程序高性能取时间问题

桃扇骨 2022-03-02 10:48 415阅读 0赞

系统调用time底层调用的是gettimeofday，因此只需关注gettimeofday的性能，而且不同Linux上的gettimeofday会存在性能差异。围绕gettimeofday的优势主要基于**rdtsc**指令，**rdtsc**和CPU核相关，因此实现时需要处理好多核问题，除非进程和CPU建立亲和关系。

并不是每个应用场景需要高精度的时间，如果精度只要求到秒，则可采用更为直接的方式达到高性能取时间的目的。

引用一专门的时间线程，这个专门的时间线程负责取时间，并提供读取时间接口。时间线程每10ms更新一次时间，即可满足秒级精度需求。

时间线程对时间的读写操作采用原子读写，开销低性能高。看一个基于libmooon的具体实现（实现简单，其它可参照）：

1) 头文件（[https://github.com/eyjian/libmooon/blob/master/include/mooon/sys/time\_thread.h][https_github.com_eyjian_libmooon_blob_master_include_mooon_sys_time_thread.h]）

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 <td style="vertical-align:top;width:426.1pt;"> //&nbsp;提供秒级时间， //&nbsp;可用于避免多个线程重复调用time(NULL) //&nbsp;注：32位平台上的毫秒级不准 class&nbsp;CTimeThread { &nbsp;&nbsp;&nbsp;&nbsp;SINGLETON_DECLARE(CTimeThread);&nbsp;//&nbsp;单例方式方便使用 &nbsp; public: &nbsp;&nbsp;&nbsp;&nbsp;CTimeThread(); &nbsp;&nbsp;&nbsp;&nbsp;~CTimeThread(); &nbsp;&nbsp;&nbsp;&nbsp;int64_t&nbsp;get_seconds()&nbsp;const;&nbsp;//&nbsp;可用，近乎精度 &nbsp;&nbsp;&nbsp;&nbsp;int64_t&nbsp;get_milliseconds()&nbsp;const;&nbsp;//&nbsp;基本可用，但不精度 &nbsp;&nbsp;&nbsp;&nbsp;void&nbsp;stop();&nbsp;//&nbsp;同一对象在stop后不能重复使用 &nbsp;&nbsp;&nbsp;&nbsp;bool&nbsp;start(uint32_t&nbsp;interval_milliseconds);&nbsp;//&nbsp;启动时间线程 &nbsp;&nbsp;&nbsp;&nbsp;void&nbsp;wait();&nbsp;//&nbsp;等待时间线程退出 &nbsp;&nbsp;&nbsp;&nbsp;void&nbsp;run();&nbsp;//&nbsp;运行时间线程 &nbsp; private: &nbsp;&nbsp;&nbsp;&nbsp;CAtomic&lt;bool&gt;&nbsp;_stop; #if&nbsp;__WORDSIZE==64 &nbsp;&nbsp;&nbsp;&nbsp;CAtomic&lt;int64_t&gt;&nbsp;_seconds;&nbsp;//&nbsp;原子值，不用加锁安全读和写 &nbsp;&nbsp;&nbsp;&nbsp;CAtomic&lt;int64_t&gt;&nbsp;_milliseconds; #else &nbsp;&nbsp;&nbsp;&nbsp;CAtomic&lt;int&gt;&nbsp;_seconds; &nbsp;&nbsp;&nbsp;&nbsp;CAtomic&lt;int&gt;&nbsp;_milliseconds; #endif&nbsp;//&nbsp;__WORDSIZE==64 &nbsp;&nbsp;&nbsp;&nbsp;uint32_t&nbsp;_interval_milliseconds; &nbsp;&nbsp;&nbsp;&nbsp;CThreadEngine*&nbsp;_engine;&nbsp;//&nbsp;moooon提供的线程引擎 }; </td> 
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2) 实现文件（[https://github.com/eyjian/libmooon/blob/master/src/sys/time\_thread.cpp][https_github.com_eyjian_libmooon_blob_master_src_sys_time_thread.cpp]）

