Yesterday I decided to diff the export tables of some core Win32 DLLs to see what’s changed between Windows 8.1 and the Windows 10 technical preview. There weren’t many changes, but the ones that were there are quite exciting IMHO. While researching these new APIs, I also stumbled across some others that were added during the Windows 8 timeframe that we should be considering as well.

Volatile Ranges

While my diff showed these APIs as new exports for Windows 10, the MSDN docs claim that these APIs are actually new for the Windows 8.1 Update. Using the OfferVirtualMemory and ReclaimVirtualMemory functions, we can now specify ranges of virtual memory that are safe to discarded under memory pressure. Later on, should we request that access be restored to that memory, the kernel will either return that virtual memory to us unmodified, or advise us that the associated pages have been discarded.

A couple of years ago we had an intern on the Perf Team who was working on bringing this capability to Linux. I am pleasantly surprised that this is now offered on Windows.

madvise(MADV_WILLNEED) for Win32

For the longest time we have been hacking around the absence of a madvise-like API on Win32. On Linux we will do a madvise(MADV_WILLNEED) on memory-mapped files when we want the kernel to read ahead. On Win32, we were opening the backing file and then doing a series of sequential reads through the file to force the kernel to cache the file data. As of Windows 8, we can now call PrefetchVirtualMemory for a similar effect.

Operation Recorder: An API for SuperFetch

The OperationStart and OperationEnd APIs are intended to record access patterns during a file I/O operation. SuperFetch will then create prefetch files for the operation, enabling prefetch capabilities above and beyond the use case of initial process startup.

Memory Pressure Notifications

This API is not actually new, but I couldn’t find any invocations of it in the Mozilla codebase. CreateMemoryResourceNotification allocates a kernel handle that becomes signalled when physical memory is running low. Gecko already has facilities for handling memory pressure events on other platforms, so we should probably add this to the Win32 port.