third_party/tcmalloc/vendor/doc/cpuprofile.html

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  18
  19 <p>This is the CPU profiler we use at Google.  There are three parts
  20 to using it: linking the library into an application, running the
  21 code, and analyzing the output.</p>
  22
  23 <p>On the off-chance that you should need to understand it, the CPU
  24 profiler data file format is documented separately,
  25 <a href="cpuprofile-fileformat.html">here</a>.
  26
  27
  28 <H1>Linking in the Library</H1>
  29
  30 <p>To install the CPU profiler into your executable, add
  31 <code>-lprofiler</code> to the link-time step for your executable.
  32 (It's also probably possible to add in the profiler at run-time using
  33 <code>LD_PRELOAD</code>, e.g.
  34 <code>% env LD_PRELOAD="/usr/lib/libprofiler.so" &lt;binary&gt;</code>,
  35 but this isn't necessarily recommended.)</p>
  36
  37 <p>This does <i>not</i> turn on CPU profiling; it just inserts the
  38 code.  For that reason, it's practical to just always link
  39 <code>-lprofiler</code> into a binary while developing; that's what we
  40 do at Google.  (However, since any user can turn on the profiler by
  41 setting an environment variable, it's not necessarily recommended to
  42 install profiler-linked binaries into a production, running
  43 system.)</p>
  44
  45
  46 <H1>Running the Code</H1>
  47
  48 <p>There are several alternatives to actually turn on CPU profiling
  49 for a given run of an executable:</p>
  50
  51 <ol>
  52   <li> <p>Define the environment variable CPUPROFILE to the filename
  53        to dump the profile to.  For instance, if you had a version of
  54        <code>/bin/ls</code> that had been linked against libprofiler,
  55        you could run:</p>
  56        <pre>% env CPUPROFILE=ls.prof /bin/ls</pre>
  57
  58   <li> <p>In your code, bracket the code you want profiled in calls to
  59        <code>ProfilerStart()</code> and <code>ProfilerStop()</code>.
  60        (These functions are declared in <code>&lt;gperftools/profiler.h&gt;</code>.)
  61        <code>ProfilerStart()</code> will take
  62        the profile-filename as an argument.</p>
  63 </ol>
  64
  65 <p>In Linux 2.6 and above, profiling works correctly with threads,
  66 automatically profiling all threads.  In Linux 2.4, profiling only
  67 profiles the main thread (due to a kernel bug involving itimers and
  68 threads).  Profiling works correctly with sub-processes: each child
  69 process gets its own profile with its own name (generated by combining
  70 CPUPROFILE with the child's process id).</p>
  71
  72 <p>For security reasons, CPU profiling will not write to a file -- and
  73 is thus not usable -- for setuid programs.</p>
  74
  75 <p>See the include-file <code>gperftools/profiler.h</code> for
  76 advanced-use functions, including <code>ProfilerFlush()</code> and
  77 <code>ProfilerStartWithOptions()</code>.</p>
  78
  79
  80 <H2>Modifying Runtime Behavior</H2>
  81
  82 <p>You can more finely control the behavior of the CPU profiler via
  83 environment variables.</p>
  84
  85 <table frame=box rules=sides cellpadding=5 width=100%>
  86
  87 <tr valign=top>
  88   <td><code>CPUPROFILE_FREQUENCY=<i>x</i></code></td>
  89   <td>default: 100</td>
  90   <td>
  91     How many interrupts/second the cpu-profiler samples.
  92   </td>
  93 </tr>
  94
  95 <tr valign=top>
  96   <td><code>CPUPROFILE_REALTIME=1</code></td>
  97   <td>default: [not set]</td>
  98   <td>
  99     If set to any value (including 0 or the empty string), use
 100     ITIMER_REAL instead of ITIMER_PROF to gather profiles.  In
 101     general, ITIMER_REAL is not as accurate as ITIMER_PROF, and also
 102     interacts badly with use of alarm(), so prefer ITIMER_PROF unless
 103     you have a reason prefer ITIMER_REAL.
