Very slow during tests

[dargue3]

Hey! I'm sure this is going to be difficult for anyone to debug without a repro, but just taking a shot in the dark.

My jest tests are running SUPER slow and when I use console.time() to see how long it's taking for MSW to return some mocked responses, I'm seeing upwards of 3 seconds on a few of the graphql queries for the MSW resolver to even be called, and then another 3 for the response to return back to the caller. When I perform the same action in the browser — 50ms.

I'm assuming MSW can only respond as fast as the CPU has time for, which leads me to believe that maybe Jest is just hogging resources so much MSW can't even find a spare tick to do its response?

[msutkowski]

@dargue3 Is this the same with a rest request, or specifically limited to graphl operations? We can create a very simple repro and see if there are any stark differences.

[msutkowski]

I did run your repro along with the DEBUG flag. I'm going to need to create a better testing environment, as it seems like the timers are out of sync. The XHR patching from DEBUG is saying it takes +/- 300ms in total, but the tests execute in about 14ms on my machine.

I'd be really surprised if Cypress was able to run things much faster, and if it is, I'd like to understand why.

[dargue3]

I've tried a lot of things to measure performance, the best thing I could really manage to do is just litter console.time() both in the tests and within the code and just measure how long things take. I had one test that would take about 26 seconds to do something that cypress can do in 1500ms.

Profiling CPU in jest tests is quite hard, flame charts don't really look like much without knowing exactly how all the internals of React works.

[kettanaito]

I tried that once and the output was so massive it was hard for me to parse.

You can scope the output by providing the name of the logger. If you're running MSW in Node.js (via setupServer), you may scope the logs be doing either:

DEBUG=XHR* npm test
DEBUG=http* npm test

Those will depend on what kind of module performs the requests (http/https or XMLHttpRequest).

@dargue3, how big is your test suite? Can you reproduce the issue in isolation (i.e. a new repo, minimal app setup)? Does this performance decrease happen to all the tests or only some of them? Can you spot some patterns as to when it happens?

[dargue3]

I'll work on it, but honestly, the more that I dig into this, the more I think I can just attribute it to jsdom not performing well with large codebases.

I have about 120 components on just 4 separate "pages", and in my tests I render a whole page and test the workflows. Basically none of my individual tests run quicker than 7 seconds or so purely do to the time it takes to render. I think whatever slowness I'm seeing with MSW can be attributed to jest + jsdom tanking the CPU. Run the same tests in the browser (like with Cypress) and everything is much much snappier. MSW shines with cypress!

[kettanaito]

MSW shines with cypress!

Feedback like this warms my heart.

Still, we cannot toss aside the possibility that there may be some issue on our end, so we'd appreciate your investigation.

[kettanaito]

There has been a memory leak issue reported and fixed in the latest release (0.29.0). There's a serious change it might have caused this behavior. It's highly recommended to update to the latest version.