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?
Related
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 am developing a 32 bit application in .NET that for various reasons cannot be compiled as a 64 bit application.
I need to run many of these concurrently and they use a lot of memory. I want to load up a Windows 7 box with tonnes of memory and consequently would like to use the 64 bit version of Windows 7 so that we can put many gigabytes of RAM on those boxes.
My question is this: The maximum memory used by each instance of my app is ~500mb. In Windows 7 64bit, these 32-bit applications will run (I assume) using the WOW64 emulation layer in Windows. As I begin to run more and more of these instances concurrently, will they all be stuck running in the bottom 2gb of ram, or will Windows allocate memory for them using all of the higher-address range of memory possible within 64-bit Windows? Is the addressable-memory limitation of 32 bit software only a per-instance limitation in this case, or will all the instances be limited to the bottom 2gb of ram?
You are confusing memory (physical address space) with virtual address space. You can put more than 4GB of memory into a 32-bit system; you don't need to move to 64-bit to gain physical address space. Each process gets its own virtual address space, so each one will get its own 2GB of user-mode address space to play with. (Or 3GB if /3GB or 4GB if running on WOW64 with /LARGEADDRESSAWARE.)
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.
Imagine the same hardware running Windows XP 32bit, or Windows XP 64bit..
(being that it's a 64bit processor currently running XP 32bit)
2gigs of ram...
Will the performance of Visual Studio benefit from going to the 64bit OS?
The hardware and ram is currently out of my control... If I could throw more hardware or ram at it I would.
For 2 GB of RAM, most likely not. The biggest advantage of 64-bit is the additional address space. With 2GB of physical RAM, 32-bit OS's can map all of physical memory.
Above 3GB 64-bit OS does give some benefit, since some of your physical memory may be unusable on 32-bit OS's due to devices mapping physical addresses for their own use.
And obviously if you want to use above 4 GB of memory you should absolutely go for a 64-bit OS.
Yes, if you have sufficient physical memory, you will gain some benefits from running Visual Studio in 64-bit Windows as described in Visual Studio: Why is there no 64 bit version? (yet):
Doesn’t being a 64 bit application save you all kinds of page faults and
so forth?
A 64 bit address space for the process
isn’t going to help you with page
faults except in maybe indirect ways,
and it will definitely hurt you in
direct ways because your data is
bigger. In contrast a 64 bit
operating system could help you a lot!
If you’re running as a 32 bit app on a
64 bit OS then you get all of the 4G
address space and all of that could be
backed by physical memory (if you have
the RAM) even without you using 64 bit
pointers yourself. You’ll see
potentially huge improvements related
to the size of the disk cache (not in
your address space) and the fact that
your working set won’t need to be
eroded in favor of other processes as
much. Transient components and data
(like C++ compilers and their big .pch
files) stay cached in physical
memory, but not in your address space.
32 bit processes accrue all these benefits just as surely as 64 bit
ones. (my emphasis added)
not unless you add more RAM.
In fact 64 bits might even slow you down because 64-bits Windows sometimes have to load both 32-bits and 64-bits versions of a same DLL in RAM.
Add more RAM is the way to go because if you are using Windows 7 or Vista they aggressively caches disk files in RAM.
From what I understand, a 32-bit process can only access 2 GB of memory on 32-bit Windows without the /3GB switch, and that some of that memory is taken up by the OS for its own diabolical reasons. This seems to mesh with my experience as we have an app that crashes when it reaches around 1.2 - 1.5 GB of RAM without memory exceptions, even though there is still plenty of memory available.
Would moving this 32-bit application to 64-bit Windows allowing it accesses more than 1.5 GB it can now? Would the application itself have to be upgraded to 64-bit?
Newer versions of Visual Studio have a new flag which make 32-bit apps "big address space aware". Basically what it does is say that if it's loaded on a 64-bit version of windows, then it will get 4GB (the limit of 32-bit pointers). This is certainly better than the 2 or 3 GB you get on 32-bit versions of windows. See http://msdn.microsoft.com/en-us/library/aa366778.aspx:
Most notably it says:
Limits on memory and address space
vary by platform, operating system,
and by whether the
IMAGE_FILE_LARGE_ADDRESS_AWARE value
of the LOADED_IMAGE structure and
4-gigabyte tuning (4GT) are in use.
IMAGE_FILE_LARGE_ADDRESS_AWARE is set
or cleared by using the
/LARGEADDRESSAWARE linker option.
Also see: http://msdn.microsoft.com/en-us/library/wz223b1z.aspx
Yes, under the right circumstances, a 32-bit process on Windows can access a full 4GB of memory, rather than the 2Gb it's normally limited to.
For this to work, you need the following:
The app must be running on a 64-bit OS
The app must be compiled with the /LARGEADDRESSAWARE flag.
The app should be tested to make sure it actually works properly in this case. ;) (specifically, code that relies on all pointers pointing to addresses below the 2GB boundary will obviously not work here)
Your app will be limited by the pointer size, in your example 32 bits.
If your app was to access more memory then you would need some sort of segmented memory architecture like we had in the 16 bit days where apps used 16bit pointers and offsets to access the full 32bit memory space.
WOW64 allows using 32-bit Windows application on 64-bit Windows, translating 32-bit pointers to real 64-bit pointers. And actually 32-bit addressing should allow accessing 4GB of memory.