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POThe cost of passing by shared_ptr
 

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  1. POThe cost of passing by shared_ptr
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    <p>I use std::tr1::shared_ptr extensively throughout my application. This includes passing objects in as function arguments. Consider the following:</p> <pre><code>class Dataset {...} void f( shared_ptr&lt; Dataset const &gt; pds ) {...} void g( shared_ptr&lt; Dataset const &gt; pds ) {...} ... </code></pre> <p>While passing a dataset object around via shared_ptr guarantees its existence inside f and g, the functions may be called millions of times, which causes a lot of shared_ptr objects being created and destroyed. Here's a snippet of the flat gprof profile from a recent run:</p> <pre> Each sample counts as 0.01 seconds. % cumulative self self total time seconds seconds calls s/call s/call name 9.74 295.39 35.12 2451177304 0.00 0.00 std::tr1::__shared_count::__shared_count(std::tr1::__shared_count const&) 8.03 324.34 28.95 2451252116 0.00 0.00 std::tr1::__shared_count::~__shared_count() </pre> <p>So, ~17% of the runtime was spent on reference counting with shared_ptr objects. Is this normal?</p> <p>A large portion of my application is single-threaded and I was thinking about re-writing some of the functions as</p> <pre><code>void f( const Dataset&amp; ds ) {...} </code></pre> <p>and replacing the calls</p> <pre><code>shared_ptr&lt; Dataset &gt; pds( new Dataset(...) ); f( pds ); </code></pre> <p>with</p> <pre><code>f( *pds ); </code></pre> <p>in places where I know for sure the object will not get destroyed while the flow of the program is inside f(). But before I run off to change a bunch of function signatures / calls, I wanted to know what the typical performance hit of passing by shared_ptr was. Seems like shared_ptr should not be used for functions that get called very often.</p> <p>Any input would be appreciated. Thanks for reading.</p> <p>-Artem</p> <p><strong>Update:</strong> After changing a handful of functions to accept <code>const Dataset&amp;</code>, the new profile looks like this:</p> <pre> Each sample counts as 0.01 seconds. % cumulative self self total time seconds seconds calls s/call s/call name 0.15 241.62 0.37 24981902 0.00 0.00 std::tr1::__shared_count::~__shared_count() 0.12 241.91 0.30 28342376 0.00 0.00 std::tr1::__shared_count::__shared_count(std::tr1::__shared_count const&) </pre> <p>I'm a little puzzled by the number of destructor calls being smaller than the number of copy constructor calls, but overall I'm very pleased with the decrease in the associated run-time. Thanks to all for their advice.</p>
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    1. USArtem Sokolov
    1. USArtem Sokolov
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    1. CORelated question: http://stackoverflow.com/questions/327573/c-passing-references-to-boostshared-ptr
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      1. POThe cost of passing by shared_ptr
      1. USFred Larson
    2. CORelated issue: on some platforms (e.g. older ARM), the reference counting requires locking a mutex. This can make shared pointers unusable (except by reference) in a real-time context.
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      1. USMike Seymour
 

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