epoll實現原理？
Epoll是LinuxIO的多路復用的機制，是select/poll的增強版本，在Linux內核fs/eventpoll.c中可以查看epoll的具體的實現。
學習任何組件，首先得知道它有什麽數據結構或者數據類型，epoll主要有兩個結構體：eventpoll和epitem。epitem是每壹個IO對應的事件，比如EPOLL_CTL_ADD操作時，就需要創建壹個epitem；eventpoll是每壹個epoll所對應的，比如epoll_create就是創建壹個eventpoll。
libevent詳解？
libevent是壹個輕量級的開源的高性能的事件觸發的網絡庫，適用於windows、linux、bsd等多種平臺，內部使用select、epoll、kqueue等系統調用管理事件機制。
libevent支持多種I/O多路復用技術（epoll、poll、dev/poll、select和kqueue等），在不同的操作系統下，做了多路復用模型的抽象，可以選擇使用不同的模型，通過事件函數提供服務。
如何在linuxsuse中配置NTP服務器？
在suse上配置ntp
$vim/etc/ntp.conf
#增加時間源
server192.168.56.1
$chkconfigntpon#在系統重啟時啟動服務
$servicentpstart#啟動ntp
$servicentpstatus#查看ntp狀態
問題：
1）為什麽在故意改了壹個錯誤的時間，ntpd沒有更新時間？
如果差異很大，需要重新doinganInitialSynchronization，IfthetimeonthelocalserverisverydifferentfromthatofitsprimarytimeserveryourNTPdaemonwilleventuallyterminateitselfleavinganerrormessageinthe/var/log/messagesfile.Youshouldrunthentpdate-ucommandtoforceyourservertobecomeinstantlysynchronizedwithitsNTPserversbeforestartingtheNTPdaemonforthefirsttime.Thentpdatecommanddoesn'truncontinuouslyinthebackground,youwillstillhavetorunthentpddaemontogetcontinuousNTPupdates.
2）/etc/ntp.conf中選擇主NTPServer
選擇了哪個server做為主server是按stratum的大小決定的？應該不是，由ntp的算法決定，如在virtualbox中的suse怎麽樣都無法選擇外部時鐘源，ntpd在幾次polltime後算法就決定使用local源，郁悶啊。由於是虛擬機中運行，時鐘和cpu的頻率有關系，跳得比真實的硬件快，在virtaulbox中ntp的算法認為local源比外部源更準確就使用了local的，解決的方法可以去掉local源，只使用外部源或者在crontab中每分鐘執行壹次ntpdate了。
即使把本機的stratum設置為比外部源更高的，過了壹段時間以後，virtualbox中的suse還是選擇了local，郁悶again
fudge127.127.1.0stratum12#notdisciplined
remoterefidsttwhenpollreachdelayoffsetjitter
==============================================================================
LOCAL(0).LOCL.15l96410.0000.0000.002
192.168.56.1139.114.32.13414u86413.739-53.0450.002
註意最開始的時候，ip地址前面是沒有符號的，poll幾次以後ntp就會選擇壹個主時間源，前面帶*號標識。
3）windows上的ntpserver配置
官方ntp.org推薦的win上的ntpserver，mandlineutilityntpqcanbeusedtocheckthestatusofaNTPdaemononeitherthelocalmachineoronaremotehost.
ntpqcanberuninaninteractivemodeorinbatchmode.Inbatchmode,ntpqexecutesacommandandreturnstothecommandprompt.Theparameter-p('peers')letsntpqprintthestatusofaNTPdaemon.Enter
ntpq-p
todisplaythestatusofthedaemononthelocalmachine,or
ntpq-pntp_server
todisplaythestatusofthedaemonontheremotehostntp_server.ThecommandshouldprintatablewithonestatuslineforeachreferencetimesourcewhichhasbeenconfiguredfortheNTPdaemononthespecifiedhost:
remoterefidsttwhenpollreachdelayoffsetjitter
=======================================================================
LOCAL(0)LOCAL(0)12l30643770.0000.0000.000
*GENERIC(0).DCFa.0-24643770.0000.0500.003
+172.16.3.103.PPS.1u36643771.306-0.0190.043
ThetableaboveshowstheoutputforaNTPdaemonwhichhas3referencetimesources:itsownlocalclock,aDCF77radioclockasrefclock-0,plusanNTPdaemononthenetwork,withIPaddress172.16.3.103.
