if there is a Rust crate that measures memory independently!
  • The test hard-codes a specific amount of memory that is allowed to be used. That means the test is prone to random test fails when the memory footprint of the server application increases deliberately. In that case the test case will need to be adjusted and fine-tuned as well.
  • Test execution time is long. Performing 30k requests takes time, so this test is annoying when doing development to just check if everything is still working. A solution to that would be excluding the test from normal cargo test runs and invoking the test explicitly on Continues Integration tools.
  • Memory measuring of a test run is fuzzy and not deterministic. That is why we have to do such a large amount of test requests - to detect a significant change in memory that is not caused by other noise.

So while there is a lot of imperfection in this test approach I think it is still very practical and useful.