Re: [PATCH] Incremental sort (was: PoC: Partial sort)

From: Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com>
To: James Coleman <jtc331(at)gmail(dot)com>
Cc: Michael Paquier <michael(at)paquier(dot)xyz>, Rafia Sabih <rafia(dot)pghackers(at)gmail(dot)com>, Peter Geoghegan <pg(at)bowt(dot)ie>, Simon Riggs <simon(at)2ndquadrant(dot)com>, Shaun Thomas <shaun(dot)thomas(at)2ndquadrant(dot)com>, Dmitry Dolgov <9erthalion6(at)gmail(dot)com>, Alexander Korotkov <a(dot)korotkov(at)postgrespro(dot)ru>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, PostgreSQL Developers <pgsql-hackers(at)lists(dot)postgresql(dot)org>, Alvaro Herrera <alvherre(at)2ndquadrant(dot)com>
Subject: Re: [PATCH] Incremental sort (was: PoC: Partial sort)
Date: 2020-03-14 19:58:22
Message-ID: 20200314195822.bc2mqyvolo2w5l2w@development
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On Sat, Mar 14, 2020 at 02:41:09PM -0400, James Coleman wrote:
>It looks like the issue is actually into the `tuplecontext`, which is
>currently a child context of `sortcontext`:
>#3 0x0000558cd153b565 in AllocSetCheck
>(context=context(at)entry=0x558cd28e0b70) at aset.c:1573
>1573 Assert(total_allocated == context->mem_allocated);
>(gdb) p total_allocated
>$1 = 16384
>(gdb) p context->mem_allocated
>$2 = 8192
>(gdb) p context->name
>$3 = 0x558cd16c8ccd "Caller tuples"
>I stuck in several more AllocSetCheck calls in aset.c and got the
>attached backtrace.

I think the problem is pretty simple - tuplesort_reset does call
tuplesort_reset, which resets the sortcontext. But that *deletes* the
tuplecontext, so the state->tuplecontext gets stale. I'd haven't looked
into the exact details, but it clearly confuses the accouting.

The attached patch fixes the issue for me - I'm not claiming it's the
right fix, but it's the simplest thing I could think of. Maybe the
tuplesort_rest should work differently, not sure.

And it seems to resolve the memory leak too - I suspect we've freed the
context (so it was not part of the tree of contexts) but the struct was
still valid and we kept allocating memory in it - but it was invisible
to MemoryContextDump etc.


Tomas Vondra
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

Attachment Content-Type Size
tuplesort-fix.patch text/plain 537 bytes

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