Rust mongodb/actix-web deps upgrade causing very high latency

Experiencing major request/response latency after upgrading a couple rust libraries.

On a very fast actix-web/mongodb service, I upgraded versions of a couple of the libraries.

Major ones are actix-web 3.3.2 (from 2.0.0) and mongodb rust driver to 1.2.0 (from 0.9.2)

A lot of code was updated to swap mongodb 0.9.2's synchronous API with 1.2.0's async API. On the upgraded service, the requests complete successfully but have very high latency.

On 2.0.0/0.9.2, requests are super fast, like less than 50ms

After upgrade, requests take upwards of 4 seconds, some longer.

On 3.3.2/1.2.0, I notice the service is spawning many more mongo db connections compared to previously. Not sure if that's related.

Any ideas?

Since the completion times are so large, one possible cause is that you may be blocking the thread.

In case it's caused by some sync code blocking async threads, I've dignosed such issues this way.

  1. Implement a wrapper for futures that complains when something is slow:
use std::task::Poll;
use std::pin::Pin;
use std::future::Future;

pub struct NonBlock<F> {
    future: F,
    label: &'static str,

impl<F: Future> NonBlock<F> {
    pub fn new(label: &'static str, future: F) -> Self {
        Self { future, label }

impl<F: Future> Future for NonBlock<F> {
    type Output = F::Output;
    fn poll(self: Pin<&mut Self>, ctx: &mut std::task::Context<'_>) -> Poll<Self::Output> {
        let start = std::time::Instant::now();
        let label = self.label;
        let projected = unsafe { self.map_unchecked_mut(|s| &mut s.future) };
        let res = projected.poll(ctx);
        let elapsed = start.elapsed().as_secs();
        if elapsed >= 1 {
            eprintln!("blocking poll: {} took {}s", label, elapsed);
  1. Wrap all expr.await with NonBlock::new("label", expr).await.

Then run the code, and it will print which future is blocking.


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