Please help me understand this :
If my machine has a Ram (Physical memory) of 2 GB , Will I still have a Virtual address space of 4 GB in a 32 bit machine ?
I have read somewhere that a 32 bit application has a max limit of 2 GB virtual address space.
If I have a total of 4 GB VAS , 1 GB is used by OS . So 3 GB is remaining for the applications to use . If I have 2 applications running both using 2 GB of memory ,I want to know if they will work since we are left with only 3GB .Is this where paging comes to picture?
On a 32-bit installation, by default, only 2 GB is made available to processes for their own use. The other 2GB are used by the operating system. On later 32-bit editions of Microsoft Windows it is possible to extend the user-mode virtual address space to 3 GiB while only 1 GiB is left for kernel-mode virtual address space by marking the programs as IMAGE_FILE_LARGE_ADDRESS_AWARE and enabling the /3GB switch in the boot.ini file.
If you wanted to run two applications each with 2GB of memory, you will run into a memory exception when either application tries to use the memory that has already been absorbed but the OS.
I got the clarification here:
http://social.msdn.microsoft.com/Forums/windowsdesktop/en-US/8d63585e-772d-42e5-a40a-72a9500ac0a7/understanding-on-32-bit-environment-application?forum=windowsgeneraldevelopmentissues#8d63585e-772d-42e5-a40a-72a9500ac0a7
Related
I thought that virtual address space was a section of RAM allocated to a specific process. But the book I'm reading says that 4 gbs is the standard limit of virtual address space. Isn't that the entire amount of RAM? If that is the case then I'm confused at what virtual address space is. Can anyone enlighten me?
That's the whole point of virtual addresses: The OS handles the physical memory, the process handles its own, virtual memory which is mapped to any memory the OS has available, not necessarily RAM.
On a 32 bit operating system the virtual address space (VAS) is, as you say, usually 4 GiB. 32 bits give you (2^32) addresses (0 ... (2^32)-1), each addressing one byte.
You could have more or less physical RAM and still have a 4-GiB-VAS for each and every process running. If you have less physical RAM, the OS would usually swap to harddrives.
The process doesn't need to know any of this, it can use the full VAS it is given by the OS and it's the OS' job to supply the physical memory.
(This is actually just a dumbed-down version of the Wikipedia article on VAS.)
I thought that a 32-bit process could use at most 2GB RAM, since half the address space is reserved for the OS (maybe 3GB with the /3GB switch). However, StarCraft II manages to use nearly 4GB and it's a 32-bit EXE. Process Explorer shows:
4,000,896 KB private bytes
3,928,164 KB working set
How does it do that and how can I do it in my own programs? Is it possible in .NET? In unmanaged C++? In managed C++ somehow?
On 64bit OS 32bit process can use almost 4GB.
Processes marked as "large address space aware" can use up to 3GB in 32bit OS and up to full address space on 64bit OS.
Here is more discussion on it: Can a 32bit process access more memory on a 64bit windows OS? .
I plan to run 32-bit Windows XP on a workstation with dual processors, based on Intel's Nehalem microarchitecture, and triple channel RAM. Even though XP is limited to 4 GB of RAM, my understanding is that it will function with more than 4 GB installed, but will only expose 4 GB (or slightly less).
My question is: Assuming that 6 GB of RAM is installed in six 1 GB modules, which physical 4 GB will Windows actually map into its address space?
In particular:
Will it use all six 1 GB modules, taking advantage of all memory channels? (My guess is yes, and that the mapping to individual modules within a group happens in hardware.)
Will it map 2 GB of address space to each of the two NUMA nodes (as each processor has it's own memory interface), or will one processor get fast access to 3 GB of RAM, while the other only has 1 GB?
Thanks!
This question was answered over at SuperUser. Because there are no other responses here, I'm responding to my own question so that the relevant information can easily be found.
Since the question was asked, I have also come across this blog post by Mark Russinovich, explaining how the Windows XP kernel handles memory.
In conclusion, it appears that what happens is that the kernel, even though it is PAE aware, truncates all physical memory addresses to 32-bit, meaning only the lowest physical 4 GB of RAM will be used. This in turn is mapped by hardware to memory modules, and corresponds to the entirety of the first module triplet (3 GB in total), and a third of the second triplet (spread across all three of its modules -- 1 GB in total).
Thus, all memory channels will be exploited, but the amount of memory will not be balanced between NUMA nodes.
Under Windows Server 2003, Enterprise Edition, SP2 (/3GB switch not enabled)
As I understand it, and I may be wrong, the maximum addressable memory for a process is 4GB.
Is that 2GB of private bytes and 2GB of virtual bytes?
Do you get "out of memory" errors when the private byte limit or virtual byte limit is reached?
