From: Andrew Gierth <andrew@tao11.riddles.org.uk>
To: Andres Freund <andres@anarazel.de>
Cc: pgsql-bugs@lists.postgresql.org
Cc: jakub.janecek@comgate.cz
Subject: Re: BUG #15486: PG11 jit on 50x slower than jit off
Date: Thu, 22 Nov 2018 06:26:12 +0000
Message-ID: <87k1l5sl7l.fsf@news-spur.riddles.org.uk> (raw)
In-Reply-To: <20181122015409.i4jd6e2ypwmbkyu5@alap3.anarazel.de>
References: <15486-05850