We can see 10~11% performance improvement as observed
previously. You might see 0.02% performance difference with
patch as regression, but that is just a run-to-run variation.
Don't the single-client numbers show about a 3% regresssion? Surely not 0.02%.
Sorry, you are right, it is ~2.66%, but in read-write pgbench tests, I
could see such fluctuation. Also patch doesn't change single-client
case. However, if you still feel that there could be impact by patch,
I can re-run the single client case once again with different combinations
like first with HEAD and then patch and vice versa.
Are these results from a single run, or median-of-three?
I mean, my basic feeling is that I would not accept a 2-3% regression in the single client case to get a 10% speedup in the case where we have 128 clients. A lot of people will not have 128 clients; quite a few will have a single session, or just a few. Sometimes just making the code more complex can hurt performance in subtle ways, e.g. by making it fit into the L1 instruction cache less well. If the numbers you have here are accurate, I'd vote to reject the patch.
Note that we already have apparently regressed single-client performance noticeably between 9.0 and 9.5:
I bet that wasn't a single patch but a series of patches which made things more complex to improve concurrency behavior, but in the process each one made the single-client case a tiny bit slower. In the end, that adds up. I think we need to put some effort into figuring out if there is a way we can get some of that single-client performance (and ideally more) back.