1 <?xml version="1.0" encoding="iso-8859-1"?>
2 <chapter id="profiling">
3 <title>Profiling</title>
4 <indexterm><primary>profiling</primary>
6 <indexterm><primary>cost-centre profiling</primary></indexterm>
8 <para> Glasgow Haskell comes with a time and space profiling
9 system. Its purpose is to help you improve your understanding of
10 your program's execution behaviour, so you can improve it.</para>
12 <para> Any comments, suggestions and/or improvements you have are
13 welcome. Recommended “profiling tricks” would be
14 especially cool! </para>
16 <para>Profiling a program is a three-step process:</para>
20 <para> Re-compile your program for profiling with the
21 <literal>-prof</literal> option, and probably one of the
22 <literal>-auto</literal> or <literal>-auto-all</literal>
23 options. These options are described in more detail in <xref
24 linkend="prof-compiler-options"/> </para>
25 <indexterm><primary><literal>-prof</literal></primary>
27 <indexterm><primary><literal>-auto</literal></primary>
29 <indexterm><primary><literal>-auto-all</literal></primary>
34 <para> Run your program with one of the profiling options, eg.
35 <literal>+RTS -p -RTS</literal>. This generates a file of
36 profiling information.</para>
37 <indexterm><primary><option>-p</option></primary><secondary>RTS
38 option</secondary></indexterm>
42 <para> Examine the generated profiling information, using one of
43 GHC's profiling tools. The tool to use will depend on the kind
44 of profiling information generated.</para>
49 <sect1 id="cost-centres">
50 <title>Cost centres and cost-centre stacks</title>
52 <para>GHC's profiling system assigns <firstterm>costs</firstterm>
53 to <firstterm>cost centres</firstterm>. A cost is simply the time
54 or space required to evaluate an expression. Cost centres are
55 program annotations around expressions; all costs incurred by the
56 annotated expression are assigned to the enclosing cost centre.
57 Furthermore, GHC will remember the stack of enclosing cost centres
58 for any given expression at run-time and generate a call-graph of
59 cost attributions.</para>
61 <para>Let's take a look at an example:</para>
64 main = print (nfib 25)
65 nfib n = if n < 2 then 1 else nfib (n-1) + nfib (n-2)
68 <para>Compile and run this program as follows:</para>
71 $ ghc -prof -auto-all -o Main Main.hs
77 <para>When a GHC-compiled program is run with the
78 <option>-p</option> RTS option, it generates a file called
79 <filename><prog>.prof</filename>. In this case, the file
80 will contain something like this:</para>
83 Fri May 12 14:06 2000 Time and Allocation Profiling Report (Final)
87 total time = 0.14 secs (7 ticks @ 20 ms)
88 total alloc = 8,741,204 bytes (excludes profiling overheads)
90 COST CENTRE MODULE %time %alloc
96 COST CENTRE MODULE entries %time %alloc %time %alloc
98 MAIN MAIN 0 0.0 0.0 100.0 100.0
99 main Main 0 0.0 0.0 0.0 0.0
100 CAF PrelHandle 3 0.0 0.0 0.0 0.0
101 CAF PrelAddr 1 0.0 0.0 0.0 0.0
102 CAF Main 6 0.0 0.0 100.0 100.0
103 main Main 1 0.0 0.0 100.0 100.0
104 nfib Main 242785 100.0 100.0 100.0 100.0
108 <para>The first part of the file gives the program name and
109 options, and the total time and total memory allocation measured
110 during the run of the program (note that the total memory
111 allocation figure isn't the same as the amount of
112 <emphasis>live</emphasis> memory needed by the program at any one
113 time; the latter can be determined using heap profiling, which we
114 will describe shortly).</para>
116 <para>The second part of the file is a break-down by cost centre
117 of the most costly functions in the program. In this case, there
118 was only one significant function in the program, namely
119 <function>nfib</function>, and it was responsible for 100%
120 of both the time and allocation costs of the program.</para>
122 <para>The third and final section of the file gives a profile
123 break-down by cost-centre stack. This is roughly a call-graph
124 profile of the program. In the example above, it is clear that
125 the costly call to <function>nfib</function> came from
126 <function>main</function>.</para>
128 <para>The time and allocation incurred by a given part of the
129 program is displayed in two ways: “individual”, which
130 are the costs incurred by the code covered by this cost centre
131 stack alone, and “inherited”, which includes the costs
132 incurred by all the children of this node.</para>
134 <para>The usefulness of cost-centre stacks is better demonstrated
135 by modifying the example slightly:</para>
138 main = print (f 25 + g 25)
140 g n = nfib (n `div` 2)
141 nfib n = if n < 2 then 1 else nfib (n-1) + nfib (n-2)
144 <para>Compile and run this program as before, and take a look at
145 the new profiling results:</para>
148 COST CENTRE MODULE scc %time %alloc %time %alloc
150 MAIN MAIN 0 0.0 0.0 100.0 100.0
151 main Main 0 0.0 0.0 0.0 0.0
152 CAF PrelHandle 3 0.0 0.0 0.0 0.0
153 CAF PrelAddr 1 0.0 0.0 0.0 0.0
154 CAF Main 9 0.0 0.0 100.0 100.0
155 main Main 1 0.0 0.0 100.0 100.0
156 g Main 1 0.0 0.0 0.0 0.2
157 nfib Main 465 0.0 0.2 0.0 0.2
158 f Main 1 0.0 0.0 100.0 99.8
159 nfib Main 242785 100.0 99.8 100.0 99.8
162 <para>Now although we had two calls to <function>nfib</function>
163 in the program, it is immediately clear that it was the call from
164 <function>f</function> which took all the time.</para>
166 <para>The actual meaning of the various columns in the output is:</para>
172 <para>The number of times this particular point in the call
173 graph was entered.</para>
178 <term>individual %time</term>
180 <para>The percentage of the total run time of the program
181 spent at this point in the call graph.</para>
186 <term>individual %alloc</term>
