??è?‘??¨??′???JVM????…3???PPT??????CMS??—?3????è???o???é??????ˉ????é?…èˉ??o????é??è???¤??o?è§￡????1?????§??-????????

???1???ˉCMS

CMS?…¨?§° Concurrent Mark Sweep?????ˉ??????1???‘?????????”¨???è?°-??…é?¤??—?3??????????????”???¨???

?|????è??1′??￡????”¨CMS??????????”???¨???é?è|??·????è???????o?????°-“XX:+UseConcMarkSweepGC”??

????”¨??o??ˉ???

GCè???¨???-?????????é?????ˉ1?—????è|??±?è??é???????????????”¨??·?o??¨?????…?è??é???—?é—′??????é????

??o??1???

??????é???—?é—′è??è?????é??????￥é???????…?-??￠??‰???–??

??|?¤–?????—?3??????°?ˉ”è???¤????????|?????1???—??o??1???èˉ???‰

?????°??o???

??1???GC???è§|??‘??o????????o????‘¨????§Old GC???è￠???¨??‰?’??????¨Old GC

??a?oo???è§￡????????¨?????￥é?“???1?????‰??￥???

?‘¨????§Old GC

?‘¨????§Old GC????‰§è?????é?è?‘?1????Background Collect????ˉ1è??1′??￡è??è??????”??????¨GC?—￥??—??-?ˉ”è?????è§?????”±?????°?o??¨?ConcurrentMarkSweepThread??a??ˉ??¤?–-???é??è?¤2s??‰??ˉ??|é?è|?è§|??‘??

è§|??‘??????

1???|?????2???‰è?????-XX:+UseCMSInitiatingOccupancyOnly???è???????o?????1????”?é???????°?????3?????ˉ??|è§|??‘?????oè???o??????ˉ?￠???|???è????a?????°????????????????¤§é—?é￠???’??￥???é???o|??‰??

2??è??1′??￡????”¨???è????°é???? CMSInitiatingOccupancyFraction???é??è?¤92%??

3???°??1…??￡???????”¨???è????°é???? CMSInitiatingPermOccupancyFraction???é??è?¤92%????‰??????ˉ????ˉ CMSClassUnloadingEnabled??

4???–°?”???￡???????????…????¤±è′￥??

????????…????¤±è′￥

è??1′??￡??ˉ??|??‰è?3?¤??????oé—′??￥??1?o3?…¨é?¨????–°?”???￡?ˉ1è±???–?????2?13???????????°è??1′??￡????ˉ1è±?????|????????¤????èˉ?????°±????—?è??è???????è??1′??￡???????”????é?2?-￠??????è??è??YGC????—?????‘?”????????¤±è′￥??

?‘¨????§Old GCè???¨?

??“?????????è?3?—????é???”¨a????è?°-??…???a???—?3??ˉ1è??1′??￡è??è??????”????è???¨???ˉ??￥èˉ′??????????????è?°??o?-??′??ˉ1è±??????…?????‰???????ˉ1è±?????????ˉ??o?o??????°??′??aè???¨??????????è????????é?…??—?3?è??è???2?è???1???????????′??aè???¨??????o?|?????????aé?¨??????

?ˉ1è±???¨???è?°è???¨???-?????1??????è?°??…??μ???????????‰?±????

1. ???è‰2?ˉ1è±????è?¨?¤oè?aèo???aè￠????è?°???
2. ??°è‰2?ˉ1è±????è?¨?¤oè?aèo?è￠????è?°????????…é?¨????”¨??aè￠??¤???????
3. é?‘è‰2?ˉ1è±????è?¨?¤oè?aèo?è￠????è?°?????…é?¨????”¨é??è￠??¤???????

???è????‘?”?Background Collect?—????Java???????ˉ1è±????????|???????

1??InitialMarking???????§???–???è?°?????′??aè???¨?STW??‰

èˉ￥é????μ????o??¨??‰§è????????è|?????????o??¤?-￥???

1. ???è?°GC Roots??ˉè?????è??1′??￡?ˉ1è±????
2. é??????–°?”???￡?ˉ1è±???????è?°??ˉè?????è??1′??￡?ˉ1è±????

