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<html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>Implementation Issues</title><meta name="generator" content="DocBook XSL Stylesheets V1.75.2" /><meta name="keywords" content=" C++ , library , profile " /><meta name="keywords" content=" ISO C++ , library " /><link rel="home" href="../spine.html" title="The GNU C++ Library Documentation" /><link rel="up" href="profile_mode.html" title="Chapter 32. Profile Mode" /><link rel="prev" href="bk01pt12ch32s04.html" title="Empirical Cost Model" /><link rel="next" href="bk01pt12ch32s06.html" title="Developer Information" /></head><body><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Implementation Issues</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="bk01pt12ch32s04.html">Prev</a> </td><th width="60%" align="center">Chapter 32. Profile Mode</th><td width="20%" align="right"> <a accesskey="n" href="bk01pt12ch32s06.html">Next</a></td></tr></table><hr /></div><div class="sect1" title="Implementation Issues"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="manual.ext.profile_mode.implementation"></a>Implementation Issues</h2></div></div></div><div class="sect2" title="Stack Traces"><div class="titlepage"><div><div><h3 class="title"><a id="manual.ext.profile_mode.implementation.stack"></a>Stack Traces</h3></div></div></div><p>
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Accurate stack traces are needed during profiling since we group events by
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call context and dynamic instance. Without accurate traces, diagnostics
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may be hard to interpret. For instance, when giving advice to the user
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it is imperative to reference application code, not library code.
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</p><p>
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Currently we are using the libc <code class="code">backtrace</code> routine to get
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stack traces.
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<code class="code">_GLIBCXX_PROFILE_STACK_DEPTH</code> can be set
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to 0 if you are willing to give up call context information, or to a small
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positive value to reduce run time overhead.
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</p></div><div class="sect2" title="Symbolization of Instruction Addresses"><div class="titlepage"><div><div><h3 class="title"><a id="manual.ext.profile_mode.implementation.symbols"></a>Symbolization of Instruction Addresses</h3></div></div></div><p>
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The profiling and analysis phases use only instruction addresses.
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An external utility such as addr2line is needed to postprocess the result.
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We do not plan to add symbolization support in the profile extension.
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This would require access to symbol tables, debug information tables,
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external programs or libraries and other system dependent information.
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</p></div><div class="sect2" title="Concurrency"><div class="titlepage"><div><div><h3 class="title"><a id="manual.ext.profile_mode.implementation.concurrency"></a>Concurrency</h3></div></div></div><p>
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Our current model is simplistic, but precise.
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We cannot afford to approximate because some of our diagnostics require
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precise matching of operations to container instance and call context.
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During profiling, we keep a single information table per diagnostic.
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There is a single lock per information table.
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</p></div><div class="sect2" title="Using the Standard Library in the Instrumentation Implementation"><div class="titlepage"><div><div><h3 class="title"><a id="manual.ext.profile_mode.implementation.stdlib-in-proflib"></a>Using the Standard Library in the Instrumentation Implementation</h3></div></div></div><p>
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As much as we would like to avoid uses of stdlibc++ within our
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instrumentation library, containers such as unordered_map are very
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appealing. We plan to use them as long as they are named properly
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to avoid ambiguity.
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</p></div><div class="sect2" title="Malloc Hooks"><div class="titlepage"><div><div><h3 class="title"><a id="manual.ext.profile_mode.implementation.malloc-hooks"></a>Malloc Hooks</h3></div></div></div><p>
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User applications/libraries can provide malloc hooks.
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When the implementation of the malloc hooks uses stdlibc++, there can
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be an infinite cycle between the profile mode instrumentation and the
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the malloc hook code.
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</p><p>
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We protect against reentrance to the profile mode instrumentation code,
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which should avoid this problem in most cases.
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The protection mechanism is thread safe and exception safe.
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This mechanism does not prevent reentrance to the malloc hook itself,
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which could still result in deadlock, if, for instance, the malloc hook
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uses non-recursive locks.
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XXX: A definitive solution to this problem would be for the profile extension
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to use a custom allocator internally, and perhaps not to use libstdc++.
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</p></div><div class="sect2" title="Construction and Destruction of Global Objects"><div class="titlepage"><div><div><h3 class="title"><a id="manual.ext.profile_mode.implementation.construction-destruction"></a>Construction and Destruction of Global Objects</h3></div></div></div><p>
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The profiling library state is initialized at the first call to a profiling
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method. This allows us to record the construction of all global objects.
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However, we cannot do the same at destruction time. The trace is written
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by a function registered by <code class="code">atexit</code>, thus invoked by
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<code class="code">exit</code>.
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</p></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="bk01pt12ch32s04.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="profile_mode.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="bk01pt12ch32s06.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">Empirical Cost Model </td><td width="20%" align="center"><a accesskey="h" href="../spine.html">Home</a></td><td width="40%" align="right" valign="top"> Developer Information</td></tr></table></div></body></html>
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