188 <para>The percentage of the total memory allocations
189 (excluding profiling overheads) of the program made by this
195 <term>inherited %time</term>
197 <para>The percentage of the total run time of the program
198 spent below this point in the call graph.</para>
203 <term>inherited %alloc</term>
205 <para>The percentage of the total memory allocations
206 (excluding profiling overheads) of the program made by this
207 call and all of its sub-calls.</para>
212 <para>In addition you can use the <option>-P</option> RTS option
213 <indexterm><primary><option>-P</option></primary></indexterm> to
214 get the following additional information:</para>
218 <term><literal>ticks</literal></term>
220 <para>The raw number of time “ticks” which were
221 attributed to this cost-centre; from this, we get the
222 <literal>%time</literal> figure mentioned
228 <term><literal>bytes</literal></term>
230 <para>Number of bytes allocated in the heap while in this
231 cost-centre; again, this is the raw number from which we get
232 the <literal>%alloc</literal> figure mentioned
238 <para>What about recursive functions, and mutually recursive
239 groups of functions? Where are the costs attributed? Well,
240 although GHC does keep information about which groups of functions
241 called each other recursively, this information isn't displayed in
242 the basic time and allocation profile, instead the call-graph is
243 flattened into a tree. The XML profiling tool (described in <xref
244 linkend="prof-xml-tool"/>) will be able to display real loops in
245 the call-graph.</para>
247 <sect2><title>Inserting cost centres by hand</title>
249 <para>Cost centres are just program annotations. When you say
250 <option>-auto-all</option> to the compiler, it automatically
251 inserts a cost centre annotation around every top-level function
252 in your program, but you are entirely free to add the cost
253 centre annotations yourself.</para>
255 <para>The syntax of a cost centre annotation is</para>
258 {-# SCC "name" #-} <expression>
261 <para>where <literal>"name"</literal> is an arbitrary string,
262 that will become the name of your cost centre as it appears
263 in the profiling output, and
264 <literal><expression></literal> is any Haskell
265 expression. An <literal>SCC</literal> annotation extends as
266 far to the right as possible when parsing.</para>
270 <sect2 id="prof-rules">
271 <title>Rules for attributing costs</title>
273 <para>The cost of evaluating any expression in your program is
274 attributed to a cost-centre stack using the following rules:</para>
278 <para>If the expression is part of the
279 <firstterm>one-off</firstterm> costs of evaluating the
280 enclosing top-level definition, then costs are attributed to
281 the stack of lexically enclosing <literal>SCC</literal>
282 annotations on top of the special <literal>CAF</literal>
287 <para>Otherwise, costs are attributed to the stack of
288 lexically-enclosing <literal>SCC</literal> annotations,
289 appended to the cost-centre stack in effect at the
290 <firstterm>call site</firstterm> of the current top-level
291 definition<footnote> <para>The call-site is just the place
292 in the source code which mentions the particular function or
293 variable.</para></footnote>. Notice that this is a recursive
298 <para>Time spent in foreign code (see <xref linkend="ffi"/>)
299 is always attributed to the cost centre in force at the
300 Haskell call-site of the foreign function.</para>
304 <para>What do we mean by one-off costs? Well, Haskell is a lazy
305 language, and certain expressions are only ever evaluated once.
306 For example, if we write:</para>
312 <para>then <varname>x</varname> will only be evaluated once (if
313 at all), and subsequent demands for <varname>x</varname> will
314 immediately get to see the cached result. The definition
315 <varname>x</varname> is called a CAF (Constant Applicative
316 Form), because it has no arguments.</para>
318 <para>For the purposes of profiling, we say that the expression
319 <literal>nfib 25</literal> belongs to the one-off costs of
320 evaluating <varname>x</varname>.</para>
322 <para>Since one-off costs aren't strictly speaking part of the
323 call-graph of the program, they are attributed to a special
324 top-level cost centre, <literal>CAF</literal>. There may be one
325 <literal>CAF</literal> cost centre for each module (the
326 default), or one for each top-level definition with any one-off
327 costs (this behaviour can be selected by giving GHC the
328 <option>-caf-all</option> flag).</para>
330 <indexterm><primary><literal>-caf-all</literal></primary>
333 <para>If you think you have a weird profile, or the call-graph
334 doesn't look like you expect it to, feel free to send it (and
335 your program) to us at
336 <email>glasgow-haskell-bugs@haskell.org</email>.</para>
340 <sect1 id="prof-compiler-options">
341 <title>Compiler options for profiling</title>
343 <indexterm><primary>profiling</primary><secondary>options</secondary></indexterm>
344 <indexterm><primary>options</primary><secondary>for profiling</secondary></indexterm>
349 <option>-prof</option>:
350 <indexterm><primary><option>-prof</option></primary></indexterm>
353 <para> To make use of the profiling system
354 <emphasis>all</emphasis> modules must be compiled and linked
355 with the <option>-prof</option> option. Any
356 <literal>SCC</literal> annotations you've put in your source
357 will spring to life.</para>
359 <para> Without a <option>-prof</option> option, your
360 <literal>SCC</literal>s are ignored; so you can compile
361 <literal>SCC</literal>-laden code without changing
367 <para>There are a few other profiling-related compilation options.
