Does prefetching a write ever affect single core performance?

Question

Some architectures have a "prefetch write" instruction to indicate to the CPU that you're going to be writing to a memory location before you actually do it. I understand that on a multicore machine this can be used by the core as a hint that it should try to get ownership of the given cache line now so that it can write to the location more quickly later. However, AFAICT that should only matter in situations when there are two cores potentially contending for the cache line. For a cache line that's only being read and written by a single core, does a prefetch write ever have any use?

Answer 1

All else being equal, Prefetch-Write has no benefit over Prefetch-Read for lines accessed only by a single core. After any kind of prefetch, the core will own the line in the Exclusive state. On a subsequent write, the line changes to the Modified state. An Exclusive-to-Modified transition is free since by definition no other core has the line. The E->M state change completes locally without snooping.

Beware that cores have their own hardware prefetch logic. An access to a line may cause the core to grab adjacent line(s) automatically. If global variables or other data reside nearby, an SMP system may experience a lot of unexpected cross-snooping.