Given a 32-bit/64-bit processor can a 4GB process run on 2GB RAM. Will it use virtual memory or it wont run at all?
This is HIGHLY platform dependent. On many 32bit OS's, no single process can ever use more than 2GB of memory, regardless of the physical memory installed or virtual memory allocated.
For example, my work computers use 32bit Linux with PAE (Physical Address Extensions) to allow it to have 16GB of RAM installed. The 2GB per process limit still applies however. Having the extra RAM simply allows me to have more individual processes running. 32bit Windows is the same way.
64bit OS's are more of a mixed bag. 64bit Linux will allow individual processes to map memory well in excess of 32GB (but again, varies from Kernel to Kernel). You will be limited only by the amount of Swap (Linux virtual memory) you have. 64bit Windows is a complete crap shoot. Certain versions will only allow 2GB per process, but most will allow >32GB limited only by the amount of Page File the user has allocated.
Microsoft provides a useful table breaking down the various memory limits on various OS versions/editions. Unfortunately there is no such table that I can find with cursory searching for Linux since it is so fragmented.
Short answer: Depends on the system.
Most 32-bit systems have a limitation of 2GB per process. If your system allows >2GB per process, then we can move on to the next part of your question.
Most modern systems use Virtual Memory. Yet, there are some constrained (and various old) systems that would just run out of space and make you cry. I believe uClinux supports both MMU and MMU-less architectures. Most 32-bit processors have a MMU (a few don't, see ARM Cortex-M0) and a handful of 16-bit or 8-bit have it as well (see Atmel ATtiny13A-MMU and Atari MMU).
Any process that needs more memory than is physically available will require a form of Memory Swap (e.g., a partition or file).
Virtual Memory is divided in pages. At some point, a page reside either in RAM or in Swap. Any attempt to access a memory page that's not loaded in RAM will trigger an interruption called Page Fault, which is handled by the kernel.
A 64-bit process needing 4GB on a 64-bit OS can generally run in 2GB of physical RAM, by using virtual memory, assuming disk swap space is available, but performance will be severely impacted if all of that memory is frequently accessed.
A 32-bit process can't address exactly 4GB of memory in practice (some address space overhead is required by the operating system), so it won't run. Depending on the OS, it can probably run a process that needs > 2GB and < 3-4GB.
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I have some conceptions I will lay out first. A 32-bit Windows application can't address more than ~3 GB of memory if I got it right. However, when it runs out of memory in its virtual address space, it will hand over data to the Windows virtual memory manager in order to make more space for whatever it needs. Windows' virtual memory manager will then write this data to a location that is in physical RAM, but outside the application's virtual address space. When the 32-bit application needs this data again, the virtual memory manager can respond swiftly by loading it from physical RAM.
I can imagine there is overhead to this. How much overhead are we talking about? A 64-bit application would be able to make a virtual address space that is large enough for everything it needs, but how much more efficient is this than shuffling around data with the memory manager?
My original question is that, is this technically rational to check the required heap-size of my Java program on Windows 7, via VisualVM, and come to this conclusion that the program will require the same amount of heap on Linux(RedHat) as well?
I don't know how the system(OS or even CPU and RAM), affect memory management of JVM.
well, the windows is my development system with 4GB of RAM and a Core 2 Due CPU, however the
Linux is the production system with 32GB of RAM and multiple powerfull processors,
Actually, my concern is that the program on Linux might need more memory. less is ok.
What is the maximum size a process can take on 32 bit processor on windows machine?
Can a process size be more than 4GB in any case?
Let's assume you are talking about virtual memory size, "process size" doesn't mean anything.
A 32-bit process can consume up to two gigabytes of address space on a 32-bit operating system.
A 32-bit process can consume up to three gigabytes of address space on a 32-bit operating system when it is booted with the /3GB boot option and it is linked with the /LARGEADDRESSAWARE linker option. This boot option is rarely available these days, particularly video adapters consume too much physical address space to leave enough room for the operating system and file system cache.
A 32-bit process can consume up to four gigabytes of address space on a 64-bit operating system, as long as it is linked with the /LARGEADDRESSAWARE linker option
A 64-bit process can consume an amount of virtual address space on a 64-bit operating system that's limited by the maximum size of the paging file. The absolute maximum is further limited by the OS edition, as documented here.
You are liable to run into other limitations. This is all well explored in Mark Russinovich' blog series titled "Pushing the limits". The virtual memory limits are explored in this one.
I hope someone with a bit of knowledge can clear this up. There's many discussions about the reasons to run a 64-bit OS (e.g. Windows 7 x64), but many people seem to think that their old x86 apps will be able to take advantage of any RAM greater than 3.5GB.
As I understand it, though, x86 apps cannot address memory that high... unless they've been specifically programmed to (which very few will have).
Can someone knowledgeable clear this up for me, once and for all? Can 32-bit apps take advantage of a system running 8GB of RAM?
E.g. If a user decided (for whatever reason) to run several x86 apps at once, filling the RAM as much as possible, would the extra addressable memory available in Windows 7 x64 be used?
Thanks!
On a 64 bit system, 32 bit applications are able to use the full 4GB virtual address space, minus about 64K. A default 32 bit windows system will only allow a 32 bit process to use 2 GB of virtual address space. By specially configuring the OS it's possible to push that limit up to 3 GB, but it's still not as good as what you would get on a 64 bit version of windows.
If you have 8GB of ram, that 8 GB can be divided up between multiple 32 bit processes, and the entire 8 GB will be utilized if necessary. However, no single 32 bit process will be allocated more than 4 GB of memory.