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 <td style="vertical-align:top;width:426.1pt;"> SINGLETON_IMPLEMENT(CTimeThread); &nbsp; CTimeThread::CTimeThread() &nbsp;&nbsp;&nbsp;&nbsp;:&nbsp;_stop(false), &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;_interval_milliseconds(1000), &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;_engine(NULL) { &nbsp;&nbsp;&nbsp;&nbsp;struct&nbsp;timeval&nbsp;tv; &nbsp;&nbsp;&nbsp;&nbsp;gettimeofday(&amp;tv,&nbsp;NULL); &nbsp; #if&nbsp;__WORDSIZE==64 &nbsp;&nbsp;&nbsp;&nbsp;_seconds&nbsp;=&nbsp;static_cast&lt;int64_t&gt;(tv.tv_sec); &nbsp;&nbsp;&nbsp;&nbsp;_milliseconds&nbsp;=&nbsp;static_cast&lt;int64_t&gt;(tv.tv_usec); #else &nbsp;&nbsp;&nbsp;&nbsp;_seconds&nbsp;=&nbsp;static_cast&lt;int&gt;(tv.tv_sec); &nbsp;&nbsp;&nbsp;&nbsp;_milliseconds&nbsp;=&nbsp;static_cast&lt;int&gt;(tv.tv_usec); #endif&nbsp;//&nbsp;__WORDSIZE==64 } &nbsp; CTimeThread::~CTimeThread() { &nbsp;&nbsp;&nbsp;&nbsp;wait(); } &nbsp; int64_t&nbsp;CTimeThread::get_seconds()&nbsp;const { #if&nbsp;__WORDSIZE==64 &nbsp;&nbsp;&nbsp;&nbsp;return&nbsp;_seconds.operator&nbsp;int64_t();&nbsp;//&nbsp;原子取时间 #else &nbsp;&nbsp;&nbsp;&nbsp;return&nbsp;_seconds.operator&nbsp;int(); #endif&nbsp;//&nbsp;__WORDSIZE==64 } &nbsp; int64_t&nbsp;CTimeThread::get_milliseconds()&nbsp;const { #if&nbsp;__WORDSIZE==64 &nbsp;&nbsp;&nbsp;&nbsp;return&nbsp;_milliseconds.operator&nbsp;int64_t(); #else &nbsp;&nbsp;&nbsp;&nbsp;return&nbsp;_milliseconds.operator&nbsp;int(); #endif&nbsp;//&nbsp;__WORDSIZE==64 } &nbsp; void&nbsp;CTimeThread::stop() { &nbsp;&nbsp;&nbsp;&nbsp;_stop&nbsp;=&nbsp;true; } &nbsp; bool&nbsp;CTimeThread::start(uint32_t&nbsp;interval_milliseconds) { &nbsp;&nbsp;&nbsp;&nbsp;try &nbsp;&nbsp;&nbsp;&nbsp;{ &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;_interval_milliseconds&nbsp;=&nbsp;interval_milliseconds; &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;_engine&nbsp;=&nbsp;new&nbsp;mooon::sys::CThreadEngine(mooon::sys::bind(&amp;CTimeThread::run,&nbsp;this)); &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;return&nbsp;true; &nbsp;&nbsp;&nbsp;&nbsp;} &nbsp;&nbsp;&nbsp;&nbsp;catch&nbsp;(sys::CSyscallException&amp;&nbsp;ex) &nbsp;&nbsp;&nbsp;&nbsp;{ &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;MYLOG_ERROR("Start&nbsp;time-thread&nbsp;failed:&nbsp;%s\n",&nbsp;ex.str().c_str()); &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;return&nbsp;false; &nbsp;&nbsp;&nbsp;&nbsp;} } &nbsp; void&nbsp;CTimeThread::wait() { &nbsp;&nbsp;&nbsp;&nbsp;if&nbsp;(_engine&nbsp;!=&nbsp;NULL) &nbsp;&nbsp;&nbsp;&nbsp;{ &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;_engine-&gt;join(); &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;delete&nbsp;_engine; &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;_engine&nbsp;=&nbsp;NULL; &nbsp;&nbsp;&nbsp;&nbsp;} } &nbsp; void&nbsp;CTimeThread::run() { &nbsp;&nbsp;&nbsp;&nbsp;struct&nbsp;timeval&nbsp;start_tv,&nbsp;exit_tv; &nbsp;&nbsp;&nbsp;&nbsp;gettimeofday(&amp;start_tv,&nbsp;NULL); &nbsp; &nbsp;&nbsp;&nbsp;&nbsp;MYLOG_INFO("Time-thread&nbsp;start&nbsp;now\n"); &nbsp;&nbsp;&nbsp;&nbsp;while&nbsp;(!_stop) &nbsp;&nbsp;&nbsp;&nbsp;{ &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;struct&nbsp;timeval&nbsp;tv; &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;gettimeofday(&amp;tv,&nbsp;NULL); &nbsp; &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;const&nbsp;int64_t&nbsp;milliseconds&nbsp;=&nbsp;static_cast&lt;int64_t&gt;(tv.tv_sec*1000&nbsp;+&nbsp;tv.tv_usec/1000); #if&nbsp;__WORDSIZE==64 &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;_seconds&nbsp;=&nbsp;static_cast&lt;int64_t&gt;(tv.tv_sec); &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;_milliseconds&nbsp;=&nbsp;milliseconds;&nbsp;//&nbsp;原子更新 #else &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;_seconds&nbsp;=&nbsp;static_cast&lt;int&gt;(tv.tv_sec); &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;_milliseconds&nbsp;=&nbsp;static_cast&lt;int&gt;(milliseconds); #endif&nbsp;//&nbsp;__WORDSIZE==64 &nbsp; &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;//&nbsp;_interval_milliseconds越小精度越高， &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;//&nbsp;一般秒级精度，值为10ms即足够，甚至100ms也够用了。 &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;CUtils::millisleep(_interval_milliseconds); &nbsp;&nbsp;&nbsp;&nbsp;} &nbsp; &nbsp;&nbsp;&nbsp;&nbsp;gettimeofday(&amp;exit_tv,&nbsp;NULL); &nbsp;&nbsp;&nbsp;&nbsp;if&nbsp;(start_tv.tv_sec&lt;=exit_tv.tv_sec&nbsp;|| &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;start_tv.tv_usec&lt;=exit_tv.tv_usec) &nbsp;&nbsp;&nbsp;&nbsp;{ &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;const&nbsp;uint64_t&nbsp;interval_milliseconds&nbsp;=&nbsp;(exit_tv.tv_sec-start_tv.tv_sec)*1000&nbsp;+&nbsp;(exit_tv.tv_usec-start_tv.tv_usec)/1000; &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;MYLOG_INFO("Time-thread&nbsp;exit&nbsp;now:&nbsp;%"&nbsp;PRIu64"ms\n",&nbsp;interval_milliseconds); &nbsp;&nbsp;&nbsp;&nbsp;} &nbsp;&nbsp;&nbsp;&nbsp;else &nbsp;&nbsp;&nbsp;&nbsp;{ &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;MYLOG_INFO("Time-thread&nbsp;exit&nbsp;now\n"); &nbsp;&nbsp;&nbsp;&nbsp;} } </td> 
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3) 使用示例：

