Should we cacheline align PGXACT?

Hackers,

originally this idea was proposed by Andres Freund while experimenting with lockfree Pin/UnpinBuffer [1].

The patch is attached as well as results of pgbench -S on 72-cores machine.  As before it shows huge benefit in this case.

For sure, we should validate that it doesn't cause performance regression in other cases.  At least we should test read-write and smaller machines.

Any other ideas?

1. https://www.postgresql.org/message-id/20160411214029.ce3fw6zxim5k6a2r@alap3.anarazel.de

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