ZUncommit is enabled by default?

[Stefan Karlsson]

Hi Sundar,

It's described on our project wiki:
https://wiki.openjdk.java.net/display/zgc/Main

---
Returning Unused Memory to the Operating System

By default, ZGC uncommits unused memory, returning it to the operating 
system. This is useful for applications and environments where memory 
footprint is a concern. This feature can be disabled using 
-XX:-ZUncommit. Furthermore, memory will not be uncommitted so that the 
heap size shrinks below the minimum heap size (-Xms). This means this 
feature will be implicitly disabled if the minimum heap size (-Xms) is 
configured to be equal to the maximum heap size (-Xmx).

An uncommit delay can be configured using -XX:ZUncommitDelay=<seconds> 
(default is 300 seconds). This delay specifies for how long memory 
should have been unused before it's eligible for uncommit.
---

Additionally, -Xms sets both -XX:InitialHeapSize and -XX:MinHeapSize, so 
if you only want to set the lower limit of the amount of committed 
memory for the heap, you can use -XX:MinHeapSize instead.

StefanK

On 2019-12-10 19:57, Sundara Mohan M wrote:
> Hi,
>   I am using following java version and see Uncommit feature is disabled by
> default
> openjdk version "13.0.1" 2019-10-15
> OpenJDK Runtime Environment AdoptOpenJDK (build 13.0.1+9)
> OpenJDK 64-Bit Server VM AdoptOpenJDK (build 13.0.1+9, mixed mode, sharing)
>
> GC logs
>
> [2019-12-10T18:47:43.171+0000][0.007s][info][gc,init] Initializing The Z
> Garbage Collector
> [2019-12-10T18:47:43.171+0000][0.007s][info][gc,init] Version: 13.0.1+9
> (release)
> [2019-12-10T18:47:43.171+0000][0.007s][info][gc,init] NUMA Support: Enabled
> [2019-12-10T18:47:43.171+0000][0.007s][info][gc,init] NUMA Nodes: 2
> [2019-12-10T18:47:43.171+0000][0.007s][info][gc,init] CPUs: 48 total, 48
> available
> [2019-12-10T18:47:43.171+0000][0.007s][info][gc,init] Memory: 128318M
> [2019-12-10T18:47:43.171+0000][0.007s][info][gc,init] Large Page Support:
> Disabled
> [2019-12-10T18:47:43.171+0000][0.007s][info][gc,init] Workers: 40 parallel,
> 7 concurrent
> [2019-12-10T18:47:43.174+0000][0.010s][info][gc,init] Address Space:
> 0x0000040000000000 - 0x0000140000000000 (16T)
> [2019-12-10T18:47:43.174+0000][0.010s][info][gc,init] Heap backed by file:
> /tmp/java_heap.112273
> [2019-12-10T18:47:43.174+0000][0.010s][info][gc,init] Min Capacity: 80000M
> [2019-12-10T18:47:43.174+0000][0.010s][info][gc,init] Initial Capacity:
> 80000M
> [2019-12-10T18:47:43.174+0000][0.010s][info][gc,init] Max Capacity: 80000M
> [2019-12-10T18:47:43.174+0000][0.010s][info][gc,init] Max Reserve: 112M
> [2019-12-10T18:47:43.174+0000][0.010s][info][gc,init] Pre-touch: Enabled
> [2019-12-10T18:47:43.174+0000][0.010s][info][gc,init] Available space on
> backing filesystem: 125929M
> *[2019-12-10T18:48:30.290+0000][47.126s][info][gc,init] Uncommit: Disabled*
> [2019-12-10T18:49:30.315+0000][107.151s][info][gc,init] Runtime Workers: 40
> parallel
> [2019-12-10T18:49:30.318+0000][107.155s][info][gc     ] Using The Z Garbage
> Collector
> ...
> Is this feature enabled by default in later release only?
>
> Thanks
> Sundar