Re: resowner "cold start" overhead

From: Heikki Linnakangas <hlinnaka(at)iki(dot)fi>
To: Kyotaro Horiguchi <horikyota(dot)ntt(at)gmail(dot)com>, andres(at)anarazel(dot)de
Cc: pgsql-hackers(at)postgresql(dot)org
Subject: Re: resowner "cold start" overhead
Date: 2022-10-31 10:05:32
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On 31/10/2022 04:28, Kyotaro Horiguchi wrote:
> At Sat, 29 Oct 2022 13:00:25 -0700, Andres Freund <andres(at)anarazel(dot)de> wrote in
>> One way to reduce the size increase would be to use the space for initialarr
>> to store variables we don't need while initialarr is used. E.g. itemsarr,
>> maxitems, lastarr are candidates. But I suspect that the code complication
>> isn't worth it.
> +1
>> A different approach could be to not go for the "embedded initial elements"
>> approach, but instead to not delete resource owners / resource arrays inside
>> ResourceOwnerDelete(). We could stash them in a bounded list of resource
>> owners, to be reused by ResourceOwnerCreate(). We do end up creating a
>> several resource owners even for the simplest queries.
> We often do end up creating several resource owners that aquires not
> an element at all . On the other hand, a few resource owners
> sometimes grown up to 2048 (several times) or 4096 (one time) elements
> druing a run of the regressiont tests. (I saw catlist, tupdesc and
> relref grown to 2048 or more elements.)
>> The advantage of that scheme is that it'd save more and that we'd only reserve
>> space for ResourceArrays that are actually used in the current workload -
>> often the majority of arrays won't be.
> Thus I believe preserving resource owners works well. Preserving
> resource arrays also would work for the time efficiency, but some
> resource owners may end up keeping large amount of memory
> unnecessarily most of the time for the backend lifetime. I guess that
> the amount is far less than the possible bloat by catcache..
>> A potential problem would be that we don't want to use the "hashing" style
>> ResourceArrays forever, I don't think they'll be as fast for other cases. But
>> we could reset the arrays when they get large.
> I'm not sure linear search (am I correct?) doesn't harm for 2048 or
> more elements. I think that the "hashing" style doesn't prevent the
> arrays from being reset (free-d) at transaction end (or at resource
> owner deletion). That allows releasing unused elements while in
> transaction but I'm not sure we need to be so keen to reclaim space
> during a transaction.

What do you think of my ResourceOwner refactoring patches [1]? Reminded
by this, I rebased and added it to the upcoming commitfest again.

With that patch, all resources are stored in the same array and hash.
The array is part of ResourceOwnerData, so it saves the allocation
overhead, like the "initialarr" that you suggested. And it always uses
the array for recently remembered resources, and spills over to the hash
for more long-lived resources.

Andres, could you repeat your benchmark with [1], to see if it helps?


- Heikki

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