Hi, On 2022-11-23 19:12:07 +0530, Bharath Rupireddy wrote: > While working on something else, I noticed that each WAL insert lock > tracks its own last important WAL record's LSN (lastImportantAt) and > both the bgwriter and checkpointer later computes the max > value/server-wide last important WAL record's LSN via > GetLastImportantRecPtr(). While doing so, each WAL insertion lock is > acquired in exclusive mode in a for loop. This seems like too much > overhead to me.
GetLastImportantRecPtr() should be a very rare operation, so it's fine for it to be expensive. The important thing is for the maintenance of the underlying data to be very cheap. > I quickly coded a patch (attached herewith) that > tracks the server-wide last important WAL record's LSN in > XLogCtlInsert (lastImportantPos) protected with a spinlock and gets > rid of lastImportantAt from each WAL insert lock. That strikes me as a very bad idea. It adds another point of contention to a very very hot code path, to make a very rare code path cheaper. Greetings, Andres Freund