On Monday, November 3, 2014 10:44:19 PM UTC-6, Atamert Ölçgen wrote:
>
> Thanks Alex!
>
> Now that I took a second look at Daniel's code, it seems assoc! is used
> like swap!, as if it would modify m in place. So I would expect, if it runs
> without errors, result to be {:a 0}.
>
Right, that is an incorrect usage - it will actually modify with changes
though, but not in expected ways (this is independent of the change we're
discussing - you can get the same behavior in a single thread modifying a
transient without reusing the return).
Given that transients are values (not reference types like ref or atom) I
> can't think of a case where they can be modified concurrently.
>
You can, but not without going pretty far out of normal Clojure code.
>
>
> On Tue, Nov 4, 2014 at 11:19 AM, Alex Miller <[email protected]
> <javascript:>> wrote:
>
>>
>>
>> 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 <[email protected]>
>>> 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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>> <javascript:>
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>
>
>
> --
> Kind Regards,
> Atamert Ölçgen
>
> -+-
> --+
> +++
>
> www.muhuk.com
>
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