Monday, November 4, 2019

Is Left-Right a generic concurrency control?

Spoiler alert, yes, Left-Right is a generic concurrency control!

We mentioned Left-Right many times before, but here it goes again.
Left-Right is a technique where two replicas of your data are kept up to date. Mutative operations execute in one replica while the other replica can be read by in-flight readers and then the role of the replicas toggles for the write to apply the same operation on the opposite replica while the new readers go on the freshly modified replica.

Through the careful usage of atomics and RCU, this can be done in such a way as to have wait-free progress for the readers and that's what Left-Right is about.
There is only one writer at a time (we can use a global lock) but it can use flat combining to aggregate requests from multiple writers and thus have data locality without causing too much contention on the writer lock. This data locality allows for a "flat" throughput of the write operations, and in the case of Persistent Memory (Romulus) is can even have high positive scaling due to saving writes to PM.

If you want to see more details on Left-Right you check out this ppt:
or see my cppcon video on this topic:

Because Left-Right is a generic concurrency control, it can be used to make a Universal Construction
or an STM
both approaches imply blocking yet starvation-free writes, and wait-free population oblivious reads.
Read-only operations can execute in parallel with the write operation because they're accessing opposite replicas.

When a user decides to utilize the Left-Right UC, all it has to do is place the user-code in a lambda and pass it to the Left-Right UC.
The code in the lambda has some minor restrictions, shared in common with all other generic approaches that give wait-freedom for reads:
- No side effects in the lambda: can't modify global variables or thread-locals;
- Deterministic code: no access to random number generator devices or random functions, unless it's a pseudo-random generator and the seed is passed to the lambda;
- No I/O; no writing to files, no sending/receiving of network packets, no hw registers being written... actually all of this can be done but then you have to use an STM version (like Romulus) which needs load and store interposing
- No exceptions: any exceptions must be catch inside the lambda (user-code). For the read-only operations it can be made such that this is not a problem (implementation detail) but for the mutative operations this is an algorithmic constraint;
Operations in Left-Right are irrevocable (they don't abort-retry) which is a nice feature for most user code.

The bad news: it uses twice as much memory. Like all good things, there are trade-offs and the wait-freedom for readers has a cost, the memory usage.

Remember the issue about cross-dependencies (write skews) from the previous post ? Left-Right doesn't have such an issue because the synchronization is implicitly a global lock. Yes, yes, it serializes mutative operations, but remember, read-only operations are wait-free and go in parallel with the mutations. As long as your workload is read-dominated, Left-Right is a good trade-off.

Another nice feature is that memory reclamation works just like inside a lock: you just call malloc/free/new/delete and it works. No need to use epoch-based reclamation explicitly (it's already implicit in the usage of the userspace RCU inside Left-Right).

On the next post we're going to talk about a fully wait-free Universal Construction.

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