èˉ￥è???¨???“??????????ˉ1è±????????|???????

2??Marking????1???‘???è?°??‰

èˉ￥é????μGC?o??¨??’??o”?”¨?o??¨??1???‘?‰§è?????é?????InitialMarkingé????μ???è?°??o??￥????-??′??ˉ1è±????????????§??-é’??’???è?°è???o??ˉ1è±???ˉè??????ˉ1è±???

?????oèˉ￥é????μ?1???‘?‰§è??????????¨è??è?????é—′??ˉè????‘?”??–°?”???￡????ˉ1è±?????????°è??1′??￡????–è…??ˉ??′??￥??¨è??1′??￡???é…??ˉ1è±?????–è…??′?–°è??1′??￡?ˉ1è±????????”¨?…3?3??-‰?-‰????ˉ1?o?è???o??ˉ1è±????é????ˉé?è|?è??è??é???–°???è?°????????|?????‰?o??ˉ1è±??°±???è￠?é?—????????‘?”????????????…??μ??

??o?o????é??é???–°???è?°????????????èˉ￥é????μ?????????è?°?ˉ1è±??‰??¨???Card???èˉ???oDirty????????-??aé??‰????è???o?Dirty Card????ˉ1è±????é???…??‰??????′??aè??1′??￡??

3??Precleaning???é￠???…?????‰

é?è???????°CMSPrecleaningEnabledé‰????…3é—-èˉ￥é????μ???é??è?¤??ˉ?”¨??????è|??????¤????o???…???

1. ?¤?????–°?”???￡?·2?????‘??°???????”¨????ˉ”?|???¨?1???‘é????μ?????¨Eden??o??-???é…??o?????aA?ˉ1è±????A?ˉ1è±?????”¨?o?????aè??1′??￡?ˉ1è±?B???è????aB?1??‰??2???‰è￠????è?°??‰?????¨è????aé????μ?°±??????è?°?ˉ1è±?B??o?′?è·??ˉ1è±???
2. ??¨?1???‘???è?°é????μ????|????è??1′??￡??-??‰?ˉ1è±???…é?¨????”¨??‘?”??????–??????????‰??¨???Card???è?°??oDirty????…????è??é???1?é??????”¨CardTable???è???ˉ????a?±?????????°?????“?????????ModUnionTalble??‰???é?è???‰????è???o?Table???é???–°???è?°é?￡?o???¨?1???‘???è?°é????μ????”¨è￠???′?–°????ˉ1è±????????????°è??1′??￡????ˉ1è±??????????°±??¨è??1′??￡????ˉ1è±???‰

4??AbortablePreclean?????ˉ??-?–-???é￠???…?????‰

èˉ￥é????μ??‘?”?????‰??????ˉ????–°?”???￡Eden??o?????…?-?????”¨é???¤§?o??????°CMSScheduleRemarkEdenSizeThreshold é??è?¤??ˉ2M????|?????–°?”???￡????ˉ1è±??¤a?°‘????°±?2???‰??…è|??‰§è??èˉ￥é????μ?????′??￥?‰§è??é???–°???è?°é????μ??

??o???1?é?è|?è????aé????μ????-???¨?????·????ˉ???1????

?????oCMS GC?????????????????ˉé???????????????”??—???????????—?é—′????‰??￥??¨èˉ￥é????μè|??°????¤§?????a???????¤????é?￡?o???¨?1???‘é????μè￠??o”?”¨?o??¨???′?–°???è??1′??￡?ˉ1è±????è????·??¨?????????é???–°???è?°é????μ?°±??ˉ??￥?°‘?¤???????o???????????—?é—′?1????????o”???é???????

??¨èˉ￥é????μ??????è|???a??ˉ????????¤????o????

1. ?¤???? From ?’? To ??o????ˉ1è±???????è?°??ˉè?????è??1′??￡?ˉ1è±?
2. ?’????????aé????μ????·????‰?????¤????Dirty Card??-????ˉ1è±?

??“????o????è????aé?è?‘??????????′??a??ˉ??????????‰“?–-è????a??a??ˉ???????????‰??‰??a???