 104   </td>
 105 </tr>
 106
 107 </table>
 108
 109
 110 <h1><a name="pprof">Analyzing the Output</a></h1>
 111
 112 <p><code>pprof</code> is the script used to analyze a profile.  It has
 113 many output modes, both textual and graphical.  Some give just raw
 114 numbers, much like the <code>-pg</code> output of <code>gcc</code>,
 115 and others show the data in the form of a dependency graph.</p>
 116
 117 <p>pprof <b>requires</b> <code>perl5</code> to be installed to run.
 118 It also requires <code>dot</code> to be installed for any of the
 119 graphical output routines, and <code>gv</code> to be installed for
 120 <code>--gv</code> mode (described below).
 121 </p>
 122
 123 <p>Here are some ways to call pprof.  These are described in more
 124 detail below.</p>
 125
 126 <pre>
 127 % pprof /bin/ls ls.prof
 128                        Enters "interactive" mode
 129 % pprof --text /bin/ls ls.prof
 130                        Outputs one line per procedure
 131 % pprof --gv /bin/ls ls.prof
 132                        Displays annotated call-graph via 'gv'
 133 % pprof --gv --focus=Mutex /bin/ls ls.prof
 134                        Restricts to code paths including a .*Mutex.* entry
 135 % pprof --gv --focus=Mutex --ignore=string /bin/ls ls.prof
 136                        Code paths including Mutex but not string
 137 % pprof --list=getdir /bin/ls ls.prof
 138                        (Per-line) annotated source listing for getdir()
 139 % pprof --disasm=getdir /bin/ls ls.prof
 140                        (Per-PC) annotated disassembly for getdir()
 141 % pprof --text localhost:1234
 142                        Outputs one line per procedure for localhost:1234
 143 % pprof --callgrind /bin/ls ls.prof
 144                        Outputs the call information in callgrind format
 145 </pre>
 146
 147
 148 <h3>Analyzing Text Output</h3>
 149
 150 <p>Text mode has lines of output that look like this:</p>
 151 <pre>
 152        14   2.1%  17.2%       58   8.7% std::_Rb_tree::find
 153 </pre>
 154
 155 <p>Here is how to interpret the columns:</p>
 156 <ol>
 157   <li> Number of profiling samples in this function
 158   <li> Percentage of profiling samples in this function
 159   <li> Percentage of profiling samples in the functions printed so far
 160   <li> Number of profiling samples in this function and its callees
 161   <li> Percentage of profiling samples in this function and its callees
 162   <li> Function name
 163 </ol>
 164
 165 <h3>Analyzing Callgrind Output</h3>
 166
 167 <p>Use <a href="http://kcachegrind.sourceforge.net">kcachegrind</a> to
 168 analyze your callgrind output:</p>
 169 <pre>
 170 % pprof --callgrind /bin/ls ls.prof > ls.callgrind
 171 % kcachegrind ls.callgrind
 172 </pre>
 173
 174 <p>The cost is specified in 'hits', i.e. how many times a function
 175 appears in the recorded call stack information. The 'calls' from
 176 function a to b record how many times function b was found in the
 177 stack traces directly below function a.</p>
 178
 179 <p>Tip: if you use a debug build the output will include file and line
 180 number information and kcachegrind will show an annotated source
 181 code view.</p>
 182
 183 <h3>Node Information</h3>
 184
 185 <p>In the various graphical modes of pprof, the output is a call graph
 186 annotated with timing information, like so:</p>
 187
 188 <A HREF="pprof-test-big.gif">
 189 <center><table><tr><td>
 190    <img src="pprof-test.gif">
 191 </td></tr></table></center>
 192 </A>
 193
 194 <p>Each node represents a procedure.  The directed edges indicate
 195 caller to callee relations.  Each node is formatted as follows:</p>
 196
 197 <center><pre>
 198 Class Name
 199 Method Name
 200 local (percentage)
 201 <b>of</b> cumulative (percentage)
 202 </pre></center>
 203
 204 <p>The last one or two lines contains the timing information.  (The
 205 profiling is done via a sampling method, where by default we take 100
 206 samples a second.  Therefor one unit of time in the output corresponds
 207 to about 10 milliseconds of execution time.) The "local" time is the
