On 02/06/2016 08:39 PM, Andres Freund wrote:
On 2016-02-06 20:34:07 +0100, Tomas Vondra wrote:
On 02/06/2016 06:47 PM, Tom Lane wrote:
* It incorporates a bespoke reimplementation of palloc into hash
joins. This is not a maintainable/scalable way to go about reducing
memory consumption. It should have been done with an arm's-length API
to a new type of memory context, IMO (probably one that supports
palloc but not pfree, repalloc, or any chunk-header-dependent

Hmmm, interesting idea. I've been thinking about doing this using a memory
context when writing the dense allocation, but was stuck in the "must
support all operations" mode, making it impossible. Disallowing some of the
operations would make it a viable approach, I guess.

FWIW, I've done that at some point. Noticeable speedups (that's what
I cared about), but a bit annoying to use. There's many random
pfree()s around, and then there's MemoryContextContains(),
GetMemoryChunkContext(), GetMemoryChunkSpace() - which all are
pretty fundamentally incompatible with such an allocator. I ended up
having a full header when assertions are enabled, to be able to
detect usage of these functions and assert out.

I didn't concentrate on improving memory usage, but IIRC it was even
noticeable for some simpler things.

I think the hassle is not that bad when most of the fragments have the same life cycle. With hashjoin that's almost exactly the case, except when we realize we need to increase the number of buckets - in that case we need to split the set of accumulated tuples in two.


Tomas Vondra                  http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

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