You are absolutely right. Sorry, for distraction. Deleted my message
immediately after I realized that, but it turns out it managed to go
already.
суббота, 30 августа 2014 г., 16:25:21 UTC+4 пользователь Colin Fleming
написал:
>
> I'm not sure I understand - the contention will be the same whether or not
> the value is calculated in the swap! call, right? And if I calculate the
> value and then pass the calculated value to swap!, that value will be
> thrown away every time the cache contains an existing value for my key.
>
>
>
> On 30 August 2014 23:13, Eldar Gabdullin <elda...@gmail.com <javascript:>>
> wrote:
>
>> 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> 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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>>>
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>
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