Recent big apps testing of -Xincgc (which enables iCMS with auto-pacing)
appear to indicate that auto-pacing may not be working too well. In one
particular case (ATG with default options?), iCMS obstinately stuck to
a duty-cycle of close to 0 and would repeatedly run into concurrent
mode failures.

here's an example, albeit from a fastdebug run:

[ParNew: 3968K->0K(4032K), 0.2749890 secs] 3968K->593K(16320K), 0.2885700 secs]
[GC [1 CMS-initial-mark: 593K(12288K)] 2757K(16320K), 0.0250730 secs]
[GC [ParNew: 3968K->0K(4032K), 0.0889470 secs] 4561K->618K(16320K), 0.1021480 secs]
[GC [ParNew: 3964K->0K(4032K), 0.0991410 secs] 4582K->2345K(16320K), 0.1068090 secs]
[GC [ParNew: 3963K->0K(4032K), 0.0759430 secs] 6308K->4137K(16320K), 0.0817950 secs]
[GC [ParNew: 3962K->0K(4032K), 0.2536390 secs] 8100K->5867K(16320K), 0.2598500 secs]
[GC [ParNew: 3962K->0K(4032K), 0.0775730 secs] 9830K->7683K(16320K), 0.0841290 secs]
[CMS-concurrent-mark: 1.511/7.588 secs]
[GC [ParNew: 3968K->0K(4032K), 0.8945680 secs] 11651K->8131K(16320K), 0.8989950 secs]
[GC [ParNew: 3964K->0K(4032K), 0.2893990 secs] 12095K->9899K(16320K), 0.2960000 secs]
[CMS-concurrent-preclean: 0.282/2.288 secs]
[CMS-concurrent-abortable-preclean: 0.001/0.103 secs]
[GC[YG occupancy: 2505 K (4032 K)][checkpointRootsFinalWork[Rescan (parallel) , 0.0391710 secs][refProcessi
ngWork[weak refs processing, 0.0002790 secs], 0.0003600 secs], 0.0404190 secs] [1 CMS-remark: 9899K(12288K)
] 12404K(16320K), 0.0406260 secs]
[GC [ParNew: 3961K->0K(4032K), 0.1722770 secs] 13860K->11734K(16320K), 0.1780030 secs]
[CMS-concurrent-sweep: 0.012/0.505 secs]
[GC [ParNew: 3965K->0K(4032K), 0.1698730 secs] 15698K->13557K(23592K), 0.1756600 secs]
[CMS-concurrent-reset: 0.040/0.777 secs]
[GC [ParNew: 3968K->0K(4032K), 0.0675070 secs] 17525K->15361K(23592K) icms_dc=5 , 0.0734810 secs]
[GC [1 CMS-initial-mark: 15361K(19560K)] 18599K(23592K), 0.0967070 secs]
[GC [ParNew: 3968K->0K(4032K), 0.1122390 secs] 19329K->17070K(23592K) icms_dc=5 , 0.1173950 secs]
[CMS-concurrent-mark: 0.202/0.363 secs]
[CMS-concurrent-preclean: 0.005/0.006 secs]
[GC[YG occupancy: 157 K (4032 K)][checkpointRootsFinalWork[Rescan (parallel) , 0.0191160 secs][refProcessin
gWork[weak refs processing, 0.0149090 secs], 0.0156820 secs], 0.0352450 secs] [1 CMS-remark: 17070K(19560K)
] 17228K(23592K), 0.0354280 secs]
[CMS-concurrent-sweep: 0.214/0.214 secs]
[CMS-concurrent-reset: 0.012/0.012 secs]
[GC [1 CMS-initial-mark: 2744K(19560K)] 5032K(23592K), 0.0903140 secs]
[GC [ParNew: 3968K->0K(4032K), 0.6902760 secs] 6712K->3814K(23592K) icms_dc=0 , 0.7114050 secs]
[CMS-concurrent-mark: 0.431/1.221 secs]
[CMS-concurrent-preclean: 0.072/0.074 secs]
[GC[YG occupancy: 376 K (4032 K)][checkpointRootsFinalWork[Rescan (parallel) , 0.1933340 secs][refProcessin
