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Re: rbtree code breaks GIN's adherence to maintenance_work_mem

From: Robert Haas <robertmhaas(at)gmail(dot)com>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: pgsql-hackers(at)postgresql(dot)org
Subject: Re: rbtree code breaks GIN's adherence to maintenance_work_mem
Date: 2010-07-31 16:34:55
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Lists: pgsql-hackers
On Sat, Jul 31, 2010 at 12:32 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> Robert Haas <robertmhaas(at)gmail(dot)com> writes:
>> On Sat, Jul 31, 2010 at 12:02 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>>> I'm tempted to suggest that making RBNode be a hidden struct containing
>>> a pointer to somebody else's datum is fundamentally the wrong way to
>>> go about things, because the extra void pointer is pure overhead,
>>> and we aren't ever going to be using these things in a context where
>>> memory usage isn't of concern.  If we refactored the API so that RBNode
>>> was intended to be the first field of some larger struct, as is done in
>>> dynahash tables for instance, we could eliminate the void pointer and
>>> the palloc inefficiency.
>> Even if we do that, is it still going to be too much of a performance
>> regression overall?
> Looking back, EntryAccumulator was 56 bytes on 64-bit machines in 8.4
> (it should have been smaller, but a poor choice of field ordering left
> a lot of pad space).  Right now it's 32 bytes, and if we stick an RBNode
> field in the front it'd be 64.  So that'd be a 14% penalty compared to
> 8.4, as opposed to the present situation which is a 100% penalty (32+80
> bytes per entry).  On 32-bit machines the numbers are 32 bytes (8.4)
> versus 20+40 (HEAD) versus 36 bytes (my proposal), so 12.5% penalty
> versus 87.5%.  (All of these numbers should be discounted by whatever
> space you want to assume the pass-by-reference key datum takes.)
> So it'd definitely be a lot better than now.  Maybe there'd be some
> remaining performance gap for non-pathological cases, but I think we
> would be willing to pay that in order to avoid bad behavior for the
> pathological cases.  It's difficult to say for sure of course
> without going to the trouble of coding and testing it.

Well, it sounds like a reasonable thing to try, then.  You going to
take a crack at it?

Robert Haas
The Enterprise Postgres Company

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