Reserves a contiguous slice of virtual memory committing just a portion of it. More...

#include <VirtualMemory.h>

Public Member Functions
bool	reserve (size_t maxCapacityInBytes)
	Reserves a large block of virtual memory of size maxCapacityInBytes. More...

bool	release ()
	Reclaims the entire virtual memory block (reserved with VirtualMemory::reserve) More...

bool	commit (size_t newCapacityBytes)
	Ensures at least newCapacityBytes to be committed / accessible from the large maxCapacityInBytes block. More...

bool	shrink (size_t newCapacityBytes)
	Reclaims all unused pages past newCapacityBytes (previously committed with VirtualMemory::commit) More...

Static Public Member Functions
static size_t	roundUpToPageSize (size_t size)
	Round up the passed in size to system memory page size. More...

static size_t	getPageSize ()
	Obtains system memory page size. More...

Public Attributes
size_t	reservedBytes = 0
	Maximum amount of reserved memory that can be committed. More...

size_t	committedBytes = 0
	Current amount of committed memory. More...

void *	memory = nullptr
	Pointer to start of reserved memory. More...

Detailed Description

Reserves a contiguous slice of virtual memory committing just a portion of it.

This class is useful on 64-bit systems where the address space is so large that it's feasible reserving large chunks of memory to commit and de-commit (shrink) as needed.
Reservation ensures that the returned address will not change and will be sized in multiples of system page size.

Note: Memory must be committed in order to be read or written, occupying physical memory pages.

Warning: This class has no defined destructor so memory MUST be released calling VirtualMemory::release

    // This test uses two pages initially and just one page later
    // On Windows and Linux default Page size is typically 4Kb
    // On macOS default page size is typically 16 Kb
    const size_t moreThanOnePageSize = VirtualMemory::getPageSize() + 1024;
    const size_t lessThanOnePageSize = VirtualMemory::getPageSize() - 1024;
    SC_TEST_EXPECT(lessThanOnePageSize > 0); // sanity check just in case
 
    void* reference = Memory::allocate(moreThanOnePageSize, 1);
    memset(reference, 1, moreThanOnePageSize);
    auto releaseLater = MakeDeferred([&] { Memory::release(reference); });
 
    VirtualMemory virtualMemory;
 
    // Reserve 2 pages of virtual memory
    SC_TEST_EXPECT(virtualMemory.reserve(2 * VirtualMemory::getPageSize()));
 
    // Request to use less than one page of virtual memory
    SC_TEST_EXPECT(virtualMemory.commit(lessThanOnePageSize));
    char* memory = static_cast<char*>(virtualMemory.memory);
 
    // Check that memory is writable and fill it with 1
    memset(memory, 1, lessThanOnePageSize);
 
    // Let's now extend this block from one to two pages
    SC_TEST_EXPECT(virtualMemory.commit(moreThanOnePageSize));
 
    // Fill the "newly committed" pages with 1
    memset(memory + lessThanOnePageSize, 1, moreThanOnePageSize - lessThanOnePageSize);
 
    // Make sure that previously reserved address is stable
    SC_TEST_EXPECT(memory == virtualMemory.memory);
 
    // Check that all allocated bytes are addressable and contain expected pattern
    SC_TEST_EXPECT(memcmp(memory, reference, moreThanOnePageSize) == 0);
 
    // Now let's de-commit everything but the first page
    SC_TEST_EXPECT(virtualMemory.shrink(lessThanOnePageSize));
 
    // Address should stay stable
    SC_TEST_EXPECT(memory == virtualMemory.memory);
    SC_TEST_EXPECT(memcmp(memory, reference, lessThanOnePageSize) == 0);
 
    // Decommit everything (not really needed if we're going to release() soon)
    SC_TEST_EXPECT(virtualMemory.shrink(0));
    SC_TEST_EXPECT(memory == virtualMemory.memory);
 
    // Finally release (don't forget, VirtualMemory has no destructor!)
    SC_TEST_EXPECT(virtualMemory.release());
    SC_TEST_EXPECT(virtualMemory.memory == nullptr);

Member Function Documentation

◆ commit()

bool SC::VirtualMemory::commit ( size_t newCapacityBytes )

Ensures at least newCapacityBytes to be committed / accessible from the large maxCapacityInBytes block.

Parameters

newCapacityBytes Indicates how much of the reserved virtual address space that must be accessible

Note: newCapacityBytes must smaller than reservedBytes (previously reserved by VirtualMemory::reserve)

◆ getPageSize()

static size_t SC::VirtualMemory::getPageSize ( )

static

Obtains system memory page size.

◆ release()

bool SC::VirtualMemory::release ( )

Reclaims the entire virtual memory block (reserved with VirtualMemory::reserve)

◆ reserve()

bool SC::VirtualMemory::reserve ( size_t maxCapacityInBytes )

Reserves a large block of virtual memory of size maxCapacityInBytes.

Parameters

maxCapacityInBytes Size of the reserved amount of virtual address space

Note: The actual memory reserved will be the rounded upper multiple of VirtualMemory::getPageSize

◆ roundUpToPageSize()

static size_t SC::VirtualMemory::roundUpToPageSize ( size_t size )

static

Round up the passed in size to system memory page size.

◆ shrink()

bool SC::VirtualMemory::shrink ( size_t newCapacityBytes )

Reclaims all unused pages past newCapacityBytes (previously committed with VirtualMemory::commit)

Member Data Documentation

◆ committedBytes

size_t SC::VirtualMemory::committedBytes = 0

Current amount of committed memory.

◆ memory

void* SC::VirtualMemory::memory = nullptr

Pointer to start of reserved memory.

◆ reservedBytes

size_t SC::VirtualMemory::reservedBytes = 0

Maximum amount of reserved memory that can be committed.

The documentation for this struct was generated from the following file:

VirtualMemory.h

Public Member Functions

Static Public Member Functions

Public Attributes