On Fri, 2017-03-10 at 08:33 -0800, Tom Herbert wrote: > Okay, now I'm confused. You're saying that when early demux was added > for IPv6 performance improved, but this patch is allowing early demux > to be disabled on the basis that it hurts performance for unconnected > UDP workloads. While it's true that early demux in the case results in > another UDP lookup, Eric's changes to make it lockless have made that > lookup very cheap. So we really need numbers to justify this patch. >
Fact that the lookup is lockless does not avoid a cache line miss. Early demux computes a hash based on the 4-tuple, and lookups a hash table with does not fit in cpu caches. A cache line miss per packet is expensive, when handling millions of UDP packets per second, (with millions of 4-tuples) > Even if the numbers were to show a benefit, we still have the problem > that this creates a bimodal performance characteristic, e.g. what if > the work load were 1/2 connected and 1/2 unconnected in real life, or > what it the user incorrectly guesses the actual workload. Maybe a > deeper solution to investigate is making early demux work with > unconnected sockets. Sure, but forcing all UDP applications to perform IP early demux is not better.
