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271 lines
12 KiB
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<head><title>The Jprof Profiler</title></head>
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<body bgcolor="#FFFFFF" text="#000000"
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link="#0000EE" vlink="#551A8B" alink="#FF0000">
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<center>
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<h1>The Jprof Profiler</h1>
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<font size="-1">
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<a href="mailto:jim_nance%yahoo.com">jim_nance@yahoo.com</a>
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</font>
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<hr>
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<a href="#introduction">Introduction</a> | <a href="#operation">Operation</a> |
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<a href="#setup">Setup</a> | <a href="#usage">Usage</a> |
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<a href="#interpretation">Interpretation</a>
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</center>
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<hr>
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<h3><a name="introduction">Introduction</a></h3>
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Jprof is a profiling tool. I am writing it because I need to find out
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where mozilla is spending its time, and there do not seem to be any
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profilers for Linux that can handle threads and/or shared libraries.
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This code is based heavily on Kipp Hickman's leaky.
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<h3><a name="operation">Operation</a></h3>
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Jprof operates by installing a timer which periodically interrupts mozilla.
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When this timer goes off, the jprof code inside mozilla walks the function call
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stack to determine which code was executing and saves the results into the
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<code>jprof-log</code> and <code>jprof-map</code> files. By collecting a large
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number of these call stacks, it is possible to deduce where mozilla is spending
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its time.
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<h3><a name="setup">Setup</a></h3>
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First, check out the jprof source code since it is not a part of the
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default pull. To do this do:
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<pre>
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cvs co mozilla/tools/jprof
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</pre>
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<p>Next, configure your mozilla with jprof support by adding
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<code>--enable-jprof</code> to your configure options (eg adding
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<code>ac_add_options --enable-jprof</code> to your <code>.mozconfig</code>) and
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making sure that you do <strong>not</strong> have the
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<code>--enable-strip</code> configure option set -- jprof needs symbols to
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operate.</p>
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<p>Finally, build mozilla with your new configuration. Now you can run jprof.</p>
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<h3><a name="usage">Usage</a></h3>
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The behavior of jprof is determined by the value of the JPROF_FLAGS environment
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variable. This environment variable can be composed of several substrings
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which have the following meanings:
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<ul>
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<li> <b>JP_START</b> : Install the signal handler, and start sending the
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timer signals.
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<li> <b>JP_DEFER</b> : Install the signal handler, but don't start sending
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the timer signals. The user must start the signals by sending the first
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one (with <code>kill -PROF</code>, or with <code>kill -ALRM</code> if
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JP_REALTIME is used, or with <code>kill -POLL</code> (also known as <code>kill -IO</code>) if JP_RTC_HZ is used).
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<li> <b>JP_FIRST=x</b> : Wait x seconds before starting the timer
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<li> <b>JP_PERIOD=y</b> : Set timer to interrupt every y seconds. Only
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values of y strictly greater than 0.001 are supported.
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<li> <b>JP_REALTIME</b> : Do the profiling in intervals of real time rather
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than intervals of time used by the mozilla process (and the kernel
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when doing work for mozilla). This could probably lead to weird
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results (you'll see whatever runs when mozilla is waiting for events),
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but is needed to see time spent in the X server.
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<li> <b>JP_RTC_HZ=freq</b> : This option, only available on Linux if the
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kernel is built with RTC support, makes jprof use the RTC timer instead of
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using its own timer. This option, like JP_REALTIME, uses intervals of real
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time. This option overrides JP_PERIOD. <code>freq</code> is the frequency
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at which the timer should fire, measured in Hz. It must be a power of 2.
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The maximal frequency allowed by the kernel can be changed by writing to
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<code>/proc/sys/dev/rtc/max-user-freq</code>; the maximum value it can be
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set to is 8192. Note that <code>/dev/rtc</code> will need to be readable
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by the Firefox process; making that file world-readable is a simple way to
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accomplish that.
