ZUncommit is enabled by default?

[Per Liden]

Hi,

On 12/10/19 9:28 PM, Sundara Mohan M wrote:
> Hi Stefan,
>       Thanks for the doc.
> 
> I tried setting min < max and seeing this error
> 
> [2019-12-10T19:20:10.559+0000][35.277s][error][gc     ] Failed to uncommit
> memory (Not supported)
> [2019-12-10T19:20:10.560+0000][35.277s][info ][gc,init] Uncommit: Disabled
> 
> Guess it's related to kernel doesn't support something
> Linux  2.6.32-754.10.1.el6.20190116.16.x86_64 #1 SMP Wed Jan 16 21:27:59
> UTC 2019 x86_64 x86_64 x86_64 GNU/Linux

Yes, your kernel is too old. As mentioned in the JEP [1], uncommitting 
memory requires fallocate(2) with FALLOC_FL_PUNCH_HOLE support, which 
first appeared in Linux 3.5 for tmpfs, and 4.3 for hugetlbfs (i.e. when 
using -XX:+UseLarePages).

I'll update the wiki to include that information.

[1] https://openjdk.java.net/jeps/351

/Per

> 
> 
> Thanks
> Sundar
> 
> On Tue, Dec 10, 2019 at 12:21 PM Stefan Karlsson <stefan.karlsson at oracle.com>
> wrote:
> 
>> 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
>>
>>