I see...
Just in case, I'd like to point out one possible deficiency in your
original snippet. Generally, it should be better move value calculation
outside of swap call, thus
reducing a chance of contention, especially when you have many threads
asking for different keys.
суббота, 30 августа 2014 г., 14:09:42 UTC+4 пользователь Colin Fleming
написал:
>
> Yes, in my case the update is atomic but doesn't strictly avoid
> recomputation (since the Atom can call the calculation function many
> times). I've given this some hammock time and I don't think that's a
> problem. However on further investigation I do have to ensure that for each
> of my keys the value that is returned is identical since it uses identity
> equality semantics, so I might have to stick with my original solution.
> Thanks for the input though, it's very interesting to see your solution.
>
>
> On 30 August 2014 19:54, Eldar Gabdullin <elda...@gmail.com <javascript:>>
> wrote:
>
>> Snippet you showed before is also not an atomic. If you want strictly to
>> avoid recomputations you need something more elaborate.
>> May be this
>>
>> (def cache (atom {}))
>>
>>
>> (defn unwrap [v]
>> (if-let [a (get v ::atom)]
>>
>> @a
>> v))
>>
>> (defn compute [cache k]
>>
>> (let [p {::atom (atom nil)}
>>
>> c (swap! cache assoc k p)
>>
>> val (get c k)]
>> (when (identical? val p)
>>
>> (swap! (::atom p) (fn [_] (calc-value k)))
>>
>> (swap! cache assoc k @(::atom p)))
>>
>> (unwrap val)))
>>
>> (defn lookup [cache k]
>>
>> (let [v (get @cache k ::nil)]
>>
>> (if (= v ::nil)
>>
>> (compute cache k)
>> (unwrap v))))
>>
>>
>>
>> суббота, 30 августа 2014 г., 11:18:51 UTC+4 пользователь Colin Fleming
>> написал:
>>>
>>> True, but only if you don't mind possibly calculating the value more
>>> than once since the update is not atomic.
>>>
>>>
>>> On 30 August 2014 18:31, Eldar Gabdullin <elda...@gmail.com> wrote:
>>>
>>>> Something like the following would be fine for me
>>>>
>>>> (def cache (atom {}))
>>>>
>>>> (defn lookup [cache k]
>>>>
>>>>
>>>>
>>>> (let [v (get @cache k ::nil)]
>>>>
>>>>
>>>>
>>>> (if (= v ::nil)
>>>>
>>>>
>>>>
>>>> (let [v (calc-value k)]
>>>>
>>>>
>>>>
>>>> (swap! cache assoc k v)
>>>>
>>>>
>>>>
>>>> v)
>>>> v)))
>>>>
>>>> (let [value (lookup cache k)]
>>>>
>>>>
>>>>
>>>> ; use value and @cache here
>>>> )
>>>>
>>>>
>>>> суббота, 30 августа 2014 г., 9:27:05 UTC+4 пользователь Colin Fleming
>>>> написал:
>>>>
>>>>> Hi all,
>>>>>
>>>>> I want to use a map to cache values based on a key. I'm planning to
>>>>> use an atom for this. My basic operation is "give me the value for this
>>>>> key" - if the value exists in the map then that value should be returned,
>>>>> otherwise a new value should be calculated, inserted in the map and then
>>>>> returned. My plan is to implement something like the following:
>>>>>
>>>>>
>>>>> (defn ensure [cache key] (if (contains? cache key) cache (assoc
>>>>> cache key (calc-value key))))(let [value (get (swap! cache ensure key)
>>>>> key)] ... do my thing with value ...)
>>>>>
>>>>>
>>>>> So 'ensure' ensures that the cache contains the value for key, the
>>>>> swap! operation returns the cache with the value and then I get it out.
>>>>> This works but feels a little clumsy, is there a better way to do this?
>>>>>
>>>>> Also, looking at the Atom source code, I see that this will cause a
>>>>> CAS operation even if the value returned from swap! is identical to the
>>>>> original value. It seems like a reasonable optimisation would be to check
>>>>> if the values are identical and not update if so - is there a reason this
>>>>> might not be a good idea?
>>>>>
>>>>> Thanks,
>>>>> Colin
>>>>>
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>
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