368 Use them <emphasis>in addition to</emphasis>
369 <option>-prof</option>. These do not have to be used consistently
370 for all modules in a program.</para>
375 <option>-auto</option>:
376 <indexterm><primary><option>-auto</option></primary></indexterm>
377 <indexterm><primary>cost centres</primary><secondary>automatically inserting</secondary></indexterm>
380 <para> GHC will automatically add
381 <function>_scc_</function> constructs for all
382 top-level, exported functions.</para>
388 <option>-auto-all</option>:
389 <indexterm><primary><option>-auto-all</option></primary></indexterm>
392 <para> <emphasis>All</emphasis> top-level functions,
393 exported or not, will be automatically
394 <function>_scc_</function>'d.</para>
400 <option>-caf-all</option>:
401 <indexterm><primary><option>-caf-all</option></primary></indexterm>
404 <para> The costs of all CAFs in a module are usually
405 attributed to one “big” CAF cost-centre. With
406 this option, all CAFs get their own cost-centre. An
407 “if all else fails” option…</para>
413 <option>-ignore-scc</option>:
414 <indexterm><primary><option>-ignore-scc</option></primary></indexterm>
417 <para>Ignore any <function>_scc_</function>
418 constructs, so a module which already has
419 <function>_scc_</function>s can be compiled
420 for profiling with the annotations ignored.</para>
428 <sect1 id="prof-time-options">
429 <title>Time and allocation profiling</title>
431 <para>To generate a time and allocation profile, give one of the
432 following RTS options to the compiled program when you run it (RTS
433 options should be enclosed between <literal>+RTS...-RTS</literal>
439 <option>-p</option> or <option>-P</option>:
440 <indexterm><primary><option>-p</option></primary></indexterm>
441 <indexterm><primary><option>-P</option></primary></indexterm>
442 <indexterm><primary>time profile</primary></indexterm>
445 <para>The <option>-p</option> option produces a standard
446 <emphasis>time profile</emphasis> report. It is written
448 <filename><replaceable>program</replaceable>.prof</filename>.</para>
450 <para>The <option>-P</option> option produces a more
451 detailed report containing the actual time and allocation
452 data as well. (Not used much.)</para>
458 <option>-px</option>:
459 <indexterm><primary><option>-px</option></primary></indexterm>
462 <para>The <option>-px</option> option generates profiling
463 information in the XML format understood by our new
464 profiling tool, see <xref linkend="prof-xml-tool"/>.</para>
471 <indexterm><primary><option>-xc</option></primary><secondary>RTS option</secondary></indexterm>
474 <para>This option makes use of the extra information
475 maintained by the cost-centre-stack profiler to provide
476 useful information about the location of runtime errors.
477 See <xref linkend="rts-options-debugging"/>.</para>
485 <sect1 id="prof-heap">
486 <title>Profiling memory usage</title>
488 <para>In addition to profiling the time and allocation behaviour
489 of your program, you can also generate a graph of its memory usage
490 over time. This is useful for detecting the causes of
491 <firstterm>space leaks</firstterm>, when your program holds on to
492 more memory at run-time that it needs to. Space leaks lead to
493 longer run-times due to heavy garbage collector activity, and may
494 even cause the program to run out of memory altogether.</para>
496 <para>To generate a heap profile from your program:</para>
500 <para>Compile the program for profiling (<xref
501 linkend="prof-compiler-options"/>).</para>
504 <para>Run it with one of the heap profiling options described
505 below (eg. <option>-hc</option> for a basic producer profile).
506 This generates the file
507 <filename><replaceable>prog</replaceable>.hp</filename>.</para>
510 <para>Run <command>hp2ps</command> to produce a Postscript
512 <filename><replaceable>prog</replaceable>.ps</filename>. The
513 <command>hp2ps</command> utility is described in detail in
514 <xref linkend="hp2ps"/>.</para>
517 <para>Display the heap profile using a postscript viewer such
518 as <application>Ghostview</application>, or print it out on a
519 Postscript-capable printer.</para>
523 <sect2 id="rts-options-heap-prof">
524 <title>RTS options for heap profiling</title>
526 <para>There are several different kinds of heap profile that can
527 be generated. All the different profile types yield a graph of
528 live heap against time, but they differ in how the live heap is
529 broken down into bands. The following RTS options select which
530 break-down to use:</para>
536 <indexterm><primary><option>-hc</option></primary><secondary>RTS option</secondary></indexterm>
539 <para>Breaks down the graph by the cost-centre stack which
540 produced the data.</para>
547 <indexterm><primary><option>-hm</option></primary><secondary>RTS option</secondary></indexterm>
550 <para>Break down the live heap by the module containing
551 the code which produced the data.</para>
558 <indexterm><primary><option>-hd</option></primary><secondary>RTS option</secondary></indexterm>
561 <para>Breaks down the graph by <firstterm>closure
562 description</firstterm>. For actual data, the description
563 is just the constructor name, for other closures it is a
564 compiler-generated string identifying the closure.</para>
571 <indexterm><primary><option>-hy</option></primary><secondary>RTS option</secondary></indexterm>
574 <para>Breaks down the graph by
575 <firstterm>type</firstterm>. For closures which have
576 function type or unknown/polymorphic type, the string will
577 represent an approximation to the actual type.</para>
584 <indexterm><primary><option>-hr</option></primary><secondary>RTS option</secondary></indexterm>
587 <para>Break down the graph by <firstterm>retainer
588 set</firstterm>. Retainer profiling is described in more
589 detail below (<xref linkend="retainer-prof"/>).</para>
596 <indexterm><primary><option>-hb</option></primary><secondary>RTS option</secondary></indexterm>
599 <para>Break down the graph by
600 <firstterm>biography</firstterm>. Biographical profiling
601 is described in more detail below (<xref
602 linkend="biography-prof"/>).</para>
607 <para>In addition, the profile can be restricted to heap data
608 which satisfies certain criteria - for example, you might want
609 to display a profile by type but only for data produced by a
610 certain module, or a profile by retainer for a certain type of
611 data. Restrictions are specified as follows:</para>
616 <option>-hc</option><replaceable>name</replaceable>,...
