/3GB switch
I have come across many a times issues with Virtual memory, /3GB,/PAE etc. So is with you I guess. Here is a brief description of how /3GB and /PAE works and where to use them.
/3GB – This is a switch in boot.ini file which increases the User mode Virtual Address Space and decreases for Kernel mode. By default, /3GB switch is not enabled, system will allocate 2GB for User mode and 2GB for Kernel mode. It means, 2GB of Virtual Address space can be used by Applications in User mode and rest can be used by Kernel mode. In other words, 2 Billion memory addresses for kernel mode and 2 billion for Applications. MS W2k, W2k3 can directly address up to 4GB of Memory address space, regardless of how much physical RAM is installed. From the process perspective, each element of virtual address conceptually refers to a byte of physical memory.
Now we see what is Virtual Address Space. Is it the same as Virtual Memory? … No. Virtual Address space is different from Virtual Memory. Virtual address space is something to do with allocating address space for each process of user mode and kernel mode. Now you can understand the advantage and disadvantage of /3GB switch.
Let’s see how it works.
Adding /3GB switch increases the performance of programs/applications that are
capable of using the /3GB switch.
For example, Active Directory is capable of using /3GB switch and this allows AD to improve its caching.
W2k – /3Gb switch allows AD caching a maximum – 1024MB
W2k – without /3GB switch, AD caching is limited to 512MB
W2k3 – AD Cache is allowed to grow more freely but remains by virtual address space.
Note1: Do not enable /3GB switch unless the application is aware of /3GB. If you enable /3GB for application who is not aware of large address spaces, that will hurt the kernel address space and do not improve the performance.
Note2: If you use /3GB with /PAE will have conflicts and kernel is limited to 16GB of memory. This is caused as the kernel virtual memory address space limitations.
/PAE - Causes Ntldr to load Ntkrnlpa.exe, which is the version of the x86 kernel that is able to take advantage of x86 PAEs. The PAE version of the kernel presents 64-bit physical addresses to device drivers, so this switch is helpful for testing device driver support for large memory systems.
Operating system Maximum memory support with PAE
Windows 2000 Advanced Server – 8 GB of physical RAM
Windows 2000 Datacenter Server – 32 GB of physical RAM
Windows XP (all versions) – 4 GB of physical RAM
* Windows Server 2003 (and SP1), Standard Edition – 4 GB of physical RAM
* Windows Server 2003, Enterprise Edition – 32 GB of physical RAM
Windows Server 2003, Datacenter Edition – 64 GB of physical RAM
Windows Server 2003 SP1, Enterprise Edition – 64 GB of physical RAM
Windows Server 2003 SP1, Datacenter Edition – 128 GB of physical RAM
* Total physical address space is limited to 4 GB on these versions of Windows.
I will cover PTEs, paged and non-paged pool in my next article.
