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Re: rbtree code breaks GIN's adherence to maintenance_work_mem

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Robert Haas <robertmhaas(at)gmail(dot)com>
Cc: pgsql-hackers(at)postgresql(dot)org
Subject: Re: rbtree code breaks GIN's adherence to maintenance_work_mem
Date: 2010-07-31 16:32:03
Message-ID: 1997.1280593923@sss.pgh.pa.us (view raw or flat)
Thread:
Lists: pgsql-hackers
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.

			regards, tom lane

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