617 <indexterm><primary><option>-hc</option></primary><secondary>RTS option</secondary></indexterm>
620 <para>Restrict the profile to closures produced by
621 cost-centre stacks with one of the specified cost centres
628 <option>-hC</option><replaceable>name</replaceable>,...
629 <indexterm><primary><option>-hC</option></primary><secondary>RTS option</secondary></indexterm>
632 <para>Restrict the profile to closures produced by
633 cost-centre stacks with one of the specified cost centres
634 anywhere in the stack.</para>
640 <option>-hm</option><replaceable>module</replaceable>,...
641 <indexterm><primary><option>-hm</option></primary><secondary>RTS option</secondary></indexterm>
644 <para>Restrict the profile to closures produced by the
645 specified modules.</para>
651 <option>-hd</option><replaceable>desc</replaceable>,...
652 <indexterm><primary><option>-hd</option></primary><secondary>RTS option</secondary></indexterm>
655 <para>Restrict the profile to closures with the specified
656 description strings.</para>
662 <option>-hy</option><replaceable>type</replaceable>,...
663 <indexterm><primary><option>-hy</option></primary><secondary>RTS option</secondary></indexterm>
666 <para>Restrict the profile to closures with the specified
673 <option>-hr</option><replaceable>cc</replaceable>,...
674 <indexterm><primary><option>-hr</option></primary><secondary>RTS option</secondary></indexterm>
677 <para>Restrict the profile to closures with retainer sets
678 containing cost-centre stacks with one of the specified
679 cost centres at the top.</para>
685 <option>-hb</option><replaceable>bio</replaceable>,...
686 <indexterm><primary><option>-hb</option></primary><secondary>RTS option</secondary></indexterm>
689 <para>Restrict the profile to closures with one of the
690 specified biographies, where
691 <replaceable>bio</replaceable> is one of
692 <literal>lag</literal>, <literal>drag</literal>,
693 <literal>void</literal>, or <literal>use</literal>.</para>
698 <para>For example, the following options will generate a
699 retainer profile restricted to <literal>Branch</literal> and
700 <literal>Leaf</literal> constructors:</para>
703 <replaceable>prog</replaceable> +RTS -hr -hdBranch,Leaf
706 <para>There can only be one "break-down" option
707 (eg. <option>-hr</option> in the example above), but there is no
708 limit on the number of further restrictions that may be applied.
709 All the options may be combined, with one exception: GHC doesn't
710 currently support mixing the <option>-hr</option> and
711 <option>-hb</option> options.</para>
713 <para>There are two more options which relate to heap
719 <option>-i<replaceable>secs</replaceable></option>:
720 <indexterm><primary><option>-i</option></primary></indexterm>
723 <para>Set the profiling (sampling) interval to
724 <replaceable>secs</replaceable> seconds (the default is
725 0.1 second). Fractions are allowed: for example
726 <option>-i0.2</option> will get 5 samples per second.
727 This only affects heap profiling; time profiles are always
728 sampled on a 1/50 second frequency.</para>
735 <indexterm><primary><option>-xt</option></primary><secondary>RTS option</secondary></indexterm>
738 <para>Include the memory occupied by threads in a heap
739 profile. Each thread takes up a small area for its thread
740 state in addition to the space allocated for its stack
741 (stacks normally start small and then grow as
744 <para>This includes the main thread, so using
745 <option>-xt</option> is a good way to see how much stack
746 space the program is using.</para>
748 <para>Memory occupied by threads and their stacks is
749 labelled as “TSO” when displaying the profile
750 by closure description or type description.</para>
757 <sect2 id="retainer-prof">
758 <title>Retainer Profiling</title>
760 <para>Retainer profiling is designed to help answer questions
761 like <quote>why is this data being retained?</quote>. We start
762 by defining what we mean by a retainer:</para>
765 <para>A retainer is either the system stack, or an unevaluated
766 closure (thunk).</para>
769 <para>In particular, constructors are <emphasis>not</emphasis>
772 <para>An object B retains object A if (i) B is a retainer object and
773 (ii) object A can be reached by recursively following pointers
774 starting from object B, but not meeting any other retainer
775 objects on the way. Each live object is retained by one or more
776 retainer objects, collectively called its retainer set, or its
777 <firstterm>retainer set</firstterm>, or its
778 <firstterm>retainers</firstterm>.</para>
780 <para>When retainer profiling is requested by giving the program
781 the <option>-hr</option> option, a graph is generated which is
782 broken down by retainer set. A retainer set is displayed as a
783 set of cost-centre stacks; because this is usually too large to
784 fit on the profile graph, each retainer set is numbered and
785 shown abbreviated on the graph along with its number, and the
786 full list of retainer sets is dumped into the file
787 <filename><replaceable>prog</replaceable>.prof</filename>.</para>
789 <para>Retainer profiling requires multiple passes over the live
790 heap in order to discover the full retainer set for each
791 object, which can be quite slow. So we set a limit on the
792 maximum size of a retainer set, where all retainer sets larger
793 than the maximum retainer set size are replaced by the special
794 set <literal>MANY</literal>. The maximum set size defaults to 8
795 and can be altered with the <option>-R</option> RTS
800 <term><option>-R</option><replaceable>size</replaceable></term>
802 <para>Restrict the number of elements in a retainer set to
803 <replaceable>size</replaceable> (default 8).</para>
809 <title>Hints for using retainer profiling</title>
811 <para>The definition of retainers is designed to reflect a
