Re:Rest in Peace, Optane
by pbalcer on Tuesday December 05, 2023 @12:21PM
(#38533859)
The point is that on PMem that is simply "sfence", and not a potentially super-expensive "fsync" syscall... Fsync is an fsync, not a memory barrier...
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They both involve flushing cache to backing stores, and waiting for confirmation of the write. It’s literally the same thing. It’s just writing a cache line to RAM is orders of magnitude faster than writing a disk sector to storage, even with NVME SSDs. Octane is/was somewhere in the middle.
Re:Rest in Peace, Optane
by Dylan16807 on Tuesday December 05, 2023 @03:14PM
(#38536356)
> They both involve flushing cache to backing stores, and waiting for confirmation of the write.
No they don't. A fence only imposes ordering. It's instant. It can increase the chance of a stall when it forbids certain optimizations, but it won't cause a stall by itself.
CLWB is a small flush, but as tanelpoder explained the more recent CPUs did not need CLWB.