Re: Unhappy about API changes in the no-fsm-for-small-rels patch

Hi,

On 2019-04-24 11:28:32 +0530, Amit Kapila wrote:
> On Tue, Apr 23, 2019 at 10:59 PM Andres Freund <andres(at)anarazel(dot)de> wrote:
> > > > On 2019-04-22 18:49:44 +0530, Amit Kapila wrote:
> > > I think we should first try to see in this new scheme (a) when to set
> > > the map, (b) when to clear it, (c) how to use. I have tried to
> > > summarize my thoughts about it, let me know what do you think about
> > > the same?
> > >
> > > When to set the map.
> > > At the beginning (the first time relation is used in the backend), the
> > > map will be clear. When the first time in the backend, we find that
> > > FSM doesn't exist and the number of blocks is lesser than
> > > HEAP_FSM_CREATION_THRESHOLD, we set the map for the total blocks that
> > > exist at that time and mark all or alternate blocks as available.
> >
> > I think the alternate blocks scheme has to go. It's not defensible.
> >
>
> Fair enough, I have changed it in the attached patch. However, I
> think we should test it once the patch is ready as we have seen a
> small performance regression due to that.

Sure, but that was because you re-scanned from scratch after every
insertion, no?

> > And sure, leaving that aside we could store one byte per block
>
> Hmm, I think you mean to say one-bit per block, right?

No, I meant byte. The normal FSM saves how much space there is for a
block, but the current local fsm doesn't. That means pages are marked as
unavailble even though other tuples would possibly fit.

> > It's possible that'd come with
> > some overhead - I've not thought sufficiently about it: I assume we'd
> > still start out in each backend assuming each page is empty, and we'd
> > then rely on RelationGetBufferForTuple() to update that. What I wonder
> > is if we'd need to check if an on-disk FSM has been created every time
> > the space on a page is reduced? I think not, if we use invalidations to
> > notify others of the existance of an on-disk FSM. There's a small race,
> > but that seems ok.

> Do you mean to say that vacuum or some backend should invalidate in
> case it first time creates the FSM for a relation?

Right.

> I think it is quite possible that the vacuum takes time to trigger
> such invalidation if there are fewer deletions. And we won't be able
> to use newly added page/s.

I'm not sure I understand what you mean by that? If the backend that
ends up creating the FSM - because it extended the relation beyond 4
pages - issues an invalidation, the time till other backends pick that
up should be minimal?

> IIUC, you are suggesting to issue invalidations when the (a) vacuum
> finds there is no FSM and page has more space now, (b) invalidation to
> notify the existence of FSM. IT seems to me that issuing more
> invalidations for the purpose of FSM can lead to an increased number
> of relation builds in the overall system. I think this can have an
> impact when there are many small relations in the system which in some
> scenarios might not be uncommon.

If this becomes an issue I'd just create a separate type of
invalidation, one that just signals that the FSM is being invalidated.

Greetings,

Andres Freund