812 common cause of space leaks: a large structure is retained by
813 an unevaluated computation, and will be released once the
814 computation is forced. A good example is looking up a value in
815 a finite map, where unless the lookup is forced in a timely
816 manner the unevaluated lookup will cause the whole mapping to
817 be retained. These kind of space leaks can often be
818 eliminated by forcing the relevant computations to be
819 performed eagerly, using <literal>seq</literal> or strictness
820 annotations on data constructor fields.</para>
822 <para>Often a particular data structure is being retained by a
823 chain of unevaluated closures, only the nearest of which will
824 be reported by retainer profiling - for example A retains B, B
825 retains C, and C retains a large structure. There might be a
826 large number of Bs but only a single A, so A is really the one
827 we're interested in eliminating. However, retainer profiling
828 will in this case report B as the retainer of the large
829 structure. To move further up the chain of retainers, we can
830 ask for another retainer profile but this time restrict the
831 profile to B objects, so we get a profile of the retainers of
835 <replaceable>prog</replaceable> +RTS -hr -hcB
838 <para>This trick isn't foolproof, because there might be other
839 B closures in the heap which aren't the retainers we are
840 interested in, but we've found this to be a useful technique
841 in most cases.</para>
845 <sect2 id="biography-prof">
846 <title>Biographical Profiling</title>
848 <para>A typical heap object may be in one of the following four
849 states at each point in its lifetime:</para>
853 <para>The <firstterm>lag</firstterm> stage, which is the
854 time between creation and the first use of the
858 <para>the <firstterm>use</firstterm> stage, which lasts from
859 the first use until the last use of the object, and</para>
862 <para>The <firstterm>drag</firstterm> stage, which lasts
863 from the final use until the last reference to the object
867 <para>An object which is never used is said to be in the
868 <firstterm>void</firstterm> state for its whole
873 <para>A biographical heap profile displays the portion of the
874 live heap in each of the four states listed above. Usually the
875 most interesting states are the void and drag states: live heap
876 in these states is more likely to be wasted space than heap in
877 the lag or use states.</para>
879 <para>It is also possible to break down the heap in one or more
880 of these states by a different criteria, by restricting a
881 profile by biography. For example, to show the portion of the
882 heap in the drag or void state by producer: </para>
885 <replaceable>prog</replaceable> +RTS -hc -hbdrag,void
888 <para>Once you know the producer or the type of the heap in the
889 drag or void states, the next step is usually to find the
893 <replaceable>prog</replaceable> +RTS -hr -hc<replaceable>cc</replaceable>...
896 <para>NOTE: this two stage process is required because GHC
897 cannot currently profile using both biographical and retainer
898 information simultaneously.</para>
901 <sect2 id="mem-residency">
902 <title>Actual memory residency</title>
904 <para>How does the heap residency reported by the heap profiler relate to
905 the actual memory residency of your program when you run it? You might
906 see a large discrepancy between the residency reported by the heap
907 profiler, and the residency reported by tools on your system
908 (eg. <literal>ps</literal> or <literal>top</literal> on Unix, or the
909 Task Manager on Windows). There are several reasons for this:</para>
913 <para>There is an overhead of profiling itself, which is subtracted
914 from the residency figures by the profiler. This overhead goes
915 away when compiling without profiling support, of course. The
916 space overhead is currently 2 extra
917 words per heap object, which probably results in
918 about a 30% overhead.</para>
922 <para>Garbage collection requires more memory than the actual
923 residency. The factor depends on the kind of garbage collection
924 algorithm in use: a major GC in the standard
925 generation copying collector will usually require 3L bytes of
926 memory, where L is the amount of live data. This is because by
927 default (see the <option>+RTS -F</option> option) we allow the old
928 generation to grow to twice its size (2L) before collecting it, and
929 we require additionally L bytes to copy the live data into. When
930 using compacting collection (see the <option>+RTS -c</option>
931 option), this is reduced to 2L, and can further be reduced by
932 tweaking the <option>-F</option> option. Also add the size of the
933 allocation area (currently a fixed 512Kb).</para>
937 <para>The stack isn't counted in the heap profile by default. See the
938 <option>+RTS -xt</option> option.</para>
942 <para>The program text itself, the C stack, any non-heap data (eg. data
943 allocated by foreign libraries, and data allocated by the RTS), and
944 <literal>mmap()</literal>'d memory are not counted in the heap profile.</para>
951 <sect1 id="prof-xml-tool">
952 <title>Graphical time/allocation profile</title>
954 <para>You can view the time and allocation profiling graph of your
955 program graphically, using <command>ghcprof</command>. This is a
956 new tool with GHC 4.08, and will eventually be the de-facto
957 standard way of viewing GHC profiles<footnote><para>Actually this
958 isn't true any more, we are working on a new tool for
959 displaying heap profiles using Gtk+HS, so
960 <command>ghcprof</command> may go away at some point in the future.</para>
963 <para>To run <command>ghcprof</command>, you need
964 <productname>uDraw(Graph)</productname> installed, which can be
966 url="http://www.informatik.uni-bremen.de/uDrawGraph/en/uDrawGraph/uDrawGraph.html"><citetitle>uDraw(Graph)</citetitle></ulink>. Install one of