It is correct that the maximum address space of a process is 4GB, in a sense. Half of the address space is, for each process, taken up by the operating system. This can be changed with the 3GB switch but it might cause system instability. So, we are left with 2GB of addressable memory for the process to use on its own. Well, not entirely. It turns out that a part of this space is taken up by other stuff such as DLLs an other common code. The actual memory available to you as a programmer is around 1.5GB - 1.7GB.
I'm not sure about how you can handle accidentally going above this limit but I know of games which crash in large multiplayer maps for this reason. Another thing to note is that a 32bit program cannot use more than the 2GB address space on a 64bit system unless they enable the /LARGEADDRESSAWARE:YES linker flag.
Mark Russinovich started a series of posts on this..
Pushing the Limits of Windows: Physical Memory
While 4GB is the licensed limit for 32-bit client SKUs, the effective limit is actually lower and dependent on the system's chipset and connected devices. The reason is that the physical address map includes not only RAM, but device memory as well, and x86 and x64 systems map all device memory below the 4GB address boundary to remain compatible with 32-bit operating systems that don't know how to handle addresses larger than 4GB. If a system has 4GB RAM and devices, like video, audio and network adapters, that implement windows into their device memory that sum to 500MB, 500MB of the 4GB of RAM will reside above the 4GB address boundary.
You can only access 2Gb of memory in total (without the 3Gb switch) on 32bit Windows platforms.
You could run multiple 32bit VMs on a 64bit OS so that each app has access to as much memory as possible if your machine has more than 4Gb.
A lot of people are just starting to hit these barriers, I guess it's easier if your app is in .net or Java as the VMs happily go up to 32Gb of memory on 64bit os.
On 32 bits, if there is enough physical memory and disk space for virtual memory, memory runs out around 3GB since the kernel reserves the address space above 0xC0000000 for itself. On a 64 bits kernel running a 64 bits application, the limit is at 8TB.
For more details, check out MSDN - Memory Limits for Windows Releases
Maximum addressable memory for a 32bit machine is 4GB, for a 64bit machine you can address loads more. (Although some 32bit machines have extension systems for accessing more, but I don't think this is worth bothering with or considering for use).
You get out of memory errors when the virtual limit is reached. On Windows Server 2003, task manager tells you the limit on the performance tab labelled 'Commit Charge Limit'.
Jeff covered this a while back on his blog in terms of 32 bit Vista.
Does the same 32 bit 4 GB memory cap that applies in 32 bit Vista apply to 32 bit Ubuntu? Are there any 32 bit operating systems that have creatively solved this problem?
Ubuntu server has PAE enabled in the kernel, the desktop version does not have this feature enabled by default.
This explains, by the way, why Ubuntu server does not work in some hardware emulators whereas the desktop edition does
Well, with windows, there's something called PAE, which means you can access up to 64 GB of memory on a windows machine. The downside is that most apps don't support actually using more than 4 GB of RAM. Only a small number of apps, like SQL Server are programmed to actually take advantage of all the extra memory.
Yes, 32 bit ubuntu has the same memory limitations.
There are exceptions to the 4GB limitation, but they are application specific... As in, Microsoft Sql Server can use 16 gigabytes with "Physical address Extensions" [PAE] configured and supported and... ugh
http://forums.microsoft.com/TechNet/ShowPost.aspx?PostID=3703755&SiteID=17
Also drivers in ubuntu and windows both reduce the amount of memory available from the 4GB address space by mapping memory from that 4GB to devices. Graphics cards are particularly bad at this, your 256MB graphics card is using up at least 256MB of your address space...
If you can [your drivers support it, and cpu is new enough] install a 64 bit os. Your 32 bit applications and games will run fine.
There seems to be some confusion around PAE. PAE is "Page Address Extension", and is by no means a Windows feature. It is a hack Intel put in their Pentium II (and newer) chips to allow machines to access 64GB of memory. On Windows, applications need to support PAE explicitely, but in the open source world, packages can be compiled and optimized to your liking. The packages that could use more than 4GB of memory on Ubuntu (and other Linux distro's) are compiled with PAE support. This includes all server-specific software.
In theory, all 32-bit OSes have that problem. You have 32 bits to do addressing.
2^32 bits / 2^10 (bits per kb) / 2^10 (kb per mb) / 2^10 (mb per gb) = 2^2 = 4gb.
Although there are some ways around it. (Look up the jump from 16-bit computing to 32-bit computing. They hit the same problem.)
Linux supports a technology called PAE that lets you use more than 4GB of memory, however I don't know whether Ubuntu has it on by default. You may need to compile a new kernel.
Edit: Some threads on the Ubuntu forums suggest that the server kernel has PAE on by default, you could try installing that.