Number/size of G1 regions?

[Yu Zhang]

Tim,

This is probably due to you did not specify the minimum heap. With jdk7u40
java -Xmx24g -XX:+UseG1GC -XX:+PrintFlagsFinal -XX:+PrintGCDetails 
-verbose:gc gives the following:
Heap
  garbage-first heap   total 516096K, used 0K [0x00000001fae00000, 
0x000000021a600000, 0x00000007fae00000)
   region size 1024K, 1 young (1024K), 0 survivors (0K)

./java -Xms24g -Xms24g -XX:+UseG1GC -XX:+PrintFlagsFinal 
-XX:+PrintGCDetails -verbose:gc gives:
Heap
  garbage-first heap   total 25165824K, used 0K [0x00000001fb000000, 
0x00000007fb000000, 0x00000007fb000000)
   region size 8192K, 1 young (8192K), 0 survivors (0K)

If the minimum heap size is not specified in command line, jvm will 
first decide the minimum based on some factors(newSize, oldSize, os 
allocatable memory, ect), then take the average of min and max heap 
(average heap size) to decide the region size.  This region size is 
decided when jvm starts, and would not change if the heap is expanded.

With smaller region size, one impact is you may see a lot of humongous 
objects.  By definition, objects size > 1/2 of the region size.  Also if 
you are seeing time spend in RS related operations, probably due to a 
lot of RS to maintain.

Thanks,
Jenny

On 7/14/2014 12:45 PM, Tim King wrote:
> Hello all!
> I have a question about the number and size of heap regions we are 
> seeing on our JVMs.  We have a 24GB heap, and I am seeing that 24524 
> 1MB regions are being allocated.  We are running  JDK 1.7.0_51.
>
> I read in this article 
> http://www.oracle.com/technetwork/articles/java/g1gc-1984535.html that 
> "the goal is to have no more than 2048 regions".
>
> Based on the size of our heap, I am expecting larger region size with 
> fewer regions.  Is this something I should be worried about, or is it 
> expected and normal? What kind of impact will this have on the 
> performance?
>
> Thank you,
> Cheers,
> -Tim
>
> Heap Configuration:
>    MinHeapFreeRatio = 40
>    MaxHeapFreeRatio = 70
>    MaxHeapSize      = 25769803776 (24576.0MB)
>    NewSize          = 1363144 (1.2999954223632812MB)
>    MaxNewSize       = 17592186044415 MB
>    OldSize          = 5452592 (5.1999969482421875MB)
>    NewRatio         = 2
>    SurvivorRatio    = 8
>    PermSize         = 268435456 (256.0MB)
>    MaxPermSize      = 536870912 (512.0MB)
>    G1HeapRegionSize = 1048576 (1.0MB)
>
> Heap Usage:
> G1 Heap:
>    regions  = 24524
>    capacity = 25715277824 (24524.0MB)
>    used     = 5114734088 (4877.7905349731445MB)
>    free     = 20600543736 (19646.209465026855MB)
>    19.889865172782354% used
> ....
>
>
>
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