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by armitron 2863 days ago
The mantra that shared memory parallelism is hard to get right to the point where such platitudes as "unless you're writing some really really really niche thing" are uttered is entirely erroneous I find, through my own experience.

There are idiot-proof thread-safe datastructures and producer/consumer APIs that map extremely well to most problems that come up in practice in the domain, that one should confidently use. Refusing to do shared memory parallelism because of the _abstract potential for havoc_ rather than any practical justifications based on the problem-at-hand is throwing out the baby with the bathwater and is not the mark of competent engineering.
2 comments
devxpy 2863 days ago
This talk (hopefully) conveys my point across

https://www.youtube.com/watch?v=9zinZmE3Ogk

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devxpy 2863 days ago
You must be some sort of programming GOD, I guess.

The problem is that its _hard_ to get right.

For example - It's not trivial to use locks when you're working at an abstraction level higher than operating systems. Most people don't even realise there is a race in their application, because locks are inherently non-enforcing. Code written in locks is also really hard to read and reason by.

Message passing just makes it a little more trivial to avoid the pitfalls associated with parallel programming.

I also found that it lets you avoid busy waiting in certain places, which is always a performance advantage :)

Can you shed some light on those "idiot-proof thread-safe datastructures"?

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twblalock 2863 days ago
I do concurrency in Java all the time with CompletableFuture and threadsafe data structures provided by various libraries, e.g. the Guava caches, and I rarely need to use locks or semaphores. It's a good set of abstractions that make concurrency pretty close to idiot-proof.

Futures in particular make it easy to write concurrent code close to the way you would write single-threaded code, because all of the threading is handled behind the scenes.

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TickleSteve 2863 days ago
busy-waiting is a valid technique for some use-cases (and gives better performance in those situations) than other techniques.

Please research your topic.

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devxpy 2863 days ago
Yes, but isn't it more CPU intensive?

(Speaking purely from experience. Don't have a fancy CS degree)

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TickleSteve 2862 days ago
It uses 100% CPU, true but when the duration of the lock is extremely small (i.e. nanoseconds->microseconds) the total CPU usage is less than arranging for an OS level context-switch. In other words, you use it when synchronising with hardware or when implementing test-and-set primitives for higher level mechanisms. Crucially, the time that the lock is held for must be very short.

Given those restrictions and use cases you get a very efficient low latency locking mechanism.

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