Win32 Programming Lesson 16: Virtual Memory

Where are we?  We’ve covered the theory of Windows memory, and poked around some  Now let’s use how to use memory

Three ways to Use Memory  Virtual Memory – Best for very large arrays and structures  Memory-mapped files – Good for managing data streams and sharing memory between applications  Heaps – Best for managing large numbers of small objects

Reserving Memory  Fairly straightforward: PVOID VirtualAlloc( PVOID pvAddress, SIZE_T dwSize, DWORD fdwAllocationType, DWORD fdwProtect );

Parameters  pvAddress: Usually NULL, but can be where you would like the memory  dwSize: How much memory you would like  dwAllocationType: Should we RESERVE memory or COMMIT it?  dwProtection: The type of memory protection on this area of memory

Before Use…  Once you have reserved memory, you need to commit it before the pages can be accessed  Same call, but you use the parameters slightly differently  You don’t have to COMMIT all the region – you just need to COMMIT pages  Why is this a Good Thing?

Can do it all in one go…  Can bitwise-OR the flags…  Just like this: PVOID pvMem = VirtualAlloc( NULL, 99 * 1024, MEM_RESERVE | MEM_COMMIT, PAGE_READWRITE ); Can also specify MEM_TOP_DOWN

Knowing When to Commit  Let’s say you’re writing a spreadsheet application (gee, there’s a good idea…)  Why doesn’t Excel crash the system when it loads a spreadsheet (well, usually…)

MEM_LARGE_PAGE  New addition to VirtualAlloc Higher performance, as we can allocate a large page of memory These pages are not pageable however SIZE_T GetLargePageMinimum();

Determining the State of an Address  Four ways… Always call COMMIT – but this is slow because it’s often (usually?) unnecessary Use VirtualQuery to map the state and commit if necessary Keep a record of which pages you’ve committed and which you have – can be complicated Use SEH to catch memory exceptions and COMMIT on error (best way)

Must Decommit  If you don’t want to kill the machine, it’s good to decommit memory BOOL VirtualFree( LPVOID pvAddress, SIZE_T dwSize, DWORD fdwFreeType ); Pass 0 for dwSize as the system already knows it Must pass MEM_RELEASE

Knowing when is the trick…  Consider our Spreadsheet example Make each cell a page (not very practical, but very simple) Keep track of which cells are on which page Implement a garbage collection function which checks the state of cells

Bug Hunting…  Sometimes protecting memory can help tremendously when hunting bugs and coding defensively  Can use VirtualProtect to change access when we’re not using memory  If a rogue pointer blats into your memory, you’re protected

Physical Storage  Knowledge of the underlying system can help tremendously when trying to improve performance  For example, imagine you have some memory which you use for short periods of time… How does the design of the system potentially slow things down?

RESETting Memory  When the system looks to free physical memory it has to write RAM to the paging file  But what if you say that the memory to swap isn’t important? That is, the changes don’t need to be kept?  If you have write-access but the changes aren’t needed you can RESET the memory

Example  PINT pnData = (PINT) VirtualAlloc( NULL, 1024, MEM_RESERVE | MEM_COMMIT, PAGE_READWRITE); pn[0] = 100; pn[1] = 200; VirtualAlloc( (PVOID) pnData, sizeof(int), MEM_RESET, PAGE_READWRITE);

MEM_RESET  One thing to remember is that the MEM_RESET flag doesn’t work if you OR it with anything else  It only makes sense used on its own

Cool Stuff: AWE  Address Windowing Extensions  Allows the program to access more memory than fits in its process space  Introduced in Windows 2000  Uses AllocateUserPhysicalPages  Then MAP to a Window: BOOL MapUserPhysicalPages( PVOID pvAddressWindow, ULONG_PTR ulRAMPages, PULONG_PTR aRAMPages);

Stack Space  Each thread stack exists in the process’ address space  Very clever – assigns a “guard page” so that the system knows when to commit more memory to the stack  When the stack grows too far, generates a EXCEPTION_STACK_OVERFLOW  Can handle via a SEH – we’ll talk about that later in the term