Windows 7 and all my Java stuff.
in [Java Programming]
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From: Thomas Pornin on 25 Nov 2009 08:33 According to DuncanIdaho <Duncan.Idaho2008(a)googlemail.com>: > If I develop against a 64bit JVM (say because I gotta have the 'latest > and greatest' no other reason) and then compile the code on a client > machine against a 32bit JVM Java source code is actually never compiled against a 64-bit or 32-bit JVM; it is compiled against _the_ Java Virtual Machine, i.e. a theoretical architecture which has no "register size". A Java compiler should produce, for a given set of source file, the exact same .class files, regardless of whether the compiler runs on a 64-bit, 32-bit or whatever system. The compiled code, i.e. the .class files (usually stored in .jar archives), can be fed unchanged to a 64-bit or a 32-bit JVM, and will run to produce the exact same results(*) on both. I routinely compile Java code on a 64-bit Linux system, and the .jar files are deployed to all kinds of systems, including 32-bit Windows and the like. This is the point of using a virtual machine as compilation target. > then the only possible problem could be that I'm using some humoungous > great data structure that needs bigger that 32 bit addresses in which > case I should probably be sacked anyway ... It is not really a problem of address size -- it is a problem of RAM size. If your code requires more than 4 GB of RAM, then it will not run on a 32-bit system since 32-bit systems can use only 4 GB(**). A system has registers wide enough to address all the RAM it can use, so you will not be limited by the register size, but by the RAM size. And if your code needs 4 GB, chances are that you will notice it... --Thomas Pornin (*) Except for performance, i.e. how much time is used to perform said operations; also, there can be some rounding issues with floating-point types in non-strictfp code. (**) With PAE, available since the Pentium Pro (that's more than twelve years ago), a 32-bit x86 system may use up to 64 GB of RAM, but not simultaneously: each single process may use only 4 GB.
From: Lew on 25 Nov 2009 09:19 Thomas Pornin wrote: > (**) With PAE, available since the Pentium Pro (that's more than twelve > years ago), a 32-bit x86 system may use up to 64 GB of RAM, but not > simultaneously: each single process may use only 4 GB. I'm aware of no 32b JVM that can specify a heap over about 1.8 GB; most are lower. 32b Windows only allows up to 2GB per process or 3GB with a switch. The JVM and permanent generation use some of that, leaving less for the heap. -- Lew
From: Nigel Wade on 25 Nov 2009 10:13 On Wed, 25 Nov 2009 09:19:16 -0500, Lew wrote: > Thomas Pornin wrote: >> (**) With PAE, available since the Pentium Pro (that's more than twelve >> years ago), a 32-bit x86 system may use up to 64 GB of RAM, but not >> simultaneously: each single process may use only 4 GB. > > I'm aware of no 32b JVM that can specify a heap over about 1.8 GB; most > are lower. 32b Windows only allows up to 2GB per process or 3GB with a > switch. The JVM and permanent generation use some of that, leaving less > for the heap. $ java -version java version "1.6.0_15" Java(TM) SE Runtime Environment (build 1.6.0_15-b03) Java HotSpot(TM) Server VM (build 14.1-b02, mixed mode) $ java -jar -Xmx3700m some.jar .... $ java -jar -Xmx3800m some.jar Error occurred during initialization of VM Could not reserve enough space for object heap Could not create the Java virtual machine. So, the limit on this particular Java application is somewhere between 3.7GB and 3.8GB of heap. I should point out that this is a 32bit JVM running on a 64bit system with 32GB of RAM. Nonetheless, the limit on a 32bit application is still 4GB despite the 64bit OS. What it does mean, though, is that any individual application has got a much greater chance of getting the max. address space because the entire system isn't limited to 4GB. The same may be true of a PAE system, I don't know and can't test it. -- Nigel Wade
From: Thomas Pornin on 25 Nov 2009 11:24 According to Lew <noone(a)lewscanon.com>: > I'm aware of no 32b JVM that can specify a heap over about 1.8 GB; > most are lower. Sun's JVM can. Right now, with Sun's JDK 1.6.0_16, 32-bit version for Linux, I can specify a 3.68 GB heap with '-Xmx3683m', and use it (I made a simple Java code which successfully stored 3683 byte[], each newly allocated of size 1048576, into an ArrayList). The trick here is that while the JDK is a 32-bit binary and runs in a 32-bit system, the Linux kernel itself is a 64-bit kernel. The limit on 2 or 3 GB per process is due to the need for the kernel code to be able to access its own code and the user address space with special access rights (not those seen from user space), which basically means that memory seen by the application is mapped twice (for the 3 GB limit, the kernel must employ various trickeries such as messing with the MMU tables, which are cumbersome and potentially expensive). When the kernel itself is a 64-bit kernel, it can map the full 32-bit address space (4 GB) twice in its own, much wider 64-bit address space; hence, application may then have their (almost) full complement of 4 GB. As far as I know, this is also what happens when a 64-bit Windows system runs a 32-bit application. Running a full 32-bit system with a 64-bit kernel is a possible compromise which is actually recommended when there are many 32-bit legacy, binary only applications to run, but the machine has more than 2 or 3 GB of RAM and the CPU supports the 64-bit amd64 mode. --Thomas Pornin
From: DuncanIdaho on 25 Nov 2009 11:27
Nigel Wade wrote: > On Wed, 25 Nov 2009 09:19:16 -0500, Lew wrote: > >> Thomas Pornin wrote: >>> (**) With PAE, available since the Pentium Pro (that's more than twelve >>> years ago), a 32-bit x86 system may use up to 64 GB of RAM, but not >>> simultaneously: each single process may use only 4 GB. >> I'm aware of no 32b JVM that can specify a heap over about 1.8 GB; most >> are lower. snip > I should point out that this is a 32bit JVM running on a 64bit system > with 32GB of RAM. Nonetheless, the limit on a 32bit application is still > 4GB despite the 64bit OS. What it does mean, though, is that any > individual application has got a much greater chance of getting the max. > address space because the entire system isn't limited to 4GB. > > The same may be true of a PAE system, I don't know and can't test it. > OK well I've ordered my new laptop with Windows 7 By the way http://java.sun.com/javase/downloads/widget/jdk6.jsp offers JDK 6 Update 17 for Windows and Windows x64 where the archive for the 64 bit version is smaller than that for the 32 bit version by around 8MB, interesting huh, also, apparently the 64 bit release can be used in 32 bit mode... Anyway, it will be interesting to see just how big the heap can be with a 64bit JVM running on a 64bit system. Roll on delivery day. -- Idaho |