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 <td style="vertical-align:top;width:426.1pt;"> #include&nbsp;&lt;mooon/sys/time_thread.h&gt; &nbsp; //&nbsp;启动时间线程 mooon::sys::CTimeThread::get_singleton()-&gt;start(10); //&nbsp;取得当前时间（单位：秒） _now&nbsp;=&nbsp;mooon::sys::CTimeThread::get_singleton()-&gt;get_seconds(); </td> 
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64位的Linux实现了vsyscall，基于vsyscall实现的gettimeofday性能已达每秒千万级别。vsyscall有局限性，只允许4个系统调用，只能分配较小的内存。VDSO（Virtual Dynamic Shared Object）和vsyscall相同，允许应用在用户空间（不经过内核）执行一些内核操作，但可提供超过4个系统调用，VDSO是glibc提供的功能。

要使VDSO生效，执行：

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 <td style="vertical-align:top;width:426.1pt;"> echo&nbsp;1&nbsp;&gt;&nbsp;/proc/sys/kernel/vsyscall64 </td> 
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参考：

1) [https://alittleresearcher.blogspot.com/2017/04/linux-vdso-and-vsyscall-history.html][https_alittleresearcher.blogspot.com_2017_04_linux-vdso-and-vsyscall-history.html]

2) [https://stackoverflow.com/questions/19938324/what-are-vdso-and-vsyscall][https_stackoverflow.com_questions_19938324_what-are-vdso-and-vsyscall]

3) [https://access.redhat.com/documentation/en-US/Red\_Hat\_Enterprise\_MRG/1.3/html/Realtime\_Tuning\_Guide/sect-Realtime\_Tuning\_Guide-General\_System\_Tuning-gettimeofday\_speedup.html][https_access.redhat.com_documentation_en-US_Red_Hat_Enterprise_MRG_1.3_html_Realtime_Tuning_Guide_sect-Realtime_Tuning_Guide-General_System_Tuning-gettimeofday_speedup.html]

[https_github.com_eyjian_libmooon_blob_master_include_mooon_sys_time_thread.h]: https://github.com/eyjian/libmooon/blob/master/include/mooon/sys/time_thread.h
[https_github.com_eyjian_libmooon_blob_master_src_sys_time_thread.cpp]: https://github.com/eyjian/libmooon/blob/master/src/sys/time_thread.cpp
[https_alittleresearcher.blogspot.com_2017_04_linux-vdso-and-vsyscall-history.html]: https://alittleresearcher.blogspot.com/2017/04/linux-vdso-and-vsyscall-history.html
[https_stackoverflow.com_questions_19938324_what-are-vdso-and-vsyscall]: https://stackoverflow.com/questions/19938324/what-are-vdso-and-vsyscall
[https_access.redhat.com_documentation_en-US_Red_Hat_Enterprise_MRG_1.3_html_Realtime_Tuning_Guide_sect-Realtime_Tuning_Guide-General_System_Tuning-gettimeofday_speedup.html]: https://access.redhat.com/documentation/en-US/Red_Hat_Enterprise_MRG/1.3/html/Realtime_Tuning_Guide/sect-Realtime_Tuning_Guide-General_System_Tuning-gettimeofday_speedup.html