gWork[weak refs processing, 0.0025520 secs], 0.0026870 secs], 0.1965770 secs] [1 CMS-remark: 3814K(19560K)]
 4191K(23592K), 0.1967470 secs]
[CMS-concurrent-sweep: 0.274/0.412 secs]
[CMS-concurrent-reset: 0.014/0.014 secs]
[GC [ParNew: 3960K->0K(4032K), 1.2903710 secs] 5777K->3359K(23592K) icms_dc=0 , 1.3076870 secs]
[GC [ParNew: 3967K->0K(4032K), 0.2974850 secs] 7327K->4981K(23592K) icms_dc=0 , 0.3101230 secs]
[GC [ParNew: 3968K->0K(4032K), 0.2411100 secs] 8949K->6761K(23592K) icms_dc=0 , 0.2490910 secs]
[GC [ParNew: 3968K->0K(4032K), 0.0765620 secs] 10729K->8584K(23592K) icms_dc=0 , 0.0829090 secs]
[GC [ParNew: 3966K->0K(4032K), 0.1574670 secs] 12551K->10407K(23592K) icms_dc=0 , 0.1639160 secs]
[GC [ParNew: 3968K->0K(4032K), 0.3044010 secs] 14375K->12192K(23592K) icms_dc=0 , 0.3113390 secs]
[GC [ParNew: 3967K->0K(4032K), 0.0722010 secs] 16160K->13997K(23592K) icms_dc=0 , 0.0813970 secs]
[GC [ParNew: 3962K->0K(4032K), 0.0725220 secs] 17959K->15770K(23592K) icms_dc=0 , 0.0815660 secs]
[GC [ParNew: 3967K->0K(4032K), 0.0704970 secs] 19738K->17583K(23592K) icms_dc=0 , 0.0794120 secs]
[GC [ParNew: 3964K->0K(4032K), 0.6074360 secs] 21547K->19411K(23720K) icms_dc=0 , 0.6162590 secs]
[GC [ParNew: 3967K->0K(4032K), 0.2511520 secs] 23378K->21233K(25512K) icms_dc=0 , 0.2591340 secs]
[GC [ParNew: 3961K->0K(4032K), 0.2097800 secs] 25194K->23048K(27304K) icms_dc=0 , 0.2184840 secs]
[GC [ParNew: 3961K->0K(4032K), 0.1982940 secs] 27010K->24876K(29224K) icms_dc=0 , 0.2253380 secs]
[GC [ParNew: 3962K->0K(4032K), 0.1602020 secs] 28839K->26697K(31016K) icms_dc=0 , 0.1692120 secs]
[GC [ParNew: 3968K->0K(4032K), 0.1772730 secs] 30665K->28469K(32808K) icms_dc=0 , 0.1864410 secs]
[GC [ParNew: 3967K->0K(4032K), 0.1046300 secs] 32437K->30299K(34600K) icms_dc=0 , 0.1140270 secs]
[GC [ParNew: 3968K->0K(4032K), 0.2668900 secs] 34267K->32129K(36520K) icms_dc=0 , 0.2757080 secs]
[GC [ParNew: 3968K->0K(4032K), 0.4998970 secs] 36097K->33936K(38312K) icms_dc=0 , 0.5083410 secs]
[GC [ParNew: 3962K->0K(4032K), 0.0939820 secs] 37898K->35654K(39976K) icms_dc=0 , 0.1022170 secs]
[GC [ParNew: 3968K->0K(4032K), 0.2714720 secs] 39622K->37475K(41768K) icms_dc=0 , 0.2802820 secs]
[GC [ParNew: 3962K->0K(4032K), 0.1874690 secs] 41437K->39304K(43688K) icms_dc=0 , 0.1957990 secs]
[GC [ParNew: 3966K->0K(4032K), 0.1373540 secs] 43270K->41133K(45480K) icms_dc=0 , 0.1461720 secs]
[GC [ParNew: 3963K->0K(4032K), 0.2667400 secs] 45096K->42906K(47272K) icms_dc=0 , 0.2870410 secs]
[GC [ParNew: 3968K->0K(4032K), 0.5858740 secs] 46874K->44675K(49064K) icms_dc=0 , 0.5960840 secs]
[GC [ParNew: 3968K->0K(4032K), 0.9313580 secs] 48643K->46403K(50728K) icms_dc=0 , 0.9408300 secs]
[GC [ParNew: 3968K->0K(4032K), 0.0755800 secs] 50371K->47972K(52392K) icms_dc=0 , 0.0855960 secs]
[GC [ParNew: 3967K->0K(4032K), 0.0903550 secs] 51940K->49761K(54184K) icms_dc=0 , 0.1005620 secs]
