On Monday, November 3, 2014 9:00:10 PM UTC-6, Atamert Ölçgen wrote:
>
>
>
> On Mon, Nov 3, 2014 at 5:57 PM, Daniel Marjenburgh <dmarje...@gmail.com
> <javascript:>> wrote:
>
>> Hi,
>>
>> I just want to address this issue (CLJ-1498
>> <http://dev.clojure.org/jira/browse/CLJ-1498>). It was accepted in
>> 1.7-alpha2 and I haven't seen a lot of discussion around it, even though
>> it's quite a big change.
>>
>> With this change the following code is possible:
>>
>
> With persistents the result would be the same, every time. If this is now
> valid Clojure, I didn't run it myself, we are sacrificing consistency. I
> don't understand what we're getting in return.
>
> Even the simple example in the ticket (with one future) doesn't make a lot
> of sense to me.
>
> Am I missing something obvious?
>
Transients always expect thread isolation. In the past this was locked to a
single thread (the one that made the transient). That restriction now
extends to being used by multiple threads, but isolation should still be
maintained for proper use.
What we are gaining is the ability to use transient collections in go
blocks with core.async, where the thread being used is just one pool out of
a thread. For example (just typing this not running it, so excuse any
typos):
(defn drain [ch coll]
(let [t (transient []]
(go-loop
(if-some [v (<! ch)]
(do (conj! t v) (recur))
(persistent! t)))))
This doesn't necessarily work in core.async because each time the <! parks,
it may be woken up with a different thread from the pool. The transient
change allows this kind of code to succeed.
>
>> (let [m (transient {:a 0})
>> futs (for [n (range 100)]
>> (future (assoc! m :a (inc (:a m)))))]
>> (mapv deref futs) ; wait until futures are done
>> (persistent! m))
>>
>>
>> The results will vary per run, where it used to throw
>> an IllegalAccessError: "Transient used by non-owner thread".
>>
>> I understand the problems of not being able to have 1 go routine access a
>> transient, even though it would be safe, but this solution feels like it's
>> throwing out the baby with the bathwater. Basically, it's doing away with
>> what the following block on clojure.org <http://clojure.org/transients>
>> says:
>>
>> *Transients enforce thread isolation**.* Because each result of a
>>> transient operation shares (mutable) structure with the previous, it would
>>> be very dangerous if more than one thread were to manipulate a transient at
>>> once. In order to prevent this, transients will detect any (read or write)
>>> use from a thread other than the one that created them and throw an
>>> exception.
>>
>>
>>
>>>
>>> This may not sound like a concurrency story, but single-thread isolation
>>> is actually a very useful concurrency semantic. The whole point of using
>>> transients is that doing so is safe, because their use is an isolated
>>> implementation detail of otherwise functional code. Having that be enforced
>>> means that some things that would normally be very hard to make safe with
>>> ordinary mutable data structures become easy.
>>
>>
>> I don't have a good solution for dealing with transients and logical
>> threads, but I would much prefer keeping the semantics of transients as
>> they are and maybe pass an option to transient to disable owner checking.
>>
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>
>
>
> --
> Kind Regards,
> Atamert Ölçgen
>
> -+-
> --+
> +++
>
> www.muhuk.com
>
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