 208 time spent executing the instructions directly contained in the
 209 procedure (and in any other procedures that were inlined into the
 210 procedure).  The "cumulative" time is the sum of the "local" time and
 211 the time spent in any callees.  If the cumulative time is the same as
 212 the local time, it is not printed.</p>
 213
 214 <p>For instance, the timing information for test_main_thread()
 215 indicates that 155 units (about 1.55 seconds) were spent executing the
 216 code in <code>test_main_thread()</code> and 200 units were spent while
 217 executing <code>test_main_thread()</code> and its callees such as
 218 <code>snprintf()</code>.</p>
 219
 220 <p>The size of the node is proportional to the local count.  The
 221 percentage displayed in the node corresponds to the count divided by
 222 the total run time of the program (that is, the cumulative count for
 223 <code>main()</code>).</p>
 224
 225 <h3>Edge Information</h3>
 226
 227 <p>An edge from one node to another indicates a caller to callee
 228 relationship.  Each edge is labelled with the time spent by the callee
 229 on behalf of the caller.  E.g, the edge from
 230 <code>test_main_thread()</code> to <code>snprintf()</code> indicates
 231 that of the 200 samples in <code>test_main_thread()</code>, 37 are
 232 because of calls to <code>snprintf()</code>.</p>
 233
 234 <p>Note that <code>test_main_thread()</code> has an edge to
 235 <code>vsnprintf()</code>, even though <code>test_main_thread()</code>
 236 doesn't call that function directly.  This is because the code was
 237 compiled with <code>-O2</code>; the profile reflects the optimized
 238 control flow.</p>
 239
 240 <h3>Meta Information</h3>
 241
 242 <p>The top of the display should contain some meta information
 243 like:</p>
 244 <pre>
 245       /tmp/profiler2_unittest
 246       Total samples: 202
 247       Focusing on: 202
 248       Dropped nodes with &lt;= 1 abs(samples)
 249       Dropped edges with &lt;= 0 samples
 250 </pre>
 251
 252 <p>This section contains the name of the program, and the total
 253 samples collected during the profiling run.  If the
 254 <code>--focus</code> option is on (see the <a href="#focus">Focus</a>
 255 section below), the legend also contains the number of samples being
 256 shown in the focused display.  Furthermore, some unimportant nodes and
 257 edges are dropped to reduce clutter.  The characteristics of the
 258 dropped nodes and edges are also displayed in the legend.</p>
 259
 260 <h3><a name=focus>Focus and Ignore</a></h3>
 261
 262 <p>You can ask pprof to generate a display focused on a particular
 263 piece of the program.  You specify a regular expression.  Any portion
 264 of the call-graph that is on a path which contains at least one node
 265 matching the regular expression is preserved.  The rest of the
 266 call-graph is dropped on the floor.  For example, you can focus on the
 267 <code>vsnprintf()</code> libc call in <code>profiler2_unittest</code>
 268 as follows:</p>
 269
 270 <pre>
 271 % pprof --gv --focus=vsnprintf /tmp/profiler2_unittest test.prof
 272 </pre>
 273 <A HREF="pprof-vsnprintf-big.gif">
 274 <center><table><tr><td>
 275    <img src="pprof-vsnprintf.gif">
 276 </td></tr></table></center>
 277 </A>
 278
 279 <p>Similarly, you can supply the <code>--ignore</code> option to
 280 ignore samples that match a specified regular expression.  E.g., if
 281 you are interested in everything except calls to
 282 <code>snprintf()</code>, you can say:</p>
 283 <pre>
 284 % pprof --gv --ignore=snprintf /tmp/profiler2_unittest test.prof
 285 </pre>
 286
 287
 288 <h3>Interactive mode</a></h3>
 289
 290 <p>By default -- if you don't specify any flags to the contrary --
 291 pprof runs in interactive mode.  At the <code>(pprof)</code> prompt,
 292 you can run many of the commands described above.  You can type
 293 <code>help</code> for a list of what commands are available in
 294 interactive mode.</p>
 295
 296 <h3><a name=options>pprof Options</a></h3>
 297
 298 For a complete list of pprof options, you can run <code>pprof
 299 --help</code>.
