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From: Jeremiah Cox on 5 May 2008 15:10
Howdy IngoWolf,You may want to explore the /3GB switch, see <a href="http://zone.ni.com/reference/en-XX/help/371361D-01/lvhowto/enable_lrg_ad_aware/" target="_blank">Extending Virtual Memory Usage for 32-bit Windows</a>.&nbsp; This takes virtual address space away from the kernel and allocates it to user mode applications. Note that fragmentation of virtual memory is still an issue on x86.&nbsp; If you have a buffer that is 128MB, the memory manager has to find a contiguous 128MB hole in the virtual address (VA) space.&nbsp; The memory manager is more likely to succeed if you try to allocate 2x 64MB buffers because it only has to find 2 64MB contiguous holes in the VA.&nbsp; It is easier to simulate this problem if we talk about buffers of say 1 GB.&nbsp; By default, Windows allocates 2GB of virtual address space for applications.&nbsp; However, Microsoft system files reserve 0x70000000-7FFFFFFF (256MB) and MS recommends that 3rd party DLLs reside at virtual address 0x60000000-6FFFFFFF.&nbsp; So, you're left with around 1.5GB of contiguous virtual space for your application (code) and buffers.&nbsp; Any code or data that loads into the middle of this address space would reduce the maximum buffer size that you could allocate.&nbsp; If a tiny DLL loads into the middle of VA, you still have roughly 1.5GB available, but the largest allocation chunk would be about 750 MB, due to the fragmentation.For example:Virtual Address Space0&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; 1.5&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; 1.75 &nbsp;&nbsp; 2.0 GB&nbsp; App&nbsp; &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; &nbsp; &nbsp;&nbsp; DLLs&nbsp; System&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; { ~1.5 GB Avail }Traditional NI-DAQ loads around address 0x21400000 for "legacy" reasons, lets take a look at what happens:Virtual Address Space0&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; 1.5&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; 1.75 &nbsp;&nbsp; 2.0 GB&nbsp; App&nbsp; &nbsp;&nbsp;&nbsp;&nbsp; TDAQ&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; &nbsp; &nbsp;&nbsp; DLLs &nbsp; System&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; { A }&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; { ~1.0 A}So, if someone wanted to use large buffers (&gt;1GB) with Traditional NI-DAQ, they would have a challenge.&nbsp; Generally they would move it somewhere else in the virtual address space (hand defragmentation) using the palbase utility.The 3GB switch makes this:Virtual Address Space0&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; 1.5&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; 1.75 &nbsp;&nbsp; 2&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; 3GB&nbsp; App&nbsp; &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; &nbsp; &nbsp;&nbsp; DLLs&nbsp; System&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; { ~1.5 GB Avail }&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; {~1GB Avail }So, with 3GB, you could not allocate a large 2 GB buffer, but you could allocate 2.5 GB of buffers . I hope that helps.JeremiahNI
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