1. ??ˉ??￥è????????¤???a??ˉ????????° CMSMaxAbortablePrecleanLoops???é??è?¤??ˉ0?????????2???‰??a??ˉ?????°???é???????
2. ?|?????‰§è??è????aé?è?‘????—?é—′è????°?o?é????CMSMaxAbortablePrecleanTime???é??è?¤??ˉ5s??????é??o??a??ˉ??
3. ?|?????–°?”???￡Eden??o?????…?-?????”¨???è????°?o?é????CMSScheduleRemarkEdenPenetration???é??è?¤50%??????é??o??a??ˉ?????è????a??????è???¤???????????‰??????ˉ?????¨è??è??Precleaning?—????Eden??o???????”¨????°??o????????1?????‰

?|??????¨??a??ˉé??o?1??‰??????‘?”??o??????YGC????ˉ1?o????é?￠???Remarké????μ??￥èˉ′????¤§?¤§???è???o??‰?????1′è????￡???è′???…????????ˉ??‘?”?YGC?1?é???oo??o??§???????‰??￥??aè???￥??￥·è??5s??…??ˉ??￥??￥?????YGC??

...
1678.150: [CMS-concurrent-preclean-start]
1678.186: [CMS-concurrent-preclean: 0.044/0.055 secs]
1678.186: [CMS-concurrent-abortable-preclean-start]
1678.365: [GC 1678.465: [ParNew: 2080530K->1464K(2044544K), 0.0127340 secs] 
1389293K->306572K(2093120K), 
0.0167509 secs]
1680.093: [CMS-concurrent-abortable-preclean: 1.052/1.907 secs] 
....

??¨???é?￠GC?—￥??—??-???1678.186??ˉ??¨?o?AbortablePrecleané????μ?????¨é??????????°2s?°±??‘?”??o??????YGC??

5??FinalMarking????1???‘é???–°???è?°???STWè???¨???‰

èˉ￥é????μ?1???‘?‰§è???????¨?1??‰?????1?è??é????μ???GC?o??¨??’??o”?”¨?o??¨?????—??‰§è??????￥??ˉ”????|???¨?‰“?‰????é—′??????è????¨?‰”?o??±‘??‰?????ˉè???o§?”??–°???????”¨?…3?3??|???????

1. è??1′??￡????–°?ˉ1è±?è￠?GC Roots????”¨
2. è??1′??￡?????a???è?°?ˉ1è±?è￠??–°?”???￡?ˉ1è±?????”¨
3. è??1′??￡?·2???è?°????ˉ1è±??￠?????–°????”¨?????‘è??1′??￡?…?????ˉ1è±?
4. ?–°?”???￡?ˉ1è±??????‘è??1′??￡????”¨è￠????é?¤
5. ?1?è??è????‰?…??????…??μ..

???è?°?ˉ1è±???-??ˉè????‰???o??·2?????¨Precleaningé????μ?’?AbortablePrecleané????μè￠??¤????è???????????-???¨?2???￥??—????¤???????????‰??￥è????‰è??è???|????????¤???????

1. é??????–°?”???￡?ˉ1è±????é???–°???è?°
2. ??1???GC Roots???é???–°???è?°
3. é?????è??1′??￡???Dirty Card???é???–°???è?°???è??é?????Dirty Card?¤§é?¨????·2?????¨cleané????μ?¤????è??

??¨??????-￥éa¤??-???é?è|?é??????–°?”???￡????…¨é?¨?ˉ1è±?????|?????–°?”???￡???????”¨??????é?????é?è|?é??????¤????????ˉ1è±??1?????¤????è???ˉ1?o?è????aé????μ?????è—?—???￥èˉ′?????ˉ??a???é??????????o??ˉè???¤§é??????ˉ1è±???ˉ????—??-??′???????è???”è???o??ˉ1è±??1???ˉè??????”¨?¤§é?????è??1′??￡?ˉ1è±????é????????¤??o”èˉ￥????”????è??1′??￡?ˉ1è±?è??2???‰è￠?????”????é?????é’??’????????°?1??￠????????°‘??‰????|??????¨AbortablePrecleané????μ??-è???¤???°?￥??????‘?”??????YGC???è????·?°±??ˉ??￥é???…??‰?????—????????ˉ1è±???

