Re: FSM versus GIN pending list bloat - Mailing list pgsql-hackers

Couple of questions here...

* the docs say "it's desirable to have pending-list cleanup occur in the background", but there is no way to invoke that, except via VACUUM. I think we need a separate function to be able to call this as a background action. If we had that, we wouldn't need much else, would we?

I thought maybe the new bgworker framework would be a way to have a backend signal a bgworker to do the cleanup when it notices the pending list is getting large.  But that wouldn't directly fix this issue, because the bgworker still wouldn't recycle that space (without further changes), only vacuum workers do that currently.

But I don't think this could be implemented as an extension, because the signalling code has to be in core, so (not having studied the matter at all) I don't know if it is good fit for bgworker.  

* why do we have two parameters: gin_pending_list_limit and fastupdate? What happens if we set gin_pending_list_limit but don't set fastupdate?

Fastupdate is on by default.  If it were turned off, then gin_pending_list_limit would be mostly irrelevant for those tables. Fastupdate could have been implemented as a magic value (0 or -1) for gin_pending_list_limit but that would break backwards compatibility (and arguably would not be a better way of doing things, anyway).

 

* how do we know how to set that parameter? Is there a way of knowing gin_pending_list_limit has been reached?

I don't think there is an easier answer to that.  The trade offs are complex and depend on things like how well cached the parts of the index needing insertions are, how many lexemes/array elements are in an average document, and how many documents inserted near the same time as each other share lexemes in common.  And of course what you need to optimize for, latency or throughput, and if latency search latency or insert latency.

This and the OP seem like 9.5 open items to me.

I don't think so.  Freeing gin_pending_list_limit from being forcibly tied to work_mem is a good thing.  Even if I don't know exactly how to set gin_pending_list_limit, I know I don't want to be 4GB just because work_mem was set there for some temporary reason.  I'm happy to leave it at its default and let its fine tuning be a topic for people who really care about every microsecond of performance.