 300
 301 <h4>Output Type</h4>
 302
 303 <p>
 304 <center>
 305 <table frame=box rules=sides cellpadding=5 width=100%>
 306 <tr valign=top>
 307   <td><code>--text</code></td>
 308   <td>
 309     Produces a textual listing.  (Note: If you have an X display, and
 310     <code>dot</code> and <code>gv</code> installed, you will probably
 311     be happier with the <code>--gv</code> output.)
 312   </td>
 313 </tr>
 314 <tr valign=top>
 315   <td><code>--gv</code></td>
 316   <td>
 317     Generates annotated call-graph, converts to postscript, and
 318     displays via gv (requres <code>dot</code> and <code>gv</code> be
 319     installed).
 320   </td>
 321 </tr>
 322 <tr valign=top>
 323   <td><code>--dot</code></td>
 324   <td>
 325     Generates the annotated call-graph in dot format and
 326     emits to stdout (requres <code>dot</code> be installed).
 327   </td>
 328 </tr>
 329 <tr valign=top>
 330   <td><code>--ps</code></td>
 331   <td>
 332     Generates the annotated call-graph in Postscript format and
 333     emits to stdout (requres <code>dot</code> be installed).
 334   </td>
 335 </tr>
 336 <tr valign=top>
 337   <td><code>--pdf</code></td>
 338   <td>
 339     Generates the annotated call-graph in PDF format and emits to
 340     stdout (requires <code>dot</code> and <code>ps2pdf</code> be
 341     installed).
 342   </td>
 343 </tr>
 344 <tr valign=top>
 345   <td><code>--gif</code></td>
 346   <td>
 347     Generates the annotated call-graph in GIF format and
 348     emits to stdout (requres <code>dot</code> be installed).
 349   </td>
 350 </tr>
 351 <tr valign=top>
 352   <td><code>--list=&lt;<i>regexp</i>&gt;</code></td>
 353   <td>
 354     <p>Outputs source-code listing of routines whose
 355     name matches &lt;regexp&gt;.  Each line
 356     in the listing is annotated with flat and cumulative
 357     sample counts.</p>
 358
 359     <p>In the presence of inlined calls, the samples
 360     associated with inlined code tend to get assigned
 361     to a line that follows the location of the
 362     inlined call.  A more precise accounting can be
 363     obtained by disassembling the routine using the
 364     --disasm flag.</p>
 365   </td>
 366 </tr>
 367 <tr valign=top>
 368   <td><code>--disasm=&lt;<i>regexp</i>&gt;</code></td>
 369   <td>
 370     Generates disassembly of routines that match
 371     &lt;regexp&gt;, annotated with flat and
 372     cumulative sample counts and emits to stdout.
 373   </td>
 374 </tr>
 375 </table>
 376 </center>
 377
 378 <h4>Reporting Granularity</h4>
 379
 380 <p>By default, pprof produces one entry per procedure.  However you can
 381 use one of the following options to change the granularity of the
 382 output.  The <code>--files</code> option seems to be particularly
 383 useless, and may be removed eventually.</p>
 384
 385 <center>
 386 <table frame=box rules=sides cellpadding=5 width=100%>
 387 <tr valign=top>
 388   <td><code>--addresses</code></td>
 389   <td>
 390      Produce one node per program address.
 391   </td>
 392 </tr>
 393   <td><code>--lines</code></td>
 394   <td>
 395      Produce one node per source line.
 396   </td>
 397 </tr>
 398   <td><code>--functions</code></td>
 399   <td>
 400      Produce one node per function (this is the default).
 401   </td>
 402 </tr>
 403   <td><code>--files</code></td>
 404   <td>
 405      Produce one node per source file.
 406   </td>
 407 </tr>
 408 </table>
 409 </center>
 410
 411 <h4>Controlling the Call Graph Display</h4>
 412
 413 <p>Some nodes and edges are dropped to reduce clutter in the output
 414 display.  The following options control this effect:</p>
 415
 416 <center>
 417 <table frame=box rules=sides cellpadding=5 width=100%>
 418 <tr valign=top>
 419   <td><code>--nodecount=&lt;n&gt;</code></td>
 420   <td>
 421     This option controls the number of displayed nodes.  The nodes
 422     are first sorted by decreasing cumulative count, and then only
 423     the top N nodes are kept.  The default value is 80.
