ChallengeWhat killed the application?

time to read 2 min | 267 words

I have been doing a lot of heavy performance testing on Raven, and I run into a lot of very strange scenarios. I found a lot of interesting stuff (runaway cache causing OutOfMemoryException, unnecessary re-parsing, etc). But one thing that I wasn’t able to resolve was the concurrency issue.

In particular, Raven would slow down and crash under load. I scoured the code, trying to figure out what was going on, but I couldn’t figure it out. It seemed that after several minutes of executing, request times would grow longer and longer, until finally the server would start raising errors on most requests.

I am ashamed to say that it took me a while to figure out what was actually going on. Can you figure it out?

Here is the client code:

Parallel.ForEach(Directory.GetFiles("Docs","*.json"), file =>
    PostTo("http://localhost:9090/bulk_docs", file);

The Docs directory contains about 90,000 files, and there is no concurrent connection limit. Average processing time for each request when running in a single threaded mode was 100 – 200 ms.

That should be enough information to figure out what is going on.

Why did the application crash?

More posts in "Challenge" series:

  1. (03 Jan 2020) Spot the bug in the stream–answer
  2. (15 Feb 2010) Where is the optimization?