968 distributions, and set your
969 <envar>UDG_HOME</envar> environment variable to point to the
970 installation directory.</para>
972 <para><command>ghcprof</command> uses an XML-based profiling log
973 format, and you therefore need to run your program with a
974 different option: <option>-px</option>. The file generated is
975 still called <filename><prog>.prof</filename>. To see the
976 profile, run <command>ghcprof</command> like this:</para>
978 <indexterm><primary><option>-px</option></primary></indexterm>
981 $ ghcprof <prog>.prof
984 <para>which should pop up a window showing the call-graph of your
985 program in glorious detail. More information on using
986 <command>ghcprof</command> can be found at <ulink
987 url="http://www.dcs.warwick.ac.uk/people/academic/Stephen.Jarvis/profiler/index.html"><citetitle>The
988 Cost-Centre Stack Profiling Tool for
989 GHC</citetitle></ulink>.</para>
994 <title><command>hp2ps</command>––heap profile to PostScript</title>
996 <indexterm><primary><command>hp2ps</command></primary></indexterm>
997 <indexterm><primary>heap profiles</primary></indexterm>
998 <indexterm><primary>postscript, from heap profiles</primary></indexterm>
999 <indexterm><primary><option>-h<break-down></option></primary></indexterm>
1004 hp2ps [flags] [<file>[.hp]]
1008 <command>hp2ps</command><indexterm><primary>hp2ps
1009 program</primary></indexterm> converts a heap profile as produced
1010 by the <option>-h<break-down></option> runtime option into a
1011 PostScript graph of the heap profile. By convention, the file to
1012 be processed by <command>hp2ps</command> has a
1013 <filename>.hp</filename> extension. The PostScript output is
1014 written to <filename><file>@.ps</filename>. If
1015 <filename><file></filename> is omitted entirely, then the
1016 program behaves as a filter.</para>
1018 <para><command>hp2ps</command> is distributed in
1019 <filename>ghc/utils/hp2ps</filename> in a GHC source
1020 distribution. It was originally developed by Dave Wakeling as part
1021 of the HBC/LML heap profiler.</para>
1023 <para>The flags are:</para>
1028 <term><option>-d</option></term>
1030 <para>In order to make graphs more readable,
1031 <command>hp2ps</command> sorts the shaded bands for each
1032 identifier. The default sort ordering is for the bands with
1033 the largest area to be stacked on top of the smaller ones.
1034 The <option>-d</option> option causes rougher bands (those
1035 representing series of values with the largest standard
1036 deviations) to be stacked on top of smoother ones.</para>
1041 <term><option>-b</option></term>
1043 <para>Normally, <command>hp2ps</command> puts the title of
1044 the graph in a small box at the top of the page. However, if
1045 the JOB string is too long to fit in a small box (more than
1046 35 characters), then <command>hp2ps</command> will choose to
1047 use a big box instead. The <option>-b</option> option
1048 forces <command>hp2ps</command> to use a big box.</para>
1053 <term><option>-e<float>[in|mm|pt]</option></term>
1055 <para>Generate encapsulated PostScript suitable for
1056 inclusion in LaTeX documents. Usually, the PostScript graph
1057 is drawn in landscape mode in an area 9 inches wide by 6
1058 inches high, and <command>hp2ps</command> arranges for this
1059 area to be approximately centred on a sheet of a4 paper.
1060 This format is convenient of studying the graph in detail,
1061 but it is unsuitable for inclusion in LaTeX documents. The
1062 <option>-e</option> option causes the graph to be drawn in
1063 portrait mode, with float specifying the width in inches,
1064 millimetres or points (the default). The resulting
1065 PostScript file conforms to the Encapsulated PostScript
1066 (EPS) convention, and it can be included in a LaTeX document
1067 using Rokicki's dvi-to-PostScript converter
1068 <command>dvips</command>.</para>
1073 <term><option>-g</option></term>
1075 <para>Create output suitable for the <command>gs</command>
1076 PostScript previewer (or similar). In this case the graph is
1077 printed in portrait mode without scaling. The output is
1078 unsuitable for a laser printer.</para>
1083 <term><option>-l</option></term>
1085 <para>Normally a profile is limited to 20 bands with
1086 additional identifiers being grouped into an
1087 <literal>OTHER</literal> band. The <option>-l</option> flag
1088 removes this 20 band and limit, producing as many bands as
1089 necessary. No key is produced as it won't fit!. It is useful
1090 for creation time profiles with many bands.</para>
1095 <term><option>-m<int></option></term>
1097 <para>Normally a profile is limited to 20 bands with
1098 additional identifiers being grouped into an
1099 <literal>OTHER</literal> band. The <option>-m</option> flag
1100 specifies an alternative band limit (the maximum is
1103 <para><option>-m0</option> requests the band limit to be
1104 removed. As many bands as necessary are produced. However no
1105 key is produced as it won't fit! It is useful for displaying
1106 creation time profiles with many bands.</para>
1111 <term><option>-p</option></term>
1113 <para>Use previous parameters. By default, the PostScript
1114 graph is automatically scaled both horizontally and
1115 vertically so that it fills the page. However, when
1116 preparing a series of graphs for use in a presentation, it
1117 is often useful to draw a new graph using the same scale,
1118 shading and ordering as a previous one. The
1119 <option>-p</option> flag causes the graph to be drawn using
1120 the parameters determined by a previous run of
1121 <command>hp2ps</command> on <filename>file</filename>. These
1122 are extracted from <filename>file@.aux</filename>.</para>
1127 <term><option>-s</option></term>
1129 <para>Use a small box for the title.</para>
1134 <term><option>-t<float></option></term>
