Friday, August 19, 2016

Userspace RCU in GlusterD

Here is a great example of how RCU can be used in a project to improve scalability and throughput, namely in GlusterD:

Atin Mukherjee talks about several interesting details on how to use RCU.
My only quirk is on slide 8 "Different Locking Primitives", RCU is not a locking primitive in the sense that it doesn't provide mutual exclusion by itself, but it's hard to explain exactly what it does, so it's ok to put it on the same slide.

He even talks about a bug in their code where they did something like:
    // do some stuff
    synchronize_rcu();   // This will deadlock

This happens because we can't call synchronize_rcu() from within a block of code protected with rcu_read_lock().

It's interesting that they went with the Bullet-Proof RCU variant, which just shows that most software engineers are willing to sacrifice a small amount of performance if it gives them more flexibility and ease-of-use (myself included).

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