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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN" "http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd">
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<html xmlns="http://www.w3.org/1999/xhtml"><head><title>Design</title><meta name="generator" content="DocBook XSL-NS Stylesheets V1.76.1"/><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.html" title="Chapter 19. Profile Mode"/><link rel="next" href="bk01pt03ch19s03.html" title="Extensions for Custom Containers"/></head><body><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Design</th></tr><tr><td align="left"><a accesskey="p" href="profile_mode.html">Prev</a> </td><th width="60%" align="center">Chapter 19. Profile Mode</th><td align="right"> <a accesskey="n" href="bk01pt03ch19s03.html">Next</a></td></tr></table><hr/></div><div class="section" title="Design"><div class="titlepage"><div><div><h2 class="title"><a id="manual.ext.profile_mode.design"/>Design</h2></div></div></div><p>
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</p><div class="table"><a id="id510967"/><p class="title"><strong>Table 19.1. Profile Code Location</strong></p><div class="table-contents"><table summary="Profile Code Location" border="1"><colgroup><col style="text-align: left" class="c1"/><col style="text-align: left" class="c2"/></colgroup><thead><tr><th style="text-align: left">Code Location</th><th style="text-align: left">Use</th></tr></thead><tbody><tr><td style="text-align: left"><code class="code">libstdc++-v3/include/std/*</code></td><td style="text-align: left">Preprocessor code to redirect to profile extension headers.</td></tr><tr><td style="text-align: left"><code class="code">libstdc++-v3/include/profile/*</code></td><td style="text-align: left">Profile extension public headers (map, vector, ...).</td></tr><tr><td style="text-align: left"><code class="code">libstdc++-v3/include/profile/impl/*</code></td><td style="text-align: left">Profile extension internals. Implementation files are
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only included from <code class="code">impl/profiler.h</code>, which is the only
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file included from the public headers.</td></tr></tbody></table></div></div><br class="table-break"/><p>
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</p><div class="section" title="Wrapper Model"><div class="titlepage"><div><div><h3 class="title"><a id="manual.ext.profile_mode.design.wrapper"/>Wrapper Model</h3></div></div></div><p>
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In order to get our instrumented library version included instead of the
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release one,
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we use the same wrapper model as the debug mode.
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We subclass entities from the release version. Wherever
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<code class="code">_GLIBCXX_PROFILE</code> is defined, the release namespace is
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<code class="code">std::__norm</code>, whereas the profile namespace is
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<code class="code">std::__profile</code>. Using plain <code class="code">std</code> translates
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into <code class="code">std::__profile</code>.
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</p><p>
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Whenever possible, we try to wrap at the public interface level, e.g.,
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in <code class="code">unordered_set</code> rather than in <code class="code">hashtable</code>,
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in order not to depend on implementation.
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</p><p>
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Mixing object files built with and without the profile mode must
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not affect the program execution. However, there are no guarantees to
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the accuracy of diagnostics when using even a single object not built with
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<code class="code">-D_GLIBCXX_PROFILE</code>.
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Currently, mixing the profile mode with debug and parallel extensions is
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not allowed. Mixing them at compile time will result in preprocessor errors.
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Mixing them at link time is undefined.
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</p></div><div class="section" title="Instrumentation"><div class="titlepage"><div><div><h3 class="title"><a id="manual.ext.profile_mode.design.instrumentation"/>Instrumentation</h3></div></div></div><p>
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Instead of instrumenting every public entry and exit point,
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we chose to add instrumentation on demand, as needed
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by individual diagnostics.
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The main reason is that some diagnostics require us to extract bits of
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internal state that are particular only to that diagnostic.
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We plan to formalize this later, after we learn more about the requirements
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of several diagnostics.
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</p><p>
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All the instrumentation points can be switched on and off using
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<code class="code">-D[_NO]_GLIBCXX_PROFILE_<diagnostic></code> options.
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With all the instrumentation calls off, there should be negligible
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overhead over the release version. This property is needed to support
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diagnostics based on timing of internal operations. For such diagnostics,
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we anticipate turning most of the instrumentation off in order to prevent
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profiling overhead from polluting time measurements, and thus diagnostics.
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</p><p>
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All the instrumentation on/off compile time switches live in
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<code class="code">include/profile/profiler.h</code>.
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</p></div><div class="section" title="Run Time Behavior"><div class="titlepage"><div><div><h3 class="title"><a id="manual.ext.profile_mode.design.rtlib"/>Run Time Behavior</h3></div></div></div><p>
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For practical reasons, the instrumentation library processes the trace
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partially
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rather than dumping it to disk in raw form. Each event is processed when
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it occurs. It is usually attached a cost and it is aggregated into
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the database of a specific diagnostic class. The cost model
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is based largely on the standard performance guarantees, but in some
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cases we use knowledge about GCC's standard library implementation.
