golanggc优化思路以及实例分析

golanggc优化思路以及实例分析⼀个即将上线的go写的⾼频服务，压测的时候发现gc特别⾼，⾼到10%-15%左右了，本⽂记录下优化gc的过程和和思路。线上环境1.10.⾸先，查看gc是否有异常，我们可以使⽤gctrace跟踪实时的gc。执⾏下⾯命令可以看到gc的实时信息。GODEBUG=gctrace=1goruncmd/agent_bin.go输出结果如下:gc45@37.801s11%:0.19+627+0.29msclock,0.38+424/621/0+0.59mscpu,356->415->225MB,453MBgoal,4Pgc46@39.126s11%:2.9+927+0.16msclock,5.8+342/925/0+0.33mscpu,361->460->275MB,450MBgoal,4Pgc47@40.847s12%:0.24+1096+0.12msclock,0.49+291/1007/0+0.24mscpu,427->559->319MB,551MBgoal,4Pgc48@42.771s12%:0.26+841+0.12msclock,0.52+377/830/0+0.24mscpu,486->561->271MB,638MBgoal,4Pgc49@44.429s12%:3.1+890+0.40msclock,6.2+492/833/0+0.81mscpu,440->528->294MB,543MBgoal,4Pgc50@46.188s12%:0.23+1165+0.13msclock,0.47+624/1158/0+0.27mscpu,471->579->323MB,589MBgoal,4Pgc51@48.252s13%:0.26+1410+0.14msclock,0.52+358/1336/9.9+0.28mscpu,506->620->343MB,646MBgoal,4Pgc52@50.942s13%:0.27+806+0.51msclock,0.55+403/805/200+1.0mscpu,549->657->340MB,687MBgoal,4Pgc53@53.014s13%:0.10+857+0.36msclock,0.21+467/851/94+0.73mscpu,546->666->351MB,681MBgoal,4Pgc45：表⽰第45次GC，共有4个P(线程)参与GC。11%:表⽰gc占时间⽐。0.19+627+0.29us：STW（stop-the-world）0.19ms,并发标记和扫描的时间627ms,STW标记的时间0.29ms。0.38+424/621/0+0.59mscpu,表⽰垃圾回收占⽤cpu时间356->415->225MB,453MBgoal,表⽰堆的⼤⼩，gc后堆的⼤⼩，存活堆的⼤⼩453MBgoal表⽰整体堆的⼤⼩为435M。根据官⽅描述，golang1.0的gc可以降到100ms以内，但是这⾥gc都超过1s了，这明显是不可以接受的，说明gc是有很⼤异常的。检查思路，⾸先利⽤pprof打出整个调⽤过程累计的堆分配图，查出到底是哪些模块堆分配异常。通过代码内嵌pprof暴露端⼝的⽅式，终端输出svg。import_"net/http/pprof"gofunc(){http.ListenAndServe(":8080",nil)}()然后终端输⼊：gotoolpprof-alloc_space-cum-svg:8080/debug/pprof/heap>heap.svg然后浏览器打开svg，找到消耗最⼤的调⽤栈：使⽤-cum，是累计函数调⽤栈的堆分配⼤⼩，因为图形会将调⽤栈很⼤的路线着重标识出来，这⾥可以明显看出是⾃⼰实现的Fetch函数的iocopyBuffer函数gc压⼒最⼤。这⾥⼤致知道来源了，那到底是我们函数哪个对象有问题？接下来我们对这个⽂件使⽤逃逸分析：直接⽤gobuild--gcflags=-m接⽂件即可：我们的源码是这样的：packagehttpclientimport("io""io/ioutil""net/http""net/url""os""sync""time""refresh_agent/utils")varclient*http.ClientvarbuffPoolsync.Poolfuncinit(){funcinit(){client=&http.Client{}http.DefaultTransport.(*http.Transport).MaxIdleConnsPerHost=2000http.DefaultTransport.(*http.Transport).MaxIdleConns=20000}typeHttpClientstruct{}funcNewHttpClient()*HttpClient{httpClient:=HttpClient{}return&httpClient}func(this*HttpClient)replaceUrl(srcUrlstring,ipstring)string{httpPrefix:="http://"parsedUrl,err:=url.Parse(srcUrl)iferr!=nil{return""}returnhttpPrefix+ip+parsedUrl.Path}func(this*HttpClient)downLoadFile(resp*http.Response)error{out,err:=os.OpenFile("/dev/null",os.O_RDWR|os.O_CREATE|os.O_APPEND,0666)deferout.Close()_,err=io.Copy(out,resp.Body)returnerr}func(this*HttpClient)Fetch(dstUrlstring,methodstring,proxyHoststring,headermap[string]string,preloadbool,timeoutint64)(*http.Response,error){//proxyHost换掉url中请求newUrl:=this.replaceUrl(dstUrl,proxyHost)req,_:=http.NewRequest(method,newUrl,nil)fork,v:=rangeheader{req.Header.Add(k,v)}req.Host=utils.GetUrlHost(dstUrl)client.Timeout=time.Duration(timeout)*time.Secondresp,err:=client.Do(req)ifresp==nil||err!=nil{returnresp,err}ifpreload{err:=this.downLoadFile(resp)iferr!