2 The Eight Myths of Erlang Performance
Some truths seem to live on well beyond their best-before date, perhaps because "information" spreads faster from person-to-person than a single release note that says, for example, that funs have become faster.
This section tries to kill the old truths (or semi-truths) that have become myths.
Funs used to be very slow, slower than apply/3. Originally, funs were implemented using nothing more than compiler trickery, ordinary tuples, apply/3, and a great deal of ingenuity.
But that is history. Funs was given its own data type in R6B and was further optimized in R7B. Now the cost for a fun call falls roughly between the cost for a call to a local function and apply/3.
List comprehensions used to be implemented using funs, and in the old days funs were indeed slow.
Nowadays, the compiler rewrites list comprehensions into an ordinary recursive function. Using a tail-recursive function with a reverse at the end would be still faster. Or would it? That leads us to the next myth.
According to the myth, recursive functions leave references to dead terms on the stack and the garbage collector has to copy all those dead terms, while tail-recursive functions immediately discard those terms.
That used to be true before R7B. In R7B, the compiler started to generate code that overwrites references to terms that will never be used with an empty list, so that the garbage collector would not keep dead values any longer than necessary.
Even after that optimization, a tail-recursive function is still most of the times faster than a body-recursive function. Why?
It has to do with how many words of stack that are used in each recursive call. In most cases, a recursive function uses more words on the stack for each recursion than the number of words a tail-recursive would allocate on the heap. As more memory is used, the garbage collector is invoked more frequently, and it has more work traversing the stack.
In R12B and later releases, there is an optimization that in many cases reduces the number of words used on the stack in body-recursive calls. A body-recursive list function and a tail-recursive function that calls lists:reverse/1 at the end will use the same amount of memory. lists:map/2, lists:filter/2, list comprehensions, and many other recursive functions now use the same amount of space as their tail-recursive equivalents.
So, which is faster? It depends. On Solaris/Sparc, the body-recursive function seems to be slightly faster, even for lists with a lot of elements. On the x86 architecture, tail-recursion was up to about 30% faster.
So, the choice is now mostly a matter of taste. If you really do need the utmost speed, you must measure. You can no longer be sure that the tail-recursive list function always is the fastest.
A tail-recursive function that does not need to reverse the list at the end is faster than a body-recursive function, as are tail-recursive functions that do not construct any terms at all (for example, a function that sums all integers in a list).
The ++ operator has, somewhat undeservedly, got a bad reputation. It probably has something to do with code like the following, which is the most inefficient way there is to reverse a list:
naive_reverse([H|T]) -> naive_reverse(T)++[H]; naive_reverse() -> .
As the ++ operator copies its left operand, the result is copied repeatedly, leading to quadratic complexity.
But using ++ as follows is not bad:
naive_but_ok_reverse([H|T], Acc) -> naive_but_ok_reverse(T, [H]++Acc); naive_but_ok_reverse(, Acc) -> Acc.
Each list element is copied only once. The growing result Acc is the right operand for the ++ operator, and it is not copied.
Experienced Erlang programmers would write as follows:
vanilla_reverse([H|T], Acc) -> vanilla_reverse(T, [H|Acc]); vanilla_reverse(, Acc) -> Acc.
This is slightly more efficient because here you do not build a list element only to copy it directly. (Or it would be more efficient if the compiler did not automatically rewrite [H]++Acc to [H|Acc].)
String handling can be slow if done improperly. In Erlang, you need to think a little more about how the strings are used and choose an appropriate representation. If you use regular expressions, use the re module in STDLIB instead of the obsolete regexp module.
The repair time is still proportional to the number of records in the file, but Dets repairs used to be much slower in the past. Dets has been massively rewritten and improved.
BEAM is a register-based virtual machine. It has 1024 virtual registers that are used for holding temporary values and for passing arguments when calling functions. Variables that need to survive a function call are saved to the stack.
BEAM is a threaded-code interpreter. Each instruction is word pointing directly to executable C-code, making instruction dispatching very fast.
That was once true, but from R6B the BEAM compiler can see that a variable is not used.