Java 7 Default GC for server

[Rohit Mohta]

Hi All,


   I have quite some questions around default GC in server mode

(a) Is the formula for calculating number GC threads is
3 + (5 * cores/8)

(b) In the GC logs, I can see the below lines printed even when the
application is idle. Is this something to do with JIT or some other JVM
internal operation?

2015-03-05T14:42:18.320+0000: 520807.126: Total time for which application
threads were stopped: 0.0000500 seconds
2015-03-05T14:42:18.320+0000: 520807.126: Application time: 0.0000240
seconds
2015-03-05T14:42:18.320+0000: 520807.126: Total time for which application
threads were stopped: 0.0000500 seconds
2015-03-05T14:42:58.405+0000: 520847.212: Application time: 40.0857170
seconds
2015-03-05T14:42:58.406+0000: 520847.212: Total time for which application
threads were stopped: 0.0001980 seconds
2015-03-05T14:42:58.406+0000: 520847.212: Application time: 0.0000250
seconds
2015-03-05T14:42:58.406+0000: 520847.212: Total time for which application
threads were stopped: 0.0000520 seconds
2015-03-05T14:43:28.406+0000: 520877.213: Application time: 30.0001550
second

(c) We have about 15 JVM's on a single server. Linux Server has 24 cores
and about 37GB of RAM. When we restart all the JVM's, they start with a
good heap size, about 700MB+. And we have no issues with that. After a day
or so, some of the processes drop down to less than 100MB of heap size and
they start doing very frequent minor and major GC. We have lot of unused
memory on the server.

Why won't GC cause expansion?
I know we can set a Xms to a minimum value, but we are curious to know why
few of them go from 750MB to 100MB, whereas some of them stay around 500MB.
Is this has to do with SizeIncrement, SizeSupplement or AdaptiveeSize
values?

Thanks,
Rohit
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