=nil{returnnil,err}}io.Copy(ioutil.Discard,resp.Body)resp.Body.Close()returnresp,nil}逃逸分析的结果如下：gobuild--gcflags=-mlibs/httpclient/httpclient.go#refresh_agent/vendor//garyburd/redigo/redisgc1@0.078s5%:0.075+24+0.040msclock,0.15+0.15/11/17+0.081mscpu,16->16->11MB,42MBgoal,2P#refresh_agent/vendor//imroc/reqgc1@0.071s3%:0.017+26+0.050msclock,0.034+0.22/6.0/22+0.10mscpu,16->17->11MB,42MBgoal,2P#refresh_agent/vendor//BurntSushi/tomlgc1@0.054s6%:0.086+43+0.062msclock,0.17+0.14/12/14+0.12mscpu,16->17->12MB,42MBgoal,2P#refresh_agent/vendor//modern-go/reflect2gc1@0.054s7%:0.070+28+0.051msclock,0.14+0.15/12/14+0.10mscpu,16->17->10MB,42MBgoal,2Pgc2@0.334s3%:0.016+33+0.055msclock,0.032+0.54/12/20+0.11mscpu,37->38->14MB,54MBgoal,2P#refresh_agent/vendor//json-iterator/gogc1@0.045s5%:0.068+27+0.040msclock,0.13+2.5/4.7/16+0.081mscpu,16->16->11MB,42MBgoal,2Pgc2@0.211s9%:0.029+107+0.12msclock,0.058+0.96/52/0+0.24mscpu,39->44->26MB,58MBgoal,2P#command-line-argumentslibs/httpclient/httpclient.go:18:6:caninlineinit.0libs/httpclient/httpclient.go:26:6:caninlineNewHttpClientlibs/httpclient/httpclient.go:19:24:&http.Clientliteralescapestoheaplibs/httpclient/httpclient.go:28:9:&httpClientescapestoheaplibs/httpclient/httpclient.go:27:2:movedtoheap:httpClientlibs/httpclient/httpclient.go:31:62:leakingparam:srcUrllibs/httpclient/httpclient.go:37:25:httpPrefix+ip+parsedUrl.Pathescapestoheaplibs/httpclient/httpclient.go:31:62:(*HttpClient).replaceUrlthisdoesnotescapelibs/httpclient/httpclient.go:31:62:(*HttpClient).replaceUrlipdoesnotescapelibs/httpclient/httpclient.go:44:18:outescapestoheaplibs/httpclient/httpclient.go:44:28:resp.Bodyescapestoheaplibs/httpclient/httpclient.go:41:59:leakingparamcontent:resplibs/httpclient/httpclient.go:41:59:(*HttpClient).downLoadFilethisdoesnotescapelibs/httpclient/httpclient.go:48:151:leakingparam:dstUrllibs/httpclient/httpclient.go:48:151:leakingparam:methodlibs/httpclient/httpclient.go:48:151:leakingparamcontent:headerlibs/httpclient/httpclient.go:69:30:resp.Bodyescapestoheaplibs/httpclient/httpclient.go:48:151:(*HttpClient).Fetchthisdoesnotescapelibs/httpclient/httpclient.go:48:151:(*HttpClient).FetchproxyHostdoesnotescape<autogenerated>:1:0:leakingparam:.anon0<autogenerated>:1:0:leakingparam:.this<autogenerated>:1:0:leakingparam:io.p<autogenerated>:1:0:leakingparam:.this<autogenerated>:1:0:os.(*File).close.thisdoesnotescape<autogenerated>:1:0:leakingparam:.this<autogenerated>:1:0:leakingparam:.this<autogenerated>:1:0:leakingparam:.this<autogenerated>:1:0:leakingparam:context.key<autogenerated>:1:0:leakingparam:.this这⾥我们发现resp.Body逃逸到了堆上。和我们原始的希望是不⼀致的，我们的⽬的，是希望将resp.body重定向到空设备中去，这⾥的对象最好是分配到栈区。这⾥看不出，到底哪⼀步反复创建对象了。继续追io.CopyBuffer,查看io.Copy的源码：funcCopy(dstWriter,srcReader)(writtenint64,errerror){returncopyBuffer(dst,src,nil)}//CopyBufferisidenticaltoCopyexceptthatitstagesthroughthe//providedbuffer(ifoneisrequired)ratherthanallocatinga//temporaryone.Ifbufisnil,oneisallocated;otherwiseifithas//zerolength,CopyBufferpanics.funcCopyBuffer(dstWriter,srcReader,buf[]byte)(writtenint64,errerror){ifbuf!=nil&&len(buf)==0{panic("emptybufferinio.CopyBuffer")}returncopyBuffer(dst,src,buf)}funccopyBuffer(dstWriter,srcReader,buf[]byte)(writtenint64,errerror){//IfthereaderhasaWriteTomethod,useittodothecopy.//Avoidsanallocationandacopy.ifwt,ok:=src.(WriterTo);ok{returnwt.WriteTo(dst)}//Similarly,ifthewriterhasaReadFrommethod,useittodothecopy.ifrt,ok:=dst.(ReaderFrom);ok{returnrt.ReadFrom(src)}size:=32*1024ifl,ok:=src.(*LimitedReader);ok&&int64(size)>l.N{ifl.N<1{size=1}else{size=int(l.N)}}ifbuf==nil{buf=make([]byte,size)#这⾥反复创建⼤⼩为32k的slice}看到调⽤栈提⽰的函数，发现，我们调⽤copy的时候，如果不传⼊buffer，内部会反复创建这个buf。基本就定位到问题了。那么我们知道某个对象会⼀直被gc回收，怎么处理呢，通⽤做法是，使⽤对象池。经过