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</ul>
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<h4>Starting and stopping jprof from JavaScript</h4>
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<p>
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A build with jprof enabled adds two functions to the Window object:
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<code>JProfStartProfiling()</code> and <code>JProfStopProfiling()</code>. When used with JP_DEFER, these
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allow one to start and stop the timer just around whatever critical section is
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being profiled
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</p>
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<h4>Examples of JPROF_FLAGS usage</h4>
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<ul>
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<li>To make the timer start firing 3 seconds after the program is started and
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fire every 25 milliseconds of program time use:
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<pre>
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setenv JPROF_FLAGS "JP_START JP_FIRST=3 JP_PERIOD=0.025" </pre>
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<li>To make the timer start on your signal and fire every 1.5 milliseconds of
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program time use:
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<pre>
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setenv JPROF_FLAGS "JP_DEFER JP_PERIOD=0.0015" </pre>
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<li>To make the timer start on your signal and fire every 10 milliseconds of
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wall-clock time use:
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<pre>
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setenv JPROF_FLAGS "JP_DEFER JP_PERIOD=0.010 JP_REALTIME" </pre>
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<li>To make the timer start on your signal and fire at 8192 Hz in wall-clock
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time use:
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<pre>
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setenv JPROF_FLAGS "JP_DEFER JP_RTC_HZ=8192" </pre>
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</ul>
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<h4>Pausing profiles</h4>
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<P>jprof can be paused at any time by sending a SIGUSR1 to mozilla (<code>kill
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-USR1</code>). This will cause the timer signals to stop and jprof-map to be
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written, but it will not close jprof-log. Combining SIGUSR1 with the JP_DEFER
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option allows profiling of one sequence of actions by starting the timer right
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before starting the actions and stopping the timer right afterward.
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<P>After a SIGUSR1, sending another timer signal (SIGPROF, SIGALRM, or SIGPOLL (aka SIGIO),
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depending on the mode) can be used to continue writing data to the same output.
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<h4>Looking at the results</h4>
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Now that we have <code>jprof-log</code> and <code>jprof-map</code> files, we
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can use the jprof executable is used to turn them into readable output. To do
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this jprof needs the name of the mozilla binary and the log file. It deduces
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the name of the map file:
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<pre>
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./jprof /home/user/mozilla/debug/dist/bin/mozilla-bin ./jprof-log > tmp.html
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</pre>
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This will generate the file <code>tmp.html</code> which you should view in a
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web browser.
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<h3><a name="interpretation">Interpretation</a></h3>
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The Jprof output is split into a flat portion and a hierarchical portion.
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There are links to each section at the top of the page. It is typically
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easier to analyze the profile by starting with the flat output and following
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the links contained in the flat output up to the hierarchical output.
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<h4><a name="flat">Flat output</a></h3>
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The flat portion of the profile indicates which functions were executing
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when the timer was going off. It is displayed as a list of functions names
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on the right and the number of times that function was interrupted on the
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left. The list is sorted by decreasing interrupt count. For example:
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<blockquote> <pre>
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Total hit count: 151603
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Count %Total Function Name
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<a href="#23081">8806 5.8 __libc_poll</a>
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<a href="#40008">2254 1.5 __i686.get_pc_thunk.bx</a>
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<a href="#21390">2053 1.4 _int_malloc</a>
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<a href="#49013">1777 1.2 nsStyleContext::GetStyleData(nsStyleStructID)</a>
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<a href="#21380">1600 1.1 __libc_malloc</a>
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<a href="#603">1552 1.0 nsCOMPtr_base::~nsCOMPtr_base()</a>
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</pre> </blockquote>
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This shows that of the 151603 times the timer fired, 1777 (1.2% of the total) were inside nsStyleContext::GetStyleData() and 1552 (1.0% of the total) were in the nsCOMPtr_base destructor.
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<p>
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In general, the functions with the highest count are the functions which
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are taking the most time.
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<P>
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The function names are linked to the entry for that function in the
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hierarchical profile, which is described in the next section.
