[erlang-questions] What is the best type as key of mnesia table?
Wed Jan 9 16:56:21 CET 2013
Q0: there is not a single best key type, and you are on the right track to
find the one that is best suited for your scenario: do a lot of experiments
and measurements :) Personally I would advise against using atoms when you
use a lot of dynamically generated keys, they are not designed for that.
Q1: the numbers you quote are the sizes for in-memory representation. Atoms
use 1 word in memory because there is a lookup table for them. When you
write the data to disc, all characters have to be stored, and the
difference between these types will be small.
On Wednesday, January 9, 2013 8:27:53 AM UTC+1, nanun wrote:
> I used _list_ as key of table.
> Now I need more faster and lighter so I tested some.
> Before I tested, I think that _atom_ is the fastest and smaller mnesia
> And I thought that _binary_ is the best candidate because _atom_ has issue
> of garbage collection.
> But test results make me confused.
> Q0. What is the best type as key?
> == test results ==
> There are three candidates; list, binary and atom.
> 100k keys;
> list keys are "k1000001" ~ "k1100000",
> binary keys are <<"k1000001">> ~ <<"k1100000">>,
> atom keys are k1000001 ~ k1100000.
> I tested both mnesia types; disc_copy and disc_only_copy
> Q1. Mnesia file sizes are strange, what's wrong?
> - disc_copy
> size table_type
> 3100089 atom.DCD
> 3300089 binary.DCD
> 3100089 list.DCD
> - disc_only_copy
> size table_type
> 5391992 list_o.DAT
> 6489808 binary_o.DAT
> 6130276 atom_o.DAT
> * Every key has same value.
> * checked the sizes after q().
> accoding to http://erlang.org/doc/efficiency_guide/advanced.html
> list : 1 word + 2 words * 8(length of a list) = 17 words
> binary : 3..6 + 8(length of a list) = 11 ~ 14 words
> atom : 1 word
> so I thought the size order will list > binary >> atom.
> Q2. Why atom is not fastest? atom is sometimes the slowest.
> atom read : 61.095
> binary read : 65.729
> list read : 84.162
> atom write : 858.18
> binary write : 913.092
> list write : 861.357
> atom_o read : 1657.427
> binary_o read : 1453.002
> list_o read : 1482.138
> atom_o write : 1765.436
> binary_o write : 2261.9
> list_o write : 2289.084
> * read and write every tables(100k)
> * tried to avoid difference of period to get a key.
> * milli seconds
> * I think that the best type is _integer_, but I want use one of them.
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