 424   </td>
 425 </tr>
 426 <tr valign=top>
 427   <td><code>--nodefraction=&lt;f&gt;</code></td>
 428   <td>
 429     This option provides another mechanism for discarding nodes
 430     from the display.  If the cumulative count for a node is
 431     less than this option's value multiplied by the total count
 432     for the profile, the node is dropped.  The default value
 433     is 0.005; i.e. nodes that account for less than
 434     half a percent of the total time are dropped.  A node
 435     is dropped if either this condition is satisfied, or the
 436     --nodecount condition is satisfied.
 437   </td>
 438 </tr>
 439 <tr valign=top>
 440   <td><code>--edgefraction=&lt;f&gt;</code></td>
 441   <td>
 442     This option controls the number of displayed edges.  First of all,
 443     an edge is dropped if either its source or destination node is
 444     dropped.  Otherwise, the edge is dropped if the sample
 445     count along the edge is less than this option's value multiplied
 446     by the total count for the profile.  The default value is
 447     0.001; i.e., edges that account for less than
 448     0.1% of the total time are dropped.
 449   </td>
 450 </tr>
 451 <tr valign=top>
 452   <td><code>--focus=&lt;re&gt;</code></td>
 453   <td>
 454     This option controls what region of the graph is displayed
 455     based on the regular expression supplied with the option.
 456     For any path in the callgraph, we check all nodes in the path
 457     against the supplied regular expression.  If none of the nodes
 458     match, the path is dropped from the output.
 459   </td>
 460 </tr>
 461 <tr valign=top>
 462   <td><code>--ignore=&lt;re&gt;</code></td>
 463   <td>
 464     This option controls what region of the graph is displayed
 465     based on the regular expression supplied with the option.
 466     For any path in the callgraph, we check all nodes in the path
 467     against the supplied regular expression.  If any of the nodes
 468     match, the path is dropped from the output.
 469   </td>
 470 </tr>
 471 </table>
 472 </center>
 473
 474 <p>The dropped edges and nodes account for some count mismatches in
 475 the display.  For example, the cumulative count for
 476 <code>snprintf()</code> in the first diagram above was 41.  However
 477 the local count (1) and the count along the outgoing edges (12+1+20+6)
 478 add up to only 40.</p>
 479
 480
 481 <h1>Caveats</h1>
 482
 483 <ul>
 484   <li> If the program exits because of a signal, the generated profile
 485        will be <font color=red>incomplete, and may perhaps be
 486        completely empty</font>.
 487   <li> The displayed graph may have disconnected regions because
 488        of the edge-dropping heuristics described above.
 489   <li> If the program linked in a library that was not compiled
 490        with enough symbolic information, all samples associated
 491        with the library may be charged to the last symbol found
 492        in the program before the library.  This will artificially
 493        inflate the count for that symbol.
 494   <li> If you run the program on one machine, and profile it on
 495        another, and the shared libraries are different on the two
 496        machines, the profiling output may be confusing: samples that
 497        fall within  shared libaries may be assigned to arbitrary
 498        procedures.
 499   <li> If your program forks, the children will also be profiled
 500        (since they inherit the same CPUPROFILE setting).  Each process
 501        is profiled separately; to distinguish the child profiles from
 502        the parent profile and from each other, all children will have
 503        their process-id appended to the CPUPROFILE name.
 504   <li> Due to a hack we make to work around a possible gcc bug, your
 505        profiles may end up named strangely if the first character of
 506        your CPUPROFILE variable has ascii value greater than 127.
 507        This should be exceedingly rare, but if you need to use such a
 508        name, just set prepend <code>./</code> to your filename:
 509        <code>CPUPROFILE=./&Auml;gypten</code>.
 510 </ul>
 511
 512
 513 <hr>
 514 <address>Sanjay Ghemawat<br>
 515 <!-- Created: Tue Dec 19 10:43:14 PST 2000 -->
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 517 Last modified: Fri May  9 14:41:29 PDT 2008
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