1136 <para>Normally trace elements which sum to a total of less
1137 than 1% of the profile are removed from the
1138 profile. The <option>-t</option> option allows this
1139 percentage to be modified (maximum 5%).</para>
1141 <para><option>-t0</option> requests no trace elements to be
1142 removed from the profile, ensuring that all the data will be
1148 <term><option>-c</option></term>
1150 <para>Generate colour output.</para>
1155 <term><option>-y</option></term>
1157 <para>Ignore marks.</para>
1162 <term><option>-?</option></term>
1164 <para>Print out usage information.</para>
1170 <sect2 id="manipulating-hp">
1171 <title>Manipulating the hp file</title>
1173 <para>(Notes kindly offered by Jan-Willhem Maessen.)</para>
1176 The <filename>FOO.hp</filename> file produced when you ask for the
1177 heap profile of a program <filename>FOO</filename> is a text file with a particularly
1178 simple structure. Here's a representative example, with much of the
1179 actual data omitted:
1182 DATE "Thu Dec 26 18:17 2002"
1183 SAMPLE_UNIT "seconds"
1194 BEGIN_SAMPLE 11695.47
1197 The first four lines (<literal>JOB</literal>, <literal>DATE</literal>, <literal>SAMPLE_UNIT</literal>, <literal>VALUE_UNIT</literal>) form a
1198 header. Each block of lines starting with <literal>BEGIN_SAMPLE</literal> and ending
1199 with <literal>END_SAMPLE</literal> forms a single sample (you can think of this as a
1200 vertical slice of your heap profile). The hp2ps utility should accept
1201 any input with a properly-formatted header followed by a series of
1207 <title>Zooming in on regions of your profile</title>
1210 You can look at particular regions of your profile simply by loading a
1211 copy of the <filename>.hp</filename> file into a text editor and deleting the unwanted
1212 samples. The resulting <filename>.hp</filename> file can be run through <command>hp2ps</command> and viewed
1218 <title>Viewing the heap profile of a running program</title>
1221 The <filename>.hp</filename> file is generated incrementally as your
1222 program runs. In principle, running <command>hp2ps</command> on the incomplete file
1223 should produce a snapshot of your program's heap usage. However, the
1224 last sample in the file may be incomplete, causing <command>hp2ps</command> to fail. If
1225 you are using a machine with UNIX utilities installed, it's not too
1226 hard to work around this problem (though the resulting command line
1227 looks rather Byzantine):
1229 head -`fgrep -n END_SAMPLE FOO.hp | tail -1 | cut -d : -f 1` FOO.hp \
1233 The command <command>fgrep -n END_SAMPLE FOO.hp</command> finds the
1234 end of every complete sample in <filename>FOO.hp</filename>, and labels each sample with
1235 its ending line number. We then select the line number of the last
1236 complete sample using <command>tail</command> and <command>cut</command>. This is used as a
1237 parameter to <command>head</command>; the result is as if we deleted the final
1238 incomplete sample from <filename>FOO.hp</filename>. This results in a properly-formatted
1239 .hp file which we feed directly to <command>hp2ps</command>.
1243 <title>Viewing a heap profile in real time</title>
1246 The <command>gv</command> and <command>ghostview</command> programs
1247 have a "watch file" option can be used to view an up-to-date heap
1248 profile of your program as it runs. Simply generate an incremental
1249 heap profile as described in the previous section. Run <command>gv</command> on your
1252 gv -watch -seascape FOO.ps
1254 If you forget the <literal>-watch</literal> flag you can still select
1255 "Watch file" from the "State" menu. Now each time you generate a new
1256 profile <filename>FOO.ps</filename> the view will update automatically.
1260 This can all be encapsulated in a little script:
1263 head -`fgrep -n END_SAMPLE FOO.hp | tail -1 | cut -d : -f 1` FOO.hp \
1265 gv -watch -seascape FOO.ps &
1267 sleep 10 # We generate a new profile every 10 seconds.
1268 head -`fgrep -n END_SAMPLE FOO.hp | tail -1 | cut -d : -f 1` FOO.hp \
1272 Occasionally <command>gv</command> will choke as it tries to read an incomplete copy of
1273 <filename>FOO.ps</filename> (because <command>hp2ps</command> is still running as an update
1274 occurs). A slightly more complicated script works around this
1275 problem, by using the fact that sending a SIGHUP to gv will cause it
1276 to re-read its input file:
1279 head -`fgrep -n END_SAMPLE FOO.hp | tail -1 | cut -d : -f 1` FOO.hp \
1285 head -`fgrep -n END_SAMPLE FOO.hp | tail -1 | cut -d : -f 1` FOO.hp \
1296 <sect1 id="ticky-ticky">
1297 <title>Using “ticky-ticky” profiling (for implementors)</title>
1298 <indexterm><primary>ticky-ticky profiling</primary></indexterm>
1300 <para>(ToDo: document properly.)</para>
1302 <para>It is possible to compile Glasgow Haskell programs so that
1303 they will count lots and lots of interesting things, e.g., number
1304 of updates, number of data constructors entered, etc., etc. We
1305 call this “ticky-ticky”
1306 profiling,<indexterm><primary>ticky-ticky
1307 profiling</primary></indexterm> <indexterm><primary>profiling,
1308 ticky-ticky</primary></indexterm> because that's the sound a Sun4
1309 makes when it is running up all those counters
1310 (<emphasis>slowly</emphasis>).</para>
1312 <para>Ticky-ticky profiling is mainly intended for implementors;
1313 it is quite separate from the main “cost-centre”
1314 profiling system, intended for all users everywhere.</para>
1316 <para>To be able to use ticky-ticky profiling, you will need to
1317 have built appropriate libraries and things when you made the
1318 system. See “Customising what libraries to build,” in
1319 the installation guide.</para>
1321 <para>To get your compiled program to spit out the ticky-ticky
1322 numbers, use a <option>-r</option> RTS
1323 option<indexterm><primary>-r RTS option</primary></indexterm>.
