diff options
Diffstat (limited to 'system/doc/efficiency_guide/myths.xml')
| -rw-r--r-- | system/doc/efficiency_guide/myths.xml | 128 |
1 files changed, 64 insertions, 64 deletions
diff --git a/system/doc/efficiency_guide/myths.xml b/system/doc/efficiency_guide/myths.xml index b1108dbab2..70d2dae88e 100644 --- a/system/doc/efficiency_guide/myths.xml +++ b/system/doc/efficiency_guide/myths.xml @@ -31,47 +31,48 @@ <file>myths.xml</file> </header> + <marker id="myths"></marker> <p>Some truths seem to live on well beyond their best-before date, - perhaps because "information" spreads more rapidly from person-to-person - faster than a single release note that notes, for instance, that funs + perhaps because "information" spreads faster from person-to-person + than a single release note that says, for example, that funs have become faster.</p> - <p>Here we try to kill the old truths (or semi-truths) that have + <p>This section tries to kill the old truths (or semi-truths) that have become myths.</p> <section> - <title>Myth: Funs are slow</title> - <p>Yes, funs used to be slow. Very slow. Slower than <c>apply/3</c>. + <title>Myth: Funs are Slow</title> + <p>Funs used to be very slow, slower than <c>apply/3</c>. Originally, funs were implemented using nothing more than compiler trickery, ordinary tuples, <c>apply/3</c>, and a great deal of ingenuity.</p> - <p>But that is ancient history. Funs was given its own data type - in the R6B release and was further optimized in the R7B release. - Now the cost for a fun call falls roughly between the cost for a call to - local function and <c>apply/3</c>.</p> + <p>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 <c>apply/3</c>.</p> </section> <section> - <title>Myth: List comprehensions are slow</title> + <title>Myth: List Comprehensions are Slow</title> <p>List comprehensions used to be implemented using funs, and in the - bad old days funs were really slow.</p> + old days funs were indeed slow.</p> - <p>Nowadays the compiler rewrites list comprehensions into an ordinary - recursive function. Of course, using a tail-recursive function with + <p>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.</p> </section> <section> - <title>Myth: Tail-recursive functions are MUCH faster - than recursive functions</title> + <title>Myth: Tail-Recursive Functions are Much Faster + Than Recursive Functions</title> <p><marker id="tail_recursive"></marker>According to the myth, recursive functions leave references - to dead terms on the stack and the garbage collector will have to - copy all those dead terms, while tail-recursive functions immediately + 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.</p> <p>That used to be true before R7B. In R7B, the compiler started @@ -79,48 +80,47 @@ be used with an empty list, so that the garbage collector would not keep dead values any longer than necessary.</p> - <p>Even after that optimization, a tail-recursive function would - still most of the time be faster than a body-recursive function. Why?</p> + <p>Even after that optimization, a tail-recursive function is + still most of the times faster than a body-recursive function. Why?</p> <p>It has to do with how many words of stack that are used in each - recursive call. In most cases, a recursive function would use more words + 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. Since more memory is used, the garbage - collector will be invoked more frequently, and it will have more work traversing + 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.</p> - <p>In R12B and later releases, there is an optimization that will + <p>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, so that a body-recursive list function and + body-recursive calls. A body-recursive list function and a tail-recursive function that calls <seealso - marker="stdlib:lists#reverse/1">lists:reverse/1</seealso> at the - end will use exactly the same amount of memory. + marker="stdlib:lists#reverse/1">lists:reverse/1</seealso> at + the end will use the same amount of memory. <c>lists:map/2</c>, <c>lists:filter/2</c>, list comprehensions, and many other recursive functions now use the same amount of space as their tail-recursive equivalents.</p> - <p>So which is faster?</p> + <p>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.</p> - <p>It depends. On Solaris/Sparc, the body-recursive function seems to - be slightly faster, even for lists with very many elements. On the x86 - architecture, tail-recursion was up to about 30 percent faster.