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Subject: Re: 32G ram
Date: Tue, 18 Dec 2012 14:57:34 -0600
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On 12/18/2012 3:19 AM, Nigel Wade wrote:
> On 17/12/12 08:39, Donkey Hottie wrote:
>> 17.12.2012 05:09, Arne Vajhøj kirjoitti:
>>> On 12/16/2012 10:05 PM, mcheung63@gmail.com wrote:
>>>>      i got 32GB ram, why i can't -Xmx2G, it got an error "Could not
>>>> create the Java Virtual Machine"
>>>>
>>>> /root>free -m
>>>>                total       used       free     shared    buffers
>>>> cached
>>>> Mem:         32387      16973      15413          0        243
>>>> 11906
>>>> -/+ buffers/cache:       4823      27563
>>>> Swap:         1906          0       1906
>>>> /root>uname -a
>>>> Linux localhost.localdomain 3.6.9-2.fc17.i686.PAE #1 SMP Tue Dec 4
>>>> 14:15:28 UTC 2012 i686 i686 i386 GNU/Linux
>>>
>>> 32 bit or 64 bit Java?
>>>
>>> Arne
>>>
>>>
>>
>> Apparently 32 bit, as the i686 OS is 32 bit PAE too.
>>
>
> 32bit Java on 32bit Linux should allow an application of up to 4GB. How
> much can of that can be allocated to the heap depends on the particular
> application. I've allocated more than 2G to heap on a 32bit Linux
> system, see this old thread on Google Groups:
> https://groups.google.com/forum/?hl=en-GB&fromgroups=#!topic/comp.lang.java.programmer/iGr3bxL9ZCA
>
>
> There may be a per-process limit. Try "ulimit -a" to see what limits the
> sysadmin has set.
>

you only get a 4GB process limit if the kernel is built with a special 
option, which gives 4GB per process but at the cost of making switching 
to/from kernel-space (such as in a system call) slower.

in a more typical setup, it is 3GB for application, 1GB for kernel.
this sets an effective upper limit of 3GB which the application can use.

32-bit Windows currently has the same limit, although older versions 
only allowed around 2GB per application (the other 2GB was: 1GB shared 
memory, 1GB kernel).