R14A添加新指令优化Ref消息的接收
May 19th, 2010
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原创文章，转载请注明： 转载自系统技术非业余研究
本文链接地址: R14A添加新指令优化Ref消息的接收
Erlang的惯用法之一就是在消息匹配的时候，如果需要唯一性，通常会通过make_ref搞个唯一的Ref来作为消息的一部分来匹配。这个惯用法用在gen_server:call或者demonitor这样的使用频度很高的函数里面。由于erlang的消息匹配是再消息队列里面挨个遍历来匹配，特别是消息队列特别长的时候，会有很大的性能瓶颈。于是新的优化出现了，以下是编译器beam_receive.erl里面的解释，写的很清楚：
%%% |
%%% In code such as: |
%%% |
%%% Ref = make_ref(), %Or erlang:monitor(process, Pid) |
%%% . |
%%% . |
%%% . |
%%% receive |
%%% {Ref,Reply} -> Reply |
%%% end. |
%%% |
%%% we know that none of the messages that exist in the message queue |
%%% before the call to make_ref/0 can be matched out in the receive |
%%% statement. Therefore we can avoid going through the entire message |
%%% queue if we introduce two new instructions (here written as |
%%% BIFs in pseudo-Erlang): |
%%% |
%%% recv_mark(SomeUniqInteger), |
%%% Ref = make_ref(), |
%%% . |
%%% . |
%%% . |
%%% recv_set(SomeUniqInteger), |
%%% receive |
%%% {Ref,Reply} -> Reply |
%%% end. |
%%% |
%%% The recv_mark/1 instruction will save the current position and |
%%% SomeUniqInteger in the process context. The recv_set |
%%% instruction will verify that SomeUniqInteger is still stored |
%%% in the process context. If it is, it will set the current pointer |
%%% for the message queue (the next message to be read out) to the |
%%% position that was saved by recv_mark/1. |
%%% |
%%% The remove_message instruction must be modified toinvalidate |
%%% the information stored by the previous recv_mark/1, in case there |
%%% is another receive executed between the calls to recv_mark/1 and |
%%% recv_set/1. |
%%% |
%%% We use a reference to a label (i.e. a position in the loaded code) |
%%% as the SomeUniqInteger. |
%%% |
结论： 点滴优化成就高性能。
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