【精品】线程的定义(introduction)

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1、海量管理资源免费下载！1概述1.1线程的定义(Introduction)Threadscanbestbedescribedas“lightweightprocesses”.ThetraditionalUNIX-stylenotionofaprocesshasbeenfoundtobeinconvenient,ifnotinadequateforseveralapplicationsindistributedsystemsdevelopment.Theneedsoftheseapplicationsarebestservedb

2、ythreads,whichgeneralizethenotionofaprocesssothatitcanbeassociatedwithmultipleactivities.ThepopularityofthreadshasresultedintheirimplementationonUNIXsystemsandthreadlibrariesarenowwidelyavailabletoprogrammersforthedevelopmentofconcurrentapplications.1.2ThreadsImple

3、mentationThreadscanbeimplementedinoneoftwoways:1.User-levelthreads:Thereisnokernelsupportformulti-threadedprocesses.Hence,thekernelonlyhasasingle-threadedprocessabstraction,butmulti-threadedprocessesareimplementedinalibraryofprocedureslinkedwithapplicationprograms.

4、Thekernelhasnoknowledgeoflightweightprocesses(threads),andthereforecannotschedulethemindependently.Athreadsrun-timelibraryorganizestheschedulingofthreads.Athreadwouldblocktheprocessandthereforeallthreadswithinitifitmadeablockingsystemcall,sotheasynchronousI/Ofacili

5、tiesofUNIXareused.Themajordisadvantageofthisschemeisthatthreadswithinaprocesscannottakeadvantageofamulti-processor.（上段译文）User-level没有核心支持的多线程的进程。因此，核心只有单线程进程概念，而多线程进程由与应用程序连接的过程库实现。核心不知道线程的存在，也就不能独立的调度这些线程了。一个线程运行库组织线程的调度。如果一个线程调用了一个阻塞的系统调用，进程可能被阻塞，当然其中的所有线程也同时被阻塞，

6、所以UNIX使用了异步I/O工具。这种机制的的最大缺点是不能发挥多处理器的优势。Theadvantagesinclude:l（系统消耗小）Certainthreadoperationsaresignificantlylesscostly.Forexample,switchingbetweenthreadsbelongingtothesameprocessdonotnecessarilyinvolveasystemcall,andhencesavethisover-head.l（可以修改以适应特殊的应用）User-levelt

7、hreadimplementationscanbecustomizedorchangedtosuittheparticularapplicationrequirements.Thisisparticularlyusefulforreal-timemultimediaprocessingetc.Also,itispossible管理资源吧·管理人自己的下载网站海量管理资源免费下载！tosupportmanymoreuser-levelthreadsthancanbydefaultbyakernel.2.Kernel-level

8、threads:Thisimplementationallowsthreadswithindifferentprocessestobescheduledaccordingtoasingleschemeofrelativeprioritizing.Thisissuitedforexploit