ZUncommit is enabled by default?

[Sundara Mohan M]

Cool, thanks!

On Wed, Dec 11, 2019 at 12:21 AM Per Liden <per.liden at oracle.com> wrote:

> 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
> >>
> >>
>