1324 See <xref linkend="runtime-control"/>.</para>
1326 <para>Compiling your program with the <option>-ticky</option>
1327 switch yields an executable that performs these counts. Here is a
1328 sample ticky-ticky statistics file, generated by the invocation
1329 <command>foo +RTS -rfoo.ticky</command>.</para>
1332 foo +RTS -rfoo.ticky
1335 ALLOCATIONS: 3964631 (11330900 words total: 3999476 admin, 6098829 goods, 1232595 slop)
1336 total words: 2 3 4 5 6+
1337 69647 ( 1.8%) function values 50.0 50.0 0.0 0.0 0.0
1338 2382937 ( 60.1%) thunks 0.0 83.9 16.1 0.0 0.0
1339 1477218 ( 37.3%) data values 66.8 33.2 0.0 0.0 0.0
1340 0 ( 0.0%) big tuples
1341 2 ( 0.0%) black holes 0.0 100.0 0.0 0.0 0.0
1342 0 ( 0.0%) prim things
1343 34825 ( 0.9%) partial applications 0.0 0.0 0.0 100.0 0.0
1344 2 ( 0.0%) thread state objects 0.0 0.0 0.0 0.0 100.0
1346 Total storage-manager allocations: 3647137 (11882004 words)
1347 [551104 words lost to speculative heap-checks]
1351 ENTERS: 9400092 of which 2005772 (21.3%) direct to the entry code
1352 [the rest indirected via Node's info ptr]
1353 1860318 ( 19.8%) thunks
1354 3733184 ( 39.7%) data values
1355 3149544 ( 33.5%) function values
1356 [of which 1999880 (63.5%) bypassed arg-satisfaction chk]
1357 348140 ( 3.7%) partial applications
1358 308906 ( 3.3%) normal indirections
1359 0 ( 0.0%) permanent indirections
1362 2137257 ( 36.4%) from entering a new constructor
1363 [the rest from entering an existing constructor]
1364 2349219 ( 40.0%) vectored [the rest unvectored]
1366 RET_NEW: 2137257: 32.5% 46.2% 21.3% 0.0% 0.0% 0.0% 0.0% 0.0% 0.0%
1367 RET_OLD: 3733184: 2.8% 67.9% 29.3% 0.0% 0.0% 0.0% 0.0% 0.0% 0.0%
1368 RET_UNBOXED_TUP: 2: 0.0% 0.0%100.0% 0.0% 0.0% 0.0% 0.0% 0.0% 0.0%
1370 RET_VEC_RETURN : 2349219: 0.0% 0.0%100.0% 0.0% 0.0% 0.0% 0.0% 0.0% 0.0%
1372 UPDATE FRAMES: 2241725 (0 omitted from thunks)
1376 0 ( 0.0%) data values
1377 34827 ( 1.6%) partial applications
1378 [2 in place, 34825 allocated new space]
1379 2206898 ( 98.4%) updates to existing heap objects (46 by squeezing)
1380 UPD_CON_IN_NEW: 0: 0 0 0 0 0 0 0 0 0
1381 UPD_PAP_IN_NEW: 34825: 0 0 0 34825 0 0 0 0 0
1383 NEW GEN UPDATES: 2274700 ( 99.9%)
1385 OLD GEN UPDATES: 1852 ( 0.1%)
1387 Total bytes copied during GC: 190096
1389 **************************************************
1390 3647137 ALLOC_HEAP_ctr
1391 11882004 ALLOC_HEAP_tot
1396 34831 ALLOC_FUN_hst_0
1397 34816 ALLOC_FUN_hst_1
1401 2382937 ALLOC_UP_THK_ctr
1404 0 E!NT_PERM_IND_ctr requires +RTS -Z
1405 [... lots more info omitted ...]
1406 0 GC_SEL_ABANDONED_ctr
1409 0 GC_FAILED_PROMOTION_ctr
1410 47524 GC_WORDS_COPIED_ctr
1413 <para>The formatting of the information above the row of asterisks
1414 is subject to change, but hopefully provides a useful
1415 human-readable summary. Below the asterisks <emphasis>all
1416 counters</emphasis> maintained by the ticky-ticky system are
1417 dumped, in a format intended to be machine-readable: zero or more
1418 spaces, an integer, a space, the counter name, and a newline.</para>
1420 <para>In fact, not <emphasis>all</emphasis> counters are
1421 necessarily dumped; compile- or run-time flags can render certain
1422 counters invalid. In this case, either the counter will simply
1423 not appear, or it will appear with a modified counter name,
1424 possibly along with an explanation for the omission (notice
1425 <literal>ENT_PERM_IND_ctr</literal> appears
1426 with an inserted <literal>!</literal> above). Software analysing
1427 this output should always check that it has the counters it
1428 expects. Also, beware: some of the counters can have
1429 <emphasis>large</emphasis> values!</para>
1436 ;;; Local Variables: ***
1438 ;;; sgml-parent-document: ("users_guide.xml" "book" "chapter") ***