Ahh that makes a lot of sense. Indeed, I'm guilty of doing a blocking >!!
inside a go-block. I was so careful to avoid other kinds of blocking calls
(like IO) that I forgot that blocking variants of core.async calls
themselves were forbidden.
Thank you for pointing this out! I will rewire things to not do this.
Per Gary's suggestion, I also think it'd be useful if core.async blocking
ops checked a dynamic var (or a property of the thread itself) and at least
warned if they are being called from a forbidden context. To resolve my
original issue, I'm considering doing this in my dev environment:
(doseq [v '[<!! >!!]]
(alter-var-root (ns-resolve 'clojure.core.async v)
(fn [f]
(fn [& args]
(if (.startsWith (.getName (Thread/currentThread))
"async-dispatch-")
(throw (Exception. (str v " called inside
async-dispatch")))
(apply f args))))))
On Tuesday, August 29, 2017 at 1:43:53 PM UTC-4, Gary Trakhman wrote:
>
> Hm, I came across a similar ordering invariant (No code called by a go
> block should ever call the blocking variants of core.async functions) while
> wrapping an imperative API, and I thought it might be useful to use
> vars/binding to enforce it. Has this or other approaches been considered
> in core.async? I could see a *fixed-thread-pool* var being set and >!!
> checking for false.
>
> An analogy in existing clojure.core would be the STM commute's 'must be
> running in a transaction' check that uses a threadlocal.
> https://github.com/clojure/clojure/blob/master/src/jvm/clojure/lang/LockingTransaction.java#L205
>
> On Tue, Aug 29, 2017 at 1:30 PM Timothy Baldridge <tbald...@gmail.com
> <javascript:>> wrote:
>
>> To add to what Alex said, look at this trace:
>> https://gist.github.com/anonymous/65049ffdd37d43df8f23630928e8fed0#file-thread-dump-out-L1337-L1372
>>
>> Here we see a go block calling mapcat, and inside the inner map something
>> is calling >!!. As Alex mentioned this can be a source of deadlocks. No
>> code called by a go block should ever call the blocking variants of
>> core.async functions (<!!, >!!, alts!!, etc.). So I'd start at the code
>> redacted in those lines and go from there.
>>
>>
>>
>> On Tue, Aug 29, 2017 at 11:09 AM, Alex Miller <al...@puredanger.com
>> <javascript:>> wrote:
>>
>>> go blocks are multiplexed over a thread pool which has (by default) 8
>>> threads. You should never perform any kind of blocking activity inside a go
>>> block, because if every go block in work happens to end up blocked, you
>>> will prevent all go blocks from making any further progress. It sounds to
>>> me like that's what has happened here. The go block threads are named
>>> "async-dispatch-<n>" and it looks like there are 8 blocked ones in your
>>> thread dump.
>>>
>>> It also looks like they are all blocking on a >!!, which is a blocking
>>> call. So I would look for a go block that contains a >!! and convert that
>>> to a >! or do something else to avoid blocking there.
>>>
>>>
>>> On Tuesday, August 29, 2017 at 11:48:25 AM UTC-5, Aaron Iba wrote:
>>>>
>>>> My company has a production system that uses core.async extensively.
>>>> We've been running it 24/7 for over a year with occasional restarts to
>>>> update things and add features, and so far core.async has been working
>>>> great.
>>>>
>>>> The other day, during a particularly high workload, the whole system
>>>> got locked up. All the channels seemed blocked at once. I was able to
>>>> connect with a REPL and poke around, and noticed strange behavior of
>>>> core.async. Specifically, the following code, when evaluated in the REPL,
>>>> blocked on the put (third expression):
>>>>
>>>> (def c (async/chan))
>>>> (go-loop []
>>>> (when-some [x (<! c)]
>>>> (println x)
>>>> (recur)))
>>>> (>!! c true)
>>>>
>>>> Whereas on any fresh system, the above expressions obviously succeed.
>>>>
>>>> Puts succeeded if they went onto the channel's buffer, but not when
>>>> they should go through to a consumer. For example with the following
>>>> expressions, evaluated in the REPL, the first put succeeded (presumably
>>>> because it went on the buffer), but subsequent puts blocked:
>>>>
>>>> (def c (async/chan 1))
>>>> (def m (async/mult c))
>>>> (def out (async/chan (async/sliding-buffer 3)))
>>>> (async/tap m out)
>>>> (>!! c true) ;; succeeds
>>>> (>!! c true) ;; blocks forever
>>>>
>>>> This leads me to wonder if core.async itself somehow got into a bad
>>>> state. It's entirely possible I caused this by misusing the API somewhere
>>>> in the codebase, but we use core.async so extensively that I wouldn't know
>>>> where to begin looking.
>>>>
>>>> I'm wondering if someone more familiar with core.async internals has an
>>>> idea about what could cause the above situation. Or if we notice it
>>>> happening again, what could I do to gather more helpful information.
>>>>
>>>> I also have a redacted thread dump, in case it's useful:
>>>>
>>>> https://gist.github.com/anonymous/65049ffdd37d43df8f23630928e8fed0
>>>>
>>>> Any help would be much appreciated,
>>>>
>>>> Aaron
>>>>
>>>> P.S. core.async has been a godsend in terms of helping us structure and
>>>> modularize our large system. Thank you to all those who contributed to
>>>> this wonderful library!
>>>>
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>>
>>
>>
>> --
>> “One of the main causes of the fall of the Roman Empire was that–lacking
>> zero–they had no way to indicate successful termination of their C
>> programs.”
>> (Robert Firth)
>>
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