</p> - - <p>So the choice is now mostly a matter of taste. If you really do need + <p>So, the choice is now mostly a matter of taste. If you really do need the utmost speed, you must <em>measure</em>. You can no longer be - absolutely sure that the tail-recursive list function will be the fastest - in all circumstances.</p> + sure that the tail-recursive list function always is the fastest.</p> - <p>Note: A tail-recursive function that does not need to reverse the - list at the end is, of course, faster than a body-recursive function, + <note><p>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 instance, a function that sums all integers in a list).</p> + (for example, a function that sums all integers in a list).</p></note> </section> <section> - <title>Myth: '++' is always bad</title> + <title>Myth: Operator "++" is Always Bad</title> - <p>The <c>++</c> operator has, somewhat undeservedly, got a very bad reputation. - It probably has something to do with code like</p> + <p>The <c>++</c> 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:</p> <p><em>DO NOT</em></p> <code type="erl"> @@ -129,12 +129,10 @@ naive_reverse([H|T]) -> naive_reverse([]) -> [].</code> - <p>which is the most inefficient way there is to reverse a list. - Since the <c>++</c> operator copies its left operand, the result - will be copied again and again and again... leading to quadratic - complexity.</p> + <p>As the <c>++</c> operator copies its left operand, the result + is copied repeatedly, leading to quadratic complexity.</p> - <p>On the other hand, using <c>++</c> like this</p> + <p>But using <c>++</c> as follows is not bad:</p> <p><em>OK</em></p> <code type="erl"> @@ -143,11 +141,11 @@ naive_but_ok_reverse([H|T], Acc) -> naive_but_ok_reverse([], Acc) -> Acc.</code> - <p>is not bad. Each list element will only be copied once. + <p>Each list element is copied only once. The growing result <c>Acc</c> is the right operand - for the <c>++</c> operator, and it will <em>not</em> be copied.</p> + for the <c>++</c> operator, and it is <em>not</em> copied.</p> - <p>Of course, experienced Erlang programmers would actually write</p> + <p>Experienced Erlang programmers would write as follows:</p> <p><em>DO</em></p> <code type="erl"> @@ -156,32 +154,34 @@ vanilla_reverse([H|T], Acc) -> vanilla_reverse([], Acc) -> Acc.</code> - <p>which is slightly more efficient because you don't build a - list element only to directly copy it. (Or it would be more efficient - if the the compiler did not automatically rewrite <c>[H]++Acc</c> + <p>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 <c>[H]++Acc</c> to <c>[H|Acc]</c>.)</p> </section> <section> - <title>Myth: Strings are slow</title> - - <p>Actually, string handling could be slow if done improperly. - In Erlang, you'll have to think a little more about how the strings - are used and choose an appropriate representation and use - the <seealso marker="stdlib:re">re</seealso> module instead of the obsolete - <c>regexp</c> module if you are going to use regular expressions.</p> + <title>Myth: Strings are Slow</title> + + <p>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 + <seealso marker="stdlib:re">re</seealso> module in STDLIB + instead of the obsolete <c>regexp</c> module.</p> </section> <section> - <title>Myth: Repairing a Dets file is very slow</title> + <title>Myth: Repairing a Dets File is Very Slow</title> <p>The repair time is still proportional to the number of records - in the file, but Dets repairs used to be much, much slower in the past. + in the file, but Dets repairs used to be much slower in the past. Dets has been massively rewritten and improved.</p> </section> <section> - <title>Myth: BEAM is a stack-based byte-code virtual machine (and therefore slow)</title> + <title>Myth: BEAM is a Stack-Based Byte-Code Virtual Machine + (and Therefore Slow)</title> <p>BEAM is a register-based virtual machine. It has 1024 virtual registers that are used for holding temporary values and for passing arguments when @@ -193,11 +193,11 @@ vanilla_reverse([], Acc) -> </section> <section> - <title>Myth: Use '_' to speed up your program when a variable is not used</title> + <title>Myth: Use "_" to Speed Up Your Program When a Variable + is Not Used</title> - <p>That was once true, but since R6B the BEAM compiler is quite capable of seeing itself + <p>That was once true, but from R6B the BEAM compiler can see that a variable is not used.</p> </section> - </chapter> |