Some remarks on VirtualAlloc and MEM_LARGE_PAGES

If you try to run the sample program demonstrating how to create a file mapping using large pages, you'll probably run into the error ERROR_NOT_ALL_ASSIGNED (Not all privileges or groups referenced are assigned to the caller) when calling Adjust­Token­Privileges. What is going on?

The Adjust­Token­Privileges enables privileges that you already have (but which are masked). Sort of like how a super hero can't use super powers while disguised as a normal mild-mannered citizen. In order to enable the Se­Lock­Memory­Privilege privilege, you must already have it. But where do you get it?

You do this by using the group policy editor. The list of privileges says that the Se­Lock­Memory­Privilege corresponds to "Lock pages in memory".

Why does allocating very large pages require permission to lock pages in memory?

Because very large pages are not pageable. This is not an inherent limitation of large pages; the processor is happy to page them in or out, but you have to do it all or nothing. In practice, you don't want a single page-out or page-in operation to consume 4MB or 16MB of disk I/O; that's a thousand times more I/O than your average paging operation. And in practice, the programs which use these large pages are "You paid $40,000 for a monster server whose sole purpose is running my one application and nothing else" type applications, like SQL Server. Those applications don't want this memory to be pageable anyway, so adding code to allow them to be pageable is not only a bunch of work, but it's a bunch of work to add something nobody who uses the feature actually wants.

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