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| <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><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 Vsnapshot" /><meta name="keywords" content="C++, library, profile" /><meta name="keywords" content="ISO C++, library" /><meta name="keywords" content="ISO C++, runtime, library" /><link rel="home" href="../index.html" title="The GNU C++ Library" /><link rel="up" href="profile_mode.html" title="Chapter 19. Profile Mode" /><link rel="prev" href="profile_mode_cost_model.html" title="Empirical Cost Model" /><link rel="next" href="profile_mode_devel.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="profile_mode_cost_model.html">Prev</a> </td><th width="60%" align="center">Chapter 19. Profile Mode</th><td width="20%" align="right"> <a accesskey="n" href="profile_mode_devel.html">Next</a></td></tr></table><hr /></div><div class="section"><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="section"><div class="titlepage"><div><div><h3 class="title"><a id="manual.ext.profile_mode.implementation.stack"></a>Stack Traces</h3></div></div></div><p> |
| Accurate stack traces are needed during profiling since we group events by |
| call context and dynamic instance. Without accurate traces, diagnostics |
| may be hard to interpret. For instance, when giving advice to the user |
| it is imperative to reference application code, not library code. |
| </p><p> |
| Currently we are using the libc <code class="code">backtrace</code> routine to get |
| stack traces. |
| <code class="code">_GLIBCXX_PROFILE_STACK_DEPTH</code> can be set |
| to 0 if you are willing to give up call context information, or to a small |
| positive value to reduce run time overhead. |
| </p></div><div class="section"><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> |
| The profiling and analysis phases use only instruction addresses. |
| An external utility such as addr2line is needed to postprocess the result. |
| We do not plan to add symbolization support in the profile extension. |
| This would require access to symbol tables, debug information tables, |
| external programs or libraries and other system dependent information. |
| </p></div><div class="section"><div class="titlepage"><div><div><h3 class="title"><a id="manual.ext.profile_mode.implementation.concurrency"></a>Concurrency</h3></div></div></div><p> |
| Our current model is simplistic, but precise. |
| We cannot afford to approximate because some of our diagnostics require |
| precise matching of operations to container instance and call context. |
| During profiling, we keep a single information table per diagnostic. |
| There is a single lock per information table. |
| </p></div><div class="section"><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> |
| As much as we would like to avoid uses of libstdc++ within our |
| instrumentation library, containers such as unordered_map are very |
| appealing. We plan to use them as long as they are named properly |
| to avoid ambiguity. |
| </p></div><div class="section"><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> |
| User applications/libraries can provide malloc hooks. |
| When the implementation of the malloc hooks uses stdlibc++, there can |
| be an infinite cycle between the profile mode instrumentation and the |
| malloc hook code. |
| </p><p> |
| We protect against reentrance to the profile mode instrumentation code, |
| which should avoid this problem in most cases. |
| The protection mechanism is thread safe and exception safe. |
| This mechanism does not prevent reentrance to the malloc hook itself, |
| which could still result in deadlock, if, for instance, the malloc hook |
| uses non-recursive locks. |
| XXX: A definitive solution to this problem would be for the profile extension |
| to use a custom allocator internally, and perhaps not to use libstdc++. |
| </p></div><div class="section"><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> |
| The profiling library state is initialized at the first call to a profiling |
| method. This allows us to record the construction of all global objects. |
| However, we cannot do the same at destruction time. The trace is written |
| by a function registered by <code class="code">atexit</code>, thus invoked by |
| <code class="code">exit</code>. |
| </p></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="profile_mode_cost_model.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="profile_mode_devel.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="../index.html">Home</a></td><td width="40%" align="right" valign="top"> Developer Information</td></tr></table></div></body></html> |
