Comment by doctor_eval | Hacker News Reader

Comment by doctor_eval | Hacker News Reader

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It’s hard to understand how I can miss my own point... yes, I know that we are not simply going back and forth; I did read the article. I know that the new implementation of green threads is more sophisticated than the original implementation.

But I find the cycle - what I called the hype cycle - from internal scheduling to external scheduling and back again, interesting, and I wonder what, if anything, we as an industry can learn from this?

ISTM that Java 1.2 could have improved on green threads instead of moving to os threads. So, is there something we can learn from these two transitions that will help us all make better decisions in the future? The use of OS threads and all the complexity that this has caused has cost the industry hundreds of thousands of hours of developer time. If we can learn some lessons from this then isn’t that a good thing?

I don't think this is a matter of hype cycle at all. There are two things that changed:

1. Threading got much faster and lightweight. This is what Java was initially trying to work around, until it didn't have to any more.

2. The problem moved to handling as many sockets concurrently as possible. Even lightweight system threads are too heavy for scaling linearly with the number of connections (too much context-switching overhead, too much space for stack, etc.)

Green-threading has become a good idea again because we now have a kernel API that is used to multiplex a lot (but not all) I/O systemcalls.

Today Go runtime uses epoll/kqueue to read from a big bunch of sockets, whenever something new happens to any of them. This takes one system thread only.

The API model of epoll/kqueue implies some way to handle concurrency in your user code: this can either be callbacks (or async/await syntactic sugar) or green threads and CSP (channels and so on.) This is why green threading is having a comeback.

(Sorry for implying you did not read the article!)

That's OK, lots of people fail to RTFA, but I really like reading about this stuff.

> Green-threading has become a good idea again because we now have a kernel API that is used to multiplex a lot (but not all) I/O systemcalls.

OK, that makes a lot of sense. I had to read up about how epoll is different from select/poll (that's how long it's been since I worked in C :). Clearly epoll was needed to make green threads efficient, but from what I can see, by the time epoll and friends were widespread, the pthread model was entrenched in Java.

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