On 07/03/2017 03:12 PM, Vitalie Spinu wrote:
On Monday, 3 July 2017 22:48:40 UTC+2, red...@gmail.com wrote:
Discussion of locks aside, doing blocking operations like io or <!! or
>!! or basically anything that looks like it blocks and isn't >!
or <!
is a very bad idea in a transducer on a channel. You will (eventually)
block the threadpool and get deadlocks.
Is this the limitation in general or only when I use the chan with
blocking transducer withing a go block? Transducers are not run on a go
block, are they?
It is kind of complicated, and I haven't traced through everything in
channels.clj recently, but assume transducers are not magic, the
computation has to occur on some real thread some where, and the best
threads available to do that are the consumer thread and the producer
thread. If those threads are both executing go blocks, and you have a
transducer doing blocking stuff on a channel they operate on, you may
not be technically "blocking" in the go block, but the channel machinery
is now blocking before it yields control back to the go block. Putting
blocking operations in a transducer then operating on the channel from
go blocks turns `!>` and '<!' from operations that don't block threads
(just park go blocks), in to operations that do block a whole OS thread,
of which the core.async threadpool that go blocks execute on only has 8
by default.
There is this sort of dance where control is handed back and forth
between channels and go blocks without tying up a thread. When you
introduce blocking in to the channel machinery (by making the channel
execute a blocking operation as things are produced or consumed) then
the dance stops and the thread waits.
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And what is not good—
Need we ask anyone to tell us these things?
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