Hi, On 2018-09-05 12:31:04 -0700, Daniel Wood wrote: > NOTE: > > In GetSnapshotData because pgxact, is declared volatile, the compiler will > not reduce the following two IF tests into a single test: > > > if (pgxact->vacuumFlags & PROC_IN_LOGICAL_DECODING) > continue; > > > if (pgxact->vacuumFlags & PROC_IN_VACUUM) > continue; > > > You can reduce the code path in the inner loop by coding this as: > > if (pgxact->vacuumFlags & (PROC_IN_LOGICAL_DECODING|PROC_IN_VACUUM)) > continue; > > > I'm still working on quantifying any gain. Note it isn't just one L1 cache > > fetch and one conditional branch eliminated. Due to the update frequency of > the pgxact cache line, for single statement TXN's, there are a certain number > of full cache misses, due to invalidation, that occurs when given pgxact is > updated between the first fetch of vacuumFlags and the 2nd fetch.
These two obviously could be combined, but I think we should just get rid of the volatiles. With a small bit of work they shouldn't be required anymore these days. Greetings, Andres Freund
