[erlang-questions] Erlang Cost Model
Theepan
vasdeveloper@REDACTED
Thu Sep 17 18:36:40 CEST 2015
ETS - O(1)
Diagraph - O(1)
Theepan
On Thu, Sep 17, 2015 at 9:26 PM, Fred Hebert <mononcqc@REDACTED> wrote:
> On 09/17, Eric des Courtis wrote:
>
>> Regardless if concurrency and parallelism plays a role *I still need to
>> know if an operation in a sequential process I am working on is O(1) or
>> O(n)* or am I missing something?
>>
>
> These values are way more worthwhile as a property of libraries than
> language, since they're far more impacted by the choice of algorithm taken
> there.
>
> Let's look at a few modules:
>
> 1. Queue
>
> "all operations has an amortized O(1) running time, except len/1, join/2,
> split/2, filter/2 and member/2 that have O(n). To minimize the size of a
> queue minimizing the amount of garbage built by queue operations, the
> queues do not contain explicit length information, and that is why len/1 is
> O(n)."
>
>
> 2. Maps
>
> undefined in the module doc, but from the EEP (
> http://www.erlang.org/eeps/eep-0043.html):
>
> "has at most O(log N) time complexity in insert and lookup operations,
> where N is the number of key-value associations."
>
> 3. Dict
>
> undefined in docs. I know that it is a very flat tree (bunch of buckets)
> acting as a hash map, so O(log N)
>
> 4. Array
>
> undefined in docs. Also a very flat tree. O(Log N), where N is the size of
> the array, and if I recall, the tree has a branching factor of 10 to
> compromise between the speed of lookups and garbage generated when
> modifying it. It's described in the source only, sadly.
>
> 5. Gb_trees
>
> Doc compares them to AVL trees, without storage overhead and with better
> performance. O(log N) it is (those are the worst cases for AVL trees)
>
> 6. Sets
>
> They're using the same mechanism as dicts, so O(Log N)
>
> 7. Gb_sets
>
> Same mechanism as gb_trees, so O(log N)
>
> 8. ETS
>
> Doc says: "These provide the ability to store very large quantities of
> data in an Erlang runtime system, and to have constant access time to the
> data. (In the case of ordered_set, see below, access time is proportional
> to the logarithm of the number of objects stored)."
>
> 9. Orddict
>
> Doc says " An orddict is a representation of a dictionary, where a list of
> pairs is used to store the keys and values. The list is ordered after the
> keys."
>
> lists are O(N) traversal and insertion, so let's consider this the best
> case.
>
> 10. ordsets
>
> implemented over orddict, O(N) too.
>
> 11. sofs
>
> despite having the most cryptic documentation, it does mention: "The
> execution time of the functions of this module is dominated by the time it
> takes to sort lists. When no sorting is needed, the execution time is in
> the worst case proportional to the sum of the sizes of the input arguments
> and the returned value. A few functions execute in constant time:
> from_external, is_empty_set, is_set, is_sofs_set, to_external, type."
>
> O(N log N) for most (optimal sort time, where N = sum(Input)), with a list
> of O(1) functions
>
> 12. Digraph
>
> Unspecified, no idea
>
> 13. lists:key* functions
>
> using lists, so O(N), but they're fast given they're implemented in C
>
> 14. Proplists
>
> Using lists, so O(N).
>
>
>
> Now with these modules somewhat covered (there's a lot of subtleties in
> the APIs making some better than others depending on the access patterns),
> it's interesting to look at basic operations we may do a lot, for example:
>
> 1. Message receiving (non-selective)
>
> O(1), though accessing the mailbox when it's very large can trigger GCs
> and mess up expectations a bit.
>
> 2. Message receiving (selective)
>
> O(N) selection where N is the mailbox size. An optimization existsw for
> common access patterns where a reference (with make_ref()) is created in
> the scope of the receive and used in all clauses, which truncates the
> existing mailbox, therefore reducing N.
>
> 3. Sorting (lists:sort)
>
> O(N log N), performing a fancypants merge sort
>
>
>
> So yeah, for a lot of data structures, the information is already there.
> For many, it's also still missing.
>
> Regards,
> Fred.
>
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