On Wed, Jan 5, 2022 at 9:46 AM Andres Freund <andres@anarazel.de> wrote:
Hi,
On 2021-12-29 11:31:51 -0800, Andres Freund wrote: > That's pretty much the same - XLogInsert() can trigger an > XLogWrite()/Flush(). > > I think it's a complete no-go to add throttling to these places. It's quite > possible that it'd cause new deadlocks, and it's almost guaranteed to have > unintended consequences (e.g. replication falling back further because > XLogFlush() is being throttled).
I thought of another way to implement this feature. What if we checked the current distance somewhere within XLogInsert(), but only set InterruptPending=true, XLogDelayPending=true. Then in ProcessInterrupts() we check if XLogDelayPending is true and sleep the appropriate time.
That way the sleep doesn't happen with important locks held / within a critical section, but we still delay close to where we went over the maximum lag. And the overhead should be fairly minimal.
+1 to the idea, this way we can fairly throttle large and smaller transactions the same way. I will work on this model and share the patch. Please note that the lock contention still exists in this case.
I'm doubtful that implementing the waits on a transactional level provides a meaningful enough amount of control - there's just too much WAL that can be generated within a transaction.
