core.async threadpool size

[icamts]

Hi all,

quite a funny question. Thread pool size in core.async is twice the number of processors plus 42.

https://github.com/clojure/core.async/blob/master/src/main/clojure/clojure/core/async/impl/exec/threadpool.clj 

42? The question is:

1) should it be 4 or 2?

2) 42 is the response to life, universe and everything

3) for some technical reason 42 is also a good minimum number for a thread pool

Luca

[Ben Mabey]

I asked Timothy Baldridge about this on IRC when the change was made that added 42 and this was the reply I got:

   we're trying to strike a balance there, we don't want people to do IO inside a go block, but if they have to we'd don't want it to totally stall out.

Here is the full conversation:

http://clojure-log.n01se.net/date/2013-08-29.html#15:45a

-Ben

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[icamts]

Thank you Ben.

Can we speculate on a go-off macro? Like in agents send / send-off to have go-off blocks executed by a cached thread pool executor?

Luca

[Ben Mabey]

If you want or need to block in a go-block then you should use the `thread` macro instead of the `go` macro.  A thread macro block will be ran in a cached thread pool as you seem to be wanting:

https://github.com/clojure/core.async/blob/master/src/main/clojure/clojure/core/async.clj#L379

Just note that you will need to use the blocking CSP operator variants, e.g. <!! instead of <!, while inside of `thread`.

-Ben

[icamts]

Thank you again Ben, now it is more clear.

Luca