A 64-bit Linux machine used as server can have multiple CPU cores and addressable memory far in exess of 4GB. For example, I would like a Java (JVM) process on such a 32GB 16-CPU Core machine to use more than 20 GB of RAM. I am under the impression that that is very much doable. Setting ulimit to right value (ulimit m to unlimited?) should help. But I have been surprised to see a post http://docs.oracle.com/cd/E19316-01/...eii/index.html that seems to indicate that there is 3GB per process limit on 64-bit Linux. I am a Java guy, but do not have deep knowledge of Linux. Could a Linux guru comment if this limit really exists or this can be increased? If it can be increased, how?

Thanks and Regards.

Hello amal_s, welcome to LQ,

as far as I know (I'm not an expert for hardware) the limitations are 3GB per process on x86-Linux with pae-kernel (the pae-kernel supports up to 64GB of RAM on a 32bit system, but limits each process to a maximum of 3GB of RAM).
On a 64bit there is no 3GB limit per process. This is true for Linux in general, but I don't know if and which possibilities a programmer has, to limit the memory for one process of his application.

You should use the report-button at your post and ask a Moderator to move your thread to the "Programming"-section of LQ here http://www.linuxquestions.org/questions/programming-9/. I suppose there are people who are capable to answer your question in more detail.

Markus

As markush said, that limit is for 32-bit OS with a pae kernel. A true 64-bit OS does not have that restriction. I regularly use 30GB+ in a single process on my machines running Fedora 64-bit.

Well, that link is to the Official docs at Oracle (formerly Sun), so should be correct.
Seems an odd limit to me, but they should know know; they own/wrote it.
Best bet is to ask Oracle themselves.

I agree that Java is the limit, not a theoretical limit.

Notice how they seem to favor Solaris. If you need to you can use it or an open version just as well as linux generally.

This value is only relevant to a 32 bit JVM running on a 64-bit kernel. A 64 bit JVM will support a 20 GB (or larger) heap.

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