Writecombined

Writecombined, or write-combining, is a memory-caching attribute used to optimize sequential writes to memory-mapped devices. It enables the CPU to accumulate small writes in a per-CPU write-combining buffer and later flush them as larger, merged transactions to memory. This can significantly increase effective write bandwidth for streaming data, such as graphics framebuffers or device queues, by reducing the per-write overhead on the memory bus.

How it works in practice: writes to a write-combining region are not stored in ordinary caches with full CPU-visible coherence. Instead, the writes are buffered and later flushed in larger blocks, typically when the buffer fills, when a memory barrier enforces ordering, or when the device fetches the data. Because WC writes are optimized for throughput rather than immediate data visibility, reads from WC regions are not reliable for retrieving recent writes, and the exact flush behavior can be platform-dependent. Therefore, WC is generally used for streaming or write-intensive operations where the data is consumed by hardware devices rather than by software immediately afterward.

Use cases and cautions: WC memory is commonly used for graphics framebuffers, video pipelines, and other devices that benefit from high-bandwidth, sequential writes. It is not suitable for irregular or read-after-write access patterns, and proper synchronization is required to ensure that data reaches the device in a timely and ordered fashion. Applications typically coordinate WC use with memory barriers or device-specific flush operations.

Implementation and availability: support for write-combining is provided at the OS and architecture level. In Windows, memory can be allocated with a write-combine caching mode (PAGE_WRITECOMBINE). In Linux and other systems, WC can be requested for specific memory regions via platform-specific APIs or page-table attributes. The exact semantics, including flushing behavior and ordering guarantees, vary by platform, so developers should consult platform documentation when using writecombined memory.