Although i don't have sources to cite, but from my knowledge: 32bit app will not be able to address more than 4GB of memory itself, unless it uses some tricks(that is very unlikely), but if you have some 32bit apps running at the same time, they can all have 4GB each, and thus two 32bit apps should be able to use all 8GB of memory. Though I'm not 100% sure.
Yes. x86 apps cannot use more than 2GB of memory at once without special tricks, but they can use any memory available.
Adding to the other (correct) answers:
Instead of the term "application" the word "process" should be used. Applications often consist of multiple processes whereas the limits discussed here apply to single processes.
Thus applications benefit from x64 that either are linked with the LARGEADDRESSAWARE flag (they can use 4 GB instead of 2 GB) or that share the load between multiple processes.
32-bit processes can work with more than 4 GB RAM even on 32-bit systems by using AWE. But a 32-bit process can only ever use 2 GB at once (4 GB with LARGEADDRESSAWARE on 64 bit respectively). AWE is primarily used by databases where it is essential for performance that the entire database fit into RAM. It works by providing a 2 GB window into a larger chunk of memory.
Here are some articles for further reading:
Windows x64 – All the Same Yet Very Different, Part 1: Virtual Memory
Windows x64 – All the Same Yet Very Different, Part 2: Kernel Memory, /3GB, PTEs, (Non-) Paged Pool
x64? My Terminal Servers Run Just Fine With 32 Bits and 8/12/16 GB RAM!
E.g. If a user decided (for whatever
reason) to run several x86 apps at
once, filling the RAM as much as
possible, would the extra addressable
memory available in Windows 7 x64 be
used?
The answer is yes. That's one of the benefits a virtual address space gives us--the ability for each process to appear (to the process) as though it's executing in a linear address space that starts at 0 and goes up from there.
As far as each of the 32-bit applications is concerned, it has its own address space from 0 to 2 gigabytes (without special tricks). The operating system handles the virtual-to-physical address translation.
On Windows, under normal circumstances a 32 bit process can only access 2GB of RAM (or 3GB with a special switch in the boot.ini file). When running a 32 bit process on a 64 bit operating system, how much memory is available? Are there any special switches or settings that can change this?
2 GB by default. If the application is large address space aware (linked with /LARGEADDRESSAWARE), it gets 4 GB (not 3 GB, see http://msdn.microsoft.com/en-us/library/aa366778.aspx)
They're still limited to 2 GB since many application depends on the top bit of pointers to be zero.
4 GB minus what is in use by the system if you link with /LARGEADDRESSAWARE.
Of course, you should be even more careful with pointer arithmetic if you set that flag.
Nobody seems to touch upon the fact that if you have many different 32-bit applications, the wow64 subsystem can map them anywhere in memory above 4G, so on a 64-bit windows with sufficient memory, you can run many more 32-bit applications than on a native 32-bit system.
A 32-bit process is still limited to the same constraints in a 64-bit OS. The issue is that memory pointers are only 32-bits wide, so the program can't assign/resolve any memory address larger than 32 bits.
An single 32-bit process under a 64-bit OS is limited to 2Gb. But if it is compiled to an EXE file with IMAGE_FILE_LARGE_ADDRESS_AWARE bit set, it then has a limit of 4 GB, not 2Gb - see https://msdn.microsoft.com/en-us/library/aa366778(VS.85).aspx
The things you hear about special boot flags, 3 GB, /3GB switches, or /userva are all about 32-bit operating systems and do not apply on 64-bit Windows.
See https://msdn.microsoft.com/en-us/library/aa366778(v=vs.85).aspx for more details.
As about the 32-bit operating systems, contrary to the belief, there is no physical limit of 4GB for 32-bit operating systems. For example, 32-bit Server Operating Systems like Microsoft Windows Server 2008 32-bit can access up to 64 GB (Windows Server 2008 Enterprise and Datacenter editions) – by means of Physical Address Extension (PAE), which was first introduced by Intel in the Pentium Pro, and later by AMD in the Athlon processor - it defines a page table hierarchy of three levels, with table entries of 64 bits each instead of 32, allowing these CPUs to directly access a physical address space larger than 4 gigabytes – so theoretically, a 32-bit OS can access 2^64 bytes theoretically, or 17,179,869,184 gigabytes, but the segment is limited by 4GB. However, due to marketing reasons, Microsoft have limited maximum accessible memory on non-server operating systems to just 4GB, or, even, 3GB effectively. Thus, a single process can access more than 4GB on a 32-bit OS - and Microsoft SQL server is an example.
32-bit processes under 64-bit Windows do not have any disadvantage comparing to 64-bit processes in using shared kernel's virtual address space (also called system space). All processes, be it 64-bit or 32-bit, under 64-bit Windows share the same 64-bit system space.
Given the fact that the system space is shared across all processes, on 32-bit Windows, processes that create large amount of handles (like threads, semaphores, files, etc.) consume system space by kernel objects and can run out of memory even if you have lot of memory available in total. In contrast, on 64-bit Windows, the kernel space is 64-bit and is not limited by 4 GB. All system calls made by 32-bit applications are converted to native 64-bit calls in the user mode.
You've got the same basic restriction when running a 32bit process under Win64. Your app runs in a 32 but subsystem which does its best to look like Win32, and this will include the memory restrictions for your process (lower 2GB for you, upper 2GB for the OS)
The limit is not 2g or 3gb its 4gb for 32bit.
The reason people think its 3gb is that the OS shows 3gb free when they really have 4gb of system ram.
Its total RAM of 4gb. So if you have a 1 gb video card that counts as part of the total ram viewed by the 32bit OS.
4Gig not 3 not 2 got it?