问题
I am trying to understand the cause for high number of CMS marks and remarks(other phases as well) averaging around 700ms even though the old gen is not even half full.Following are the GC configurations and stats from GCViewer.
-Xms3g
-Xmx3g
-XX:NewSize=1800m
-XX:MaxNewSize=1800m
-XX:MaxPermSize=256m
-XX:SurvivorRatio=8
-XX:+UseConcMarkSweepGC
-XX:+CMSClassUnloadingEnabled
Summary using GC Viewer: http://i.imgur.com/0IIbNUr.png
GC Log
152433.761: [GC [1 CMS-initial-mark: 284761K(1302528K)] 692884K(2961408K), 0.3367298 secs] [Times: user=0.33 sys=0.00, real=0.34 secs]
152434.098: [CMS-concurrent-mark-start]
152434.417: [CMS-concurrent-mark: 0.318/0.318 secs] [Times: user=1.38 sys=0.02, real=0.32 secs]
152434.417: [CMS-concurrent-preclean-start]
152434.426: [CMS-concurrent-preclean: 0.008/0.009 secs] [Times: user=0.02 sys=0.00, real=0.01 secs]
152434.426: [CMS-concurrent-abortable-preclean-start]
CMS: abort preclean due to time 152439.545: [CMS-concurrent-abortable-preclean: 4.157/5.119 secs] [Times: user=5.82 sys=0.20, real=5.12 secs]
152439.549: [GC[YG occupancy: 996751 K (1658880 K)]152439.550: [Rescan (parallel) , 0.5383841 secs]152440.088: [weak refs processing, 0.0070783 secs]152440.095: [class unloading, 0.0777632 secs]152440.173: [scrub symbol & string tables, 0.0416825 secs] [1 CMS-remark: 284761K(1302528K)] 1281512K(2961408K), 0.6771800 secs] [Times: user=3.35 sys=0.02, real=0.68 secs]
152440.227: [CMS-concurrent-sweep-start]
152440.613: [CMS-concurrent-sweep: 0.382/0.386 secs] [Times: user=0.39 sys=0.01, real=0.39 secs]
152440.613: [CMS-concurrent-reset-start]
152440.617: [CMS-concurrent-reset: 0.004/0.004 secs] [Times: user=0.01 sys=0.00, real=0.00 secs]
152441.719: [GC [1 CMS-initial-mark: 284757K(1302528K)] 1320877K(2961408K), 0.7720557 secs] [Times: user=0.78 sys=0.01, real=0.77 secs]
152442.492: [CMS-concurrent-mark-start]
回答1:
CMS remark has to scan the young generation, since your young generation is so large this takes some time. Depending on the java version (which you did not specify!) you may have to enable parallel remarking (CMSParallelRemarkEnabled).
Enabling CMSScavengeBeforeRemark may also reduce the amount of memory that needs to be scanned during remarks.
And simply shrinking the new generation and taking the hit of a few more promotions that then get cleaned out by the concurrent old gen GC may work too.
I don't think incremental mode fixes anything here, it just drastically alters the behavior of CMS that it masks your original issue.
来源：https://stackoverflow.com/questions/32302679/high-number-of-cms-mark-remark-pauses-even-though-old-gen-is-not-half-full