Ifthefirstcharacterofalineisnotblankthenitcontainsaqualifierforthecorrespondingreferencetimesource.Immediatelyafterthedaemonhasbeenstartedallqualifiersareblank.TheNTPdaemonneedsseveralpollingcyclestochecktheavailabletimesourcesanddeclareoneofthemasthereferenceitsynchronizesto.
Anasterisk*inthefirstcolumnmarksthereferencetimesourcewhichiscurrentlypreferredbytheNTPdaemon,the+charactermarkshighqualitycandidatesforthereferencetimewhichcouldbeusedifthecurrentlyselectedreferencetimesourceshouldbecomeunavailable.
ThecolumnremotedisplaystheIPaddressorthehostnameofthereferencetimesource,whereLOCALreferstothelocalclock.Therefidshowsthetypeofthereferenceclock,wheree.g.LOCALorLCLreferstothelocalclockagain,.DCFa.referstoastandardDCF77timesource,and.PPS.indicatesthatthereferenceclockisdisciplinedbyahardwarepulse-per-secondsignal.Otheridentifiersarepossible,dependingonthetypeofthereferenceclock.
Thecolumnstreflectsthestratumnumberofthereferencetimesource.Intheexampleabove,thelocalclockhasstratum12,theremotetimeserverat172.16.3.103hasstratum1whichisthebestyoucanseeacrossthenetwork,andthelocalradioclockhasstratum0,sotheradioclockiscurrentlybeingpreferred.
Everytimeawhencountreachesthepollnumberinthesameline,theNTPdaemonqueriesthetimefromthecorrespondingtimesourceandresetsthewhencountto0.Thequeryresultsofeachpollingcyclearefilteredandusedasameasurefortheclock'squalityandreachability.
Thecolumnreachshowsifareferencetimesourcecouldbereachedatthelastpollingintervals,i.e.datacouldbereadfromthereferencetimesource,andthereferencetimesourcewassynchronized.Thevaluemustbeinterpretedasan8bitshiftregisterwhosecontentsisforhistoricalreasonsdisplayedasoctalvalues.IftheNTPdaemonhasjustbeenstarted,thevalueis0.Eachtimeaquerywassuccessfula'1'isshiftedinfromtheright,soafterthedaemonhasbeenstartedthesequenceofreachnumbersis0,1,3,7,17,37,77,177,377.Themaximumvalue377meansthattheeightlastquerieswerecompletedsuccessfully.
Queriesareconsideredsuccessfulifdatacanbereceivedfromthetimesource,andthetimesourceinturnclaimstobesynchronizedtosomeothertimesource.Incaseofahardwarereferenceclockthismeansthequeryconsideredunsuccessfulifthehardwarereferenceclockisnotsynchronizedtoitsincomingtimesignal,e.g.becausetheclock'santennahasbeendisconnected,orifnodatacanbereceivede.g.becausetheserialcabletoanexternaldevicehasbeendisconnected.
TheNTPdaemonmusthavereachedareferencetimesourceseveraltimes(reachnot0)beforeitselectsapreferredtimesourceandputsanasteriskinthefirstcolumn.
Thecolumnsdelay,offsetandjittershowsometimingvalueswhicharederivedfromthequeryresults.Insomeversionsofntpqthelastcolumnislabeleddisp(fordispersion)insteadofjitter.Allvaluesareininmilliseconds.Thedelayvalueisderivedfromtheroundtriptimeofthequeries.Theoffsetvalueshowsthedifferencebetweenthereferencetimeandthesystemclock.Thejittervalueindicatesthemagnitudeofjitterbetweenseveraltimequeries.