what is a store buffer?

architecture hardware intel cpu-architecture
harish reddy picture harish reddy · Jun 19, 2012 · Viewed 17.8k times · Source

can anyone explain what is load buffer and how it's different from invalidation queues. and also difference between store buffers and write combining buffers? The paper by Paul E Mckenny http://www.rdrop.com/users/paulmck/scalability/paper/whymb.2010.07.23a.pdf explains very nicely about the store buffers and invalidation queues but unfortunately doesn't talk about write combining buffers

Answer

Nathan Binkert picture Nathan Binkert · Jun 21, 2012

An invalidate queue is more like a store buffer, but it's part of the memory system, not the CPU. Basically it is a queue that keeps track of invalidations and ensures that they complete properly so that a cache can take ownership of a cache line so it can then write that line. A load queue is a speculative structure that keeps track of in-flight loads in the out of order processor. For example, the following can occur

  1. CPU speculatively issue a load from X
  2. That load was in program order after a store to Y, but the address of Y is not resolved yet, so the store does not proceed.
  3. Y is resolved and it turns out to be equal to X. At the time that the store to Y is resolved, that store searches the load queue for speculative loads that have issued, but are present after the store to Y in program order. It will notice the load to X (which is equal to Y) and have to squash those instructions starting with load X and following.

A store buffer is a speculative structure that exists in the CPU, just like the load queue and is for allowing the CPU to speculate on stores. A write combining buffer is part of the memory system and essentially takes a bunch of small writes (think 8 byte writes) and packs them into a single larger transaction (a 64-byte cache line) before sending them to the memory system. These writes are not speculative and are part of the coherence protocol. The goal is to save bus bandwidth. Typically, a write combining buffer is used for uncached writes to I/O devices (often for graphics cards). It's typical in I/O devices to do a bunch of programming of device registers by doing 8 byte writes and the write combining buffer allows those writes to be combined into larger transactions when shipping them out past the cache.

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