[GC [ParNew: 3968K->0K(4032K), 0.7428410 secs] 53729K->51565K(55976K) icms_dc=0 , 0.7532310 secs]
[GC [ParNew: 3962K->0K(4032K), 0.0705430 secs] 55527K->52312K(56744K) icms_dc=0 , 0.0817450 secs]
[GC [ParNew: 3967K->0K(4032K), 0.3009520 secs] 56280K->54151K(58536K) icms_dc=0 , 0.3111420 secs]
[GC [ParNew: 3968K->0K(4032K), 0.4323650 secs] 58119K->55789K(60200K) icms_dc=0 , 0.4516510 secs]
[GC [ParNew: 3968K->0K(4032K), 0.9389240 secs] 59757K->57596K(61992K) icms_dc=0 , 0.9482630 secs]
[GC [ParNew: 3964K->0K(4032K), 0.0914410 secs] 61561K->59145K(63528K) icms_dc=0 , 0.1015890 secs]
[GC [ParNew: 3965K->0K(4032K), 0.1055800 secs] 63111K->60932K(65448K) icms_dc=0 , 0.1166580 secs]
[GC [ParNew: 3966K->3966K(4032K), 0.0006820 secs][CMS (concurrent mode failure): 60932K->3930K(61416K), 0.6
157990 secs] 64898K->3930K(65448K) icms_dc=0 , 0.6273560 secs]
[GC [1 CMS-initial-mark: 3930K(61440K)] 3952K(65472K), 0.0283330 secs]
[GC [ParNew: 3968K->0K(4032K), 0.3344450 secs] 7898K->5670K(65472K) icms_dc=0 , 0.3438230 secs]
[GC [ParNew: 3967K->0K(4032K), 0.3422460 secs] 9638K->7381K(65472K) icms_dc=0 , 0.3518630 secs]
[GC [ParNew: 3968K->0K(4032K), 0.3301040 secs] 11349K->9101K(65472K) icms_dc=0 , 0.3395650 secs]
[GC [ParNew: 3964K->0K(4032K), 0.1738440 secs] 13066K->10822K(65472K) icms_dc=0 , 0.1833610 secs]
[GC [ParNew: 3968K->0K(4032K), 0.2554660 secs] 14790K->12529K(65472K) icms_dc=0 , 0.2721270 secs]
[GC [ParNew: 3967K->0K(4032K), 2.3883440 secs] 16497K->14306K(65472K) icms_dc=0 , 2.3923040 secs]
[GC [ParNew: 3965K->0K(4032K), 0.1843220 secs] 18272K->15948K(65472K) icms_dc=0 , 0.1938690 secs]
[GC [ParNew: 3968K->0K(4032K), 0.5584250 secs] 19916K->17455K(65472K) icms_dc=0 , 0.5668850 secs]
[GC [ParNew: 3965K->0K(4032K), 0.1456860 secs] 21421K->18931K(65472K) icms_dc=0 , 0.1547750 secs]
[GC [ParNew: 3966K->0K(4032K), 0.1473050 secs] 22898K->20349K(65472K) icms_dc=0 , 0.1553750 secs]
[GC [ParNew: 3968K->0K(4032K), 0.0591310 secs] 24317K->21624K(65472K) icms_dc=0 , 0.0664610 secs]
[GC [ParNew: 3968K->0K(4032K), 0.0980760 secs] 25592K->22580K(65472K) icms_dc=0 , 0.1033190 secs]
[GC [ParNew: 3962K->0K(4032K), 0.2098910 secs] 26543K->23154K(65472K) icms_dc=0 , 0.2134340 secs]
Time = 59227 ms (168.84 requests/s; average latency = 592 ms)
0 errors out of 10000 requests
[CMS-concurrent-mark: 1.611/13.618 secs]
[CMS-concurrent-preclean: 0.018/0.018 secs]
[CMS-concurrent-abortable-preclean: 0.000/0.000 secs]
[GC[YG occupancy: 2123 K (4032 K)][checkpointRootsFinalWork[Rescan (parallel) , 0.3407410 secs][refProcessi
ngWork[weak refs processing, 0.0002730 secs], 0.0004020 secs], 0.3416230 secs] [1 CMS-remark: 23154K(61440K
)] 25278K(65472K), 0.3418150 secs]
[CMS-concurrent-sweep: 0.078/0.078 secs]
[CMS-concurrent-reset: 0.016/0.016 secs]

etc.

The same (or worse) anomalous behaviour manifested on the server as well.