PHP 7 vs HHVM
什么是 HHVM?
在2008年，Facebook 启动了一项工作，计划开发一个工具 将 PHP 脚本转换成 C++，这样就可以被编译后在 web 服务器上运行。目的是节省服务器资源，这是一个很重要的目标，因为 Facebook 的用户量正在快速增长。从这个意义上讲，这个项目是成功的，因为它可以让服务器处理之前五到六倍的请求量。
时间回到2010年，Facebook 的服务器需求已经增加了很多，是时候考虑用新的创新来提升更大的效率。基于这个需求，Facebook 开发了 HHVM。
HHVM 使用了 Just-In-Time (JIT) 编译方式将 PHP 代码转换成某种字节码。接下来把字节码再转换成机器码并进行优化，让它尽可能快的运行。
什么是PHP 7?
PHP 7 是 PHP 社区对 HHVM 的回应。PHP 7 发布的预览版本号称比之前的 PHP 5 的性能要提升100%。
你可能会问 PHP 的版本直接从 PHP5 跳到了 PHP7，答案是这样的：PHP6 的开发开始于2005年，但是它的进展过于缓慢又出现了很多的问题，这样 PHP6 在它还没有发布正式版本之前就有了不好的名声。所以PHP社区决定给这个语言的新版本直接命名为 PHP7。
真正的问题不是 PHP5 和 PHP7 之间的比较，因为已经很明显了 PHP7 提供了更快速的运行速度。但是我们想比较的是 PHP7 和 HHVM。很多专家已经分别使用了这两个方法来处理PHP代码，并且揭示了一些有趣的结论。
PHP7缓存有opcode,
HHVM缓存有bytecode(hhbc即hiphop bytecode).
PHP7并没有引入JIT,PHP7的性能提升在于采用了更好的内存结构,比如在栈上为zval分配内存取代PHP5在堆上分配, 直接操作zval取代PHP5用(二级)指针操作,减少了内存分配循环. PHP7执行WordPress使用的CPU指令数约为PHP5的1/4,计算密集型的bench.php的CPU指令数也能减少一半以上. http://rango.swoole.com/archives/440
HHVM是一个包含多个线程的单进程服务,它不像PHP-FPM有1个主进程和多个工作进程(Nginx也是这种进程模型),也就是说HHVM因为线程问题发生崩溃或僵死时无法自动恢复,而PHP-FPM的主进程却可以重启工作进程. 百度转HHVM遇到的问题:崩溃/内存泄露/兼容性/死锁/性能(JIT热身/没有触发JIT). http://lamp.baidu.com/2014/11/04/hhvm-in-baidu/
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