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</p><p>
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Information is indexed by (1) call stack and (2) instance id or address
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to be able to understand and summarize precise creation-use-destruction
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dynamic chains. Although the analysis is sensitive to dynamic instances,
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the reports are only sensitive to call context. Whenever a dynamic instance
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is destroyed, we accumulate its effect to the corresponding entry for the
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call stack of its constructor location.
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</p><p>
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For details, see
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<a class="link" href="http://dx.doi.org/10.1109/CGO.2009.36">paper presented at
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CGO 2009</a>.
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</p></div><div class="section" title="Analysis and Diagnostics"><div class="titlepage"><div><div><h3 class="title"><a id="manual.ext.profile_mode.design.analysis"/>Analysis and Diagnostics</h3></div></div></div><p>
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Final analysis takes place offline, and it is based entirely on the
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generated trace and debugging info in the application binary.
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See section Diagnostics for a list of analysis types that we plan to support.
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</p><p>
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The input to the analysis is a table indexed by profile type and call stack.
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The data type for each entry depends on the profile type.
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</p></div><div class="section" title="Cost Model"><div class="titlepage"><div><div><h3 class="title"><a id="manual.ext.profile_mode.design.cost-model"/>Cost Model</h3></div></div></div><p>
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While it is likely that cost models become complex as we get into
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more sophisticated analysis, we will try to follow a simple set of rules
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at the beginning.
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</p><div class="itemizedlist"><ul class="itemizedlist"><li class="listitem"><p><span class="emphasis"><em>Relative benefit estimation:</em></span>
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The idea is to estimate or measure the cost of all operations
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in the original scenario versus the scenario we advise to switch to.
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For instance, when advising to change a vector to a list, an occurrence
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of the <code class="code">insert</code> method will generally count as a benefit.
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Its magnitude depends on (1) the number of elements that get shifted
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and (2) whether it triggers a reallocation.
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</p></li><li class="listitem"><p><span class="emphasis"><em>Synthetic measurements:</em></span>
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We will measure the relative difference between similar operations on
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different containers. We plan to write a battery of small tests that
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compare the times of the executions of similar methods on different
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containers. The idea is to run these tests on the target machine.
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If this training phase is very quick, we may decide to perform it at
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library initialization time. The results can be cached on disk and reused
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across runs.
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</p></li><li class="listitem"><p><span class="emphasis"><em>Timers:</em></span>
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We plan to use timers for operations of larger granularity, such as sort.
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For instance, we can switch between different sort methods on the fly
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and report the one that performs best for each call context.
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</p></li><li class="listitem"><p><span class="emphasis"><em>Show stoppers:</em></span>
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We may decide that the presence of an operation nullifies the advice.
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For instance, when considering switching from <code class="code">set</code> to
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<code class="code">unordered_set</code>, if we detect use of operator <code class="code">++</code>,
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we will simply not issue the advice, since this could signal that the use
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care require a sorted container.</p></li></ul></div></div><div class="section" title="Reports"><div class="titlepage"><div><div><h3 class="title"><a id="manual.ext.profile_mode.design.reports"/>Reports</h3></div></div></div><p>
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There are two types of reports. First, if we recognize a pattern for which
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we have a substitute that is likely to give better performance, we print
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the advice and estimated performance gain. The advice is usually associated
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to a code position and possibly a call stack.
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</p><p>
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Second, we report performance characteristics for which we do not have
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a clear solution for improvement. For instance, we can point to the user
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the top 10 <code class="code">multimap</code> locations
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which have the worst data locality in actual traversals.
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Although this does not offer a solution,
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it helps the user focus on the key problems and ignore the uninteresting ones.
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</p></div><div class="section" title="Testing"><div class="titlepage"><div><div><h3 class="title"><a id="manual.ext.profile_mode.design.testing"/>Testing</h3></div></div></div><p>
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First, we want to make sure we preserve the behavior of the release mode.
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You can just type <code class="code">"make check-profile"</code>, which
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builds and runs the whole test suite in profile mode.
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</p><p>
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Second, we want to test the correctness of each diagnostic.
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We created a <code class="code">profile</code> directory in the test suite.
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Each diagnostic must come with at least two tests, one for false positives
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and one for false negatives.
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</p></div></div><div class="navfooter"><hr/><table width="100%" summary="Navigation footer"><tr><td align="left"><a accesskey="p" href="profile_mode.html">Prev</a> </td><td align="center"><a accesskey="u" href="profile_mode.html">Up</a></td><td align="right"> <a accesskey="n" href="bk01pt03ch19s03.html">Next</a></td></tr><tr><td align="left" valign="top">Chapter 19. Profile Mode </td><td align="center"><a accesskey="h" href="../index.html">Home</a></td><td align="right" valign="top"> Extensions for Custom Containers</td></tr></table></div></body></html>
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