Hi, > 3 авг. 2021 г., в 03:01, Andres Freund <and...@anarazel.de> написал(а): > > Hi, > > On 2021-08-03 00:07:23 +0300, Michail Nikolaev wrote: >> The main idea is simple optimistic optimization - store offset to next >> valid entry. So, in most cases, we could just skip all the gaps. >> Of course, it adds some additional impact for workloads without long >> (few seconds) transactions but it is almost not detectable (because of >> CPU caches). > > I'm doubtful that that's really the right direction. For workloads that > are replay heavy we already often can see the cost of maintaining the > known xids datastructures show up significantly - not surprising, given > the datastructure. And for standby workloads with active primaries the > cost of searching through the array in all backends is noticeable as > well. I think this needs a bigger data structure redesign.
KnownAssignedXids implements simple membership test idea. What kind of redesign would you suggest? Proposed optimisation makes it close to optimal, but needs eventual compression. Maybe use a hashtable of running transactions? It will be slightly faster when adding\removing single transactions. But much worse when doing KnownAssignedXidsRemove(). Maybe use a tree? (AVL\RB or something like that) It will be slightly better, because it does not need eventual compression like exiting array. Best regards, Andrey Borodin.
