Re: Slow standby snapshot

Hello, Andrey.

Thanks for your feedback.

> Current patch addresses another problem. In presence of old enough transaction enumeration of KnownAssignedXids with
sharedlock prevents adding new transactions with exclusive lock. And recovery effectively pauses.
 

Actually, I see two problems here (caused by the presence of old long
transactions). The first one is lock contention which causes recovery
pauses. The second one - just high CPU usage on standby by
KnownAssignedXidsGetAndSetXmin.

> All in all, I think using proposed "KnownAssignedXidsNext" patch solves real problem and the problem of binary
searchesshould be addressed by compressing KnownAssignedXids more often.
 

I updated the patch a little. KnownAssignedXidsGetAndSetXmin now
causes fewer cache misses because some values are stored in variables
(registers). I think it is better to not lean on the compiler here
because of `volatile` args.
Also, I have added some comments.

Best regards,
Michail.