Date: Sat, 14 Mar 2020 20:58:22 +0100
From: Tomas Vondra <tomas.vondra@2ndquadrant.com>
To: James Coleman <jtc331@gmail.com>
Cc: Michael Paquier <michael@paquier.xyz>,
	Rafia Sabih <rafia.pghackers@gmail.com>,
	Peter Geoghegan <pg@bowt.ie>, Simon Riggs <simon@2ndquadrant.com>,
	Shaun Thomas <shaun.thomas@2ndquadrant.com>,
	Dmitry Dolgov <9erthalion6@gmail.com>,
	Alexander Korotkov <a.korotkov@postgrespro.ru>,
	Tom Lane <tgl@sss.pgh.pa.us>,
	PostgreSQL Developers <pgsql-hackers@lists.postgresql.org>,
	Alvaro Herrera <alvherre@2ndquadrant.com>
Subject: Re: [PATCH] Incremental sort (was: PoC: Partial sort)
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@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.

regards

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

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diff --git a/src/backend/utils/sort/tuplesort.c b/src/backend/utils/sort/tuplesort.c
index c2bd38f39f..91c0189577 100644
--- a/src/backend/utils/sort/tuplesort.c
+++ b/src/backend/utils/sort/tuplesort.c
@@ -1419,6 +1419,10 @@ tuplesort_reset(Tuplesortstate *state)
 	state->slabFreeHead = NULL;
 	state->growmemtuples = true;
 
+	state->tuplecontext = AllocSetContextCreate(state->sortcontext,
+										 "Caller tuples",
+										 ALLOCSET_DEFAULT_SIZES);
+
 	if (state->memtupsize < INITIAL_MEMTUPSIZE)
 	{
 		if (state->memtuples)

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