Windows® Internals, Sixth Edition, Part 2

Running TestLimit a few more times would easily exhaust physical memory on a 32-bit system, and this limitation results in one of the primary limits on systemwide 32-bit thread count.

DPC Stack

Finally, Windows keeps a per-processor DPC stack available for use by the system whenever DPCs are executing, an approach that isolates the DPC code from the current thread’s kernel stack (which is unrelated to the DPC’s actual operation because DPCs run in arbitrary thread context). The DPC stack is also configured as the initial stack for handling the SYSENTER or SYSCALL instruction during a system call. The CPU is responsible for switching the stack when SYSENTER or SYSCALL is executed, based on one of the model-specific registers (MSRs), but Windows does not want to reprogram the MSR for every context switch, because that is an expensive operation. Windows therefore configures the per-processor DPC stack pointer in the MSR.

Virtual Address Descriptors

The memory manager uses a demand-paging algorithm to know when to load pages into memory, waiting until a thread references an address and incurs a page fault before retrieving the page from disk. Like copy-on-write, demand paging is a form of lazy evaluation—waiting to perform a task until it is required.

The memory manager uses lazy evaluation not only to bring pages into memory but also to construct the page tables required to describe new pages. For example, when a thread commits a large region of virtual memory with VirtualAlloc or VirtualAllocExNuma, the memory manager could immediately construct the page tables required to access the entire range of allocated memory. But what if some of that range is never accessed? Creating page tables for the entire range would be a wasted effort. Instead, the memory manager waits to create a page table until a thread incurs a page fault, and then it creates a page table for that page. This method significantly improves performance for processes that reserve and/or commit a lot of memory but access it sparsely.

The virtual address space that would be occupied by such as-yet-nonexistent page tables is charged to the process page file quota and to the system commit charge. This ensures that space will be available for them should they be actually created. With the lazy-evaluation algorithm, allocating even large blocks of memory is a fast operation. When a thread allocates memory, the memory manager must respond with a range of addresses for the thread to use. To do this, the memory manager maintains another set of data structures to keep track of which virtual addresses have been reserved in the process’s address space and which have not. These data structures are known as virtual address descriptors (VADs). VADs are allocated in nonpaged pool.

Process VADs

For each process, the memory manager maintains a set of VADs that describes the status of the process’s address space. VADs are organized into a self-balancing AVL tree (named after its inventors, Adelson-Velskii and Landis) that optimally balances the tree. This results in, on average, the fewest number of comparisons when searching for a VAD corresponding with a virtual address. There is one virtual address descriptor for each virtually contiguous range of not-free virtual addresses that all have the same characteristics (reserved versus committed versus mapped, memory access protection, and so on). A diagram of a VAD tree is shown in Figure 10-32.

Figure 10-32. Virtual address descriptors

When a process reserves address space or maps a view of a section, the memory manager creates a VAD to store any information supplied by the allocation request, such as the range of addresses being reserved, whether the range will be shared or private, whether a child process can inherit the contents of the range, and the page protection applied to pages in the range.

When a thread first accesses an address, the memory manager must create a PTE for the page containing the address. To do so, it finds the VAD whose address range contains the accessed address and uses the information it finds to fill in the PTE. If the address falls outside the range covered by the VAD or in a range of addresses that are reserved but not committed, the memory manager knows that the thread didn’t allocate the memory before attempting to use it and therefore generates an access violation.

EXPERIMENT: Viewing Virtual Address Descriptors