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<h4><a name="hier">Hierarchical output</a></h4>
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The hierarchical output is divided up into sections, with each section
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corresponding to one function. A typical section looks something like
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this:
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<blockquote><pre>
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<A href="#29355">141300 PL_ProcessPendingEvents</A>
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<A href="#29372"> 927 PL_ProcessEventsBeforeID</A>
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29358 0 <a name=29358> 142227</a> <b>PL_HandleEvent</b>
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<A href="#28546"> 92394 nsInputStreamReadyEvent::EventHandler(PLEvent*)</A>
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<A href="#41572"> 49181 HandlePLEvent(ReflowEvent*)</A>
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<A href="#29537"> 481 handleTimerEvent(TimerEventType*)</A>
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<A href="#34494"> 158 nsTransportStatusEvent::HandleEvent(PLEvent*)</A>
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<A href="#29359"> 9 PL_DestroyEvent</A>
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<A href="#20319"> 4 __restore_rt</A>
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</pre></blockquote>
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The information this block tells us is:
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<ul>
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<li>There were 0 profiler hits <em>in</em> <code>PL_HandleEvent</code>
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<li>There were 142227 profiler hits <em>under</em> <code>PL_HandleEvent</code>. Of these:
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<ul>
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<li>92394 were in or under <code>nsInputStreamReadyEvent::EventHandler</code>
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<li>49181 were in or under <code>HandlePLEvent(ReflowEvent*)</code>
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<li>481 were in or under <code>handleTimerEvent</code>
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<li>158 were in or under <code>nsTransportStatusEvent::HandleEvent</code>
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<li>9 were in or under <code>PL_DestroyEvent</code>
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<li>4 were in or under <code>__restore_rt</code>
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</ul>
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<li>Of these 142227 calls into <code>PL_HandleEvent</code>:
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<ul>
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<li>141300 came from <code>PL_ProcessPendingEvents</code>
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<li>927 came from <code>PL_ProcessEventsBeforeID</code>
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</ul>
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</ul>
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The rest of this section explains how to read this information off from the jprof output.
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<p>This block corresponds to the function <code>PL_HandleEvent</code>, which is
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therefore bolded and not a link. The name of this function is preceded by
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three numbers which have the following meaning. The number on the left (29358)
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is the index number, and is not important. The center number (0) is the number
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of times this function was interrupted by the timer. The last number (142227)
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is the number of times this function was in the call stack when the timer went
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off. That is, the timer went off while we were in code that was ultimately
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called from <code>PL_HandleEvent</code>.
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<p>For our example we can see that our function was in the call stack for
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142227 interrupt ticks, but we were never the function that was running when
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the interrupt arrived.
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<P>
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The functions listed above the line for <code>PL_HandleEvent</code> are its
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callers. The numbers to the left of these function names are the numbers of
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times these functions were in the call stack as callers of
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<code>PL_HandleEvent</code>. In our example, we were called 927 times by
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<code>PL_ProcessEventsBeforeID</code> and 141300 times by
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<code>PL_ProcessPendingEvents</code>.
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<P>
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The functions listed below the line for <code>PL_HandleEvent</code> are its
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callees. The numbers to the left of the function names are the numbers of
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times these functions were in the callstack as callees of <code>PL_HandleEvent</code>. In our example, of the 142227 profiler hits under <code>PL_HandleEvent</code> 92394 were under <code>nsInputStreamReadyEvent::EventHandler</code>, 49181 were under <code>HandlePLEvent(ReflowEvent*)</code>, and so forth.
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<h3>Bugs</h3>
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Jprof has only been tested under Red Hat Linux 6.0, 6.1, and 6.2. It does
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not work under 6.0, though it is possible hack up the source code and make
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it work there. The way I determine the stack trace from inside the
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signal handler is tightly bound to the version of glibc that is running.
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If you know of a more portable way to get this information please let
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me know.
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<h3>Update</h3>
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<ul>
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<li>Ben Bucksch reports that installing the Red Hat 6.1 glibc rpms on a Red Hat
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6.0 system allows jprof to work, and does not seem to break anything except
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gdm (the Gnome login program), and that can be fixed by installing the RH 6.1
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gdb rpm.</li>
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<li>David Baron reports that jprof works under RedHat 6.0 if one uncomments
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the <code>#define JPROF_PTHREAD_HACK</code> near the beginning of
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<code>libmalloc.cpp</code>.</li>
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</ul>
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</body>
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</html>
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