?|??????¨AbortablePrecleané????μ?2???￥??—????‰§è???????YGC??????1???????

CMS??—?3???-???????o?????a?????°???CMSScavengeBeforeRemark???é??è?¤?1??2???‰????ˉ????|????????ˉèˉ￥?????°?????¨?‰§è??èˉ￥é????μ?1??‰?????????o???è§|??‘?????YGC?????ˉ??￥????°‘?–°?”???￡?ˉ1è±????é??????—?é—′???????”?????1???′????o?????1??

???è?????è???§??????°??‰?????‰???????????ˉé??????o?Remarké????μ??????é???—?é—′???????????ˉ??¨?–°?”???￡?ˉ1è±?????°‘?????…??μ????1??¤??o??????YGC???????ˉ???????ˉ??¨AbortablePrecleané????μ?·2?????‘?”??o??????YGC???????????¨èˉ￥é????μ????????????è§|??‘???????

?‰??￥??????é?è|?????????

?????¨Old GC

è????a?????¨Old GC???è???¨????è§|??‘???????ˉ”è??è????????

1. YGCè???¨???‘?”?Promotion Failed???è??è??ˉ1è??1′??￡è??è??????”?
2. ?ˉ”?|??‰§è???o?System.gc()????‰??????ˉ?2???‰?????°ExplicitGCInvokesConcurrent
3. ?…??????…??μ…

?|????è§|??‘?o??????¨Old GC???è???—??‘¨????§Old GC?-￡??¨?‰§è?????é?￡?1?????¤oè???‘¨????§Old GC????‰§è???????????????a?—??????aè????‰???§???¨Old GC??¨è??è????‰????1?è?°??? concurrent mode failure ??–è… concurrent mode interrupted??

?????¨GC???§??—????é?è|???¤?–-??????GC??ˉ??|è|??ˉ1è??1′??￡?????oé—′è??è??Compact????????oé???—?é—′????‘¨????§GC???é?????¤§é??????￠??‰???oé—′??‰?????¤?–-é?è?‘?????°?|???????

*should_compact =
 UseCMSCompactAtFullCollection &&
 ((_full_gcs_since_conc_gc >= CMSFullGCsBeforeCompaction) ||
 GCCause::is_user_requested_gc(gch->gc_cause()) ||
 gch->incremental_collection_will_fail(true /* consult_young */));

??¨??‰?§???…??μ??????è??è???????????

1. ?…???-?????°UseCMSCompactAtFullCollection(é??è?¤true)?’? CMSFullGCsBeforeCompaction(é??è?¤0)????‰??￥é??è?¤?ˉ????????????¨GCé??????ˉ1è??1′??￡?????…?-???oé—′è??è????????????°±??ˉ????ˉ1è±??§???¨??°??…?-???????·|è?1??
2. ??“????o?????ˉ”?|??‰§è???o?System.gc()????‰??????ˉ?2???‰?????°ExplicitGCInvokesConcurrent????1????è??è??????????
3. ?|?????–°?”???￡???????????…???????¤±è′￥??

??|????????¨????????—?3?????§°??oMSC??????è?°-??…???-?????????é???”¨????o??¨?????…¨??????????–1???è??è?????????”?é????????????—?é—′???é?????é??…

é?￡?????|????????¨????????—?3???ˉ???1???·????‘￠???

?????|????????¨???????‰§è??é?è?‘???Foreground Collect?????′??aè???¨?????ˉ1?‘¨????§Old GC??￥èˉ′????°‘?o?Precleaning?’?AbortablePreclean??¤??aé????μ????…????è???¨?é???·?????¤???

?|?????‰§è??System.gc()???è???”?·?????o??????°ExplicitGCInvokesConcurrent???è???—??1?????±??o??????¨GC???????????¨è???‘¨????§Old GC???è??è??????ˉ”?|????????‰§è??è???????????1????????-‰2s????￡??￥?????????è???ˉ???é????ˉ??¨?‘¨????§Old GC??