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RYZEN thoughts?
Author: Peter Date: 2017-04-11 22:29
The most plausible sounding explanation I've read is that Ryzen lives and dies by its large L2 caches.

Neither its IPC nor its top clock speeds can quite beat Intel, but it can keep substantially larger program and data working set fragments in its 512 kB L2s than Intel chips can in their 256 kB ones and can avoid memory stalls for some programs more easily.

By the same token, thread rescheduling can kill performance, especially if one is migrated to the opposite CCX, since the large L2 cache takes inherently longer to re-warm, and Ryzen's internal datapaths are only 32B wide (compared to 64B for post-Haswell) even within a CCX, and the Infinity Fabric crossbar is only clocked at half-nominal DDR4 interface rates, for something in the neighborhood of only 40-50 GB/s bandwidth in each direction. Re-warming an L2 cache for a thread moved to an adjacent core within a CCX could take 4x as many clocks as for an Intel chip and perhaps 10x-12x longer for migrations to a remote CCX.

The programs that do better in Ryzen will probably be the ones that can both keep thread migration under control and that can keep their working sets under control, not using so much that 512 kB L2s and 8 MB L3 cluster still can't keep up.

 
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