[FIXED] What does "bw: SpinningDown" mean in a RedisTimeoutException?

Issue

What does "bw: SpinningDown" mean in this error -

Timeout performing GET (5000ms), next: GET foo!bar!baz, inst: 5, qu: 0, qs: 0, aw: False, bw: SpinningDown, ....

Does it mean that the Redis server instance is spinning down, or something else?

Solution

It means something else actually. The abbreviation bw stands for Backlog-Writer, which contains the status of what the backlog is doing in Redis.

For this particular status: SpinningDown, you actually left out the important bits that relate to it.

There are 4 values being tracked for workers being Busy, Free, Min and Max. Let's take these hypothetical values: Busy=250,Free=750,Min=200,Max=1000

In this case there are 50 more existing (busy) threads than the minimum.

The cost of spinning up a new thread is high, especially if you hit the .NET-provided global thread pool limit. In which case only 1 new thread is created every 500ms due to throttling.

So once the Backlog is done processing an item, instead of just exiting the thread, it will keep it in a waiting state (SpinningDown) for 5 seconds. If during that time there still is more Backlog to process, the same thread will process another item from the Backlog.

If no Backlog item needed to be processed in those 5 seconds, the thread will be exited, which will eventually lead to a decrease in Busy (existing) threads.

This only happens for threads above the Min count of course, as those will be kept alive even if there is no work to do.

Answered By - Huron

Answer Checked By - Senaida (PHPFixing Volunteer)