通过shared_ptr的成本 [英] The cost of passing by shared_ptr

问题描述

我在我的应用程序中广泛使用std :: tr1 :: shared_ptr。这包括作为函数参数传递对象。请考虑以下内容：

I use std::tr1::shared_ptr extensively throughout my application. This includes passing objects in as function arguments. Consider the following:

class Dataset {...}

void f( shared_ptr< Dataset const > pds ) {...}
void g( shared_ptr< Dataset const > pds ) {...}
...

当通过shared_ptr传递数据集对象时，保证它在f和g中的存在，函数可能被调用百万次，这会导致创建很多shared_ptr对象，销毁。以下是最近一次运行的gprof配置文件的片段：

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:

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()

，约17％的运行时花费在带有shared_ptr对象的引用计数。这是正常的吗？

So, ~17% of the runtime was spent on reference counting with shared_ptr objects. Is this normal?

我的应用程序的大部分是单线程的，我正在考虑重写一些函数

A large portion of my application is single-threaded and I was thinking about re-writing some of the functions as

void f( const Dataset& ds ) {...}

并替换调用

shared_ptr< Dataset > pds( new Dataset(...) );
f( pds );

f( *pds );

在我知道的地方，确保对象不会被销毁， F（）。但在我跑掉改变一堆函数签名/调用之前，我想知道shared_ptr传递的典型性能。看起来像shared_ptr不应该用于被频繁调用的函数。

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.

任何输入都将不胜感激。非常感谢阅读。

Any input would be appreciated. Thanks for reading.

-Artem

更新：函数接受 const Dataset& ，新配置文件如下所示：

Update: After changing a handful of functions to accept const Dataset&, the new profile looks like this:

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&)

有点困惑的析构函数调用的数量小于复制构造函数调用的数量，但总体来说，我非常高兴关联运行时间的减少。

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.

推荐答案

始终通过您的 shared_ptr const 参考：

void f(const shared_ptr<Dataset const>& pds) {...} 
void g(const shared_ptr<Dataset const>& pds) {...} 

修改： 关于他人提及的安全问题：

Regarding the safety issues mentioned by others:

< shared_ptr 在整个应用程序中，通过值将占用大量的时间（我看到它走50 +％）。

• 使用<$当参数不应为空时， c> c <$ c> const T& 而不是 const shared_ptr & 。 b $ b
• 使用 const shared_ptr< T const>& 比 const T * 一个问题。

• When using shared_ptr heavily throughout an application, passing by value will take up a tremendous amount of time (I've seen it go 50+%).
• Use const T& instead of const shared_ptr<T const>& when the argument shall not be null.
• Using const shared_ptr<T const>& is safer than const T* when performance is an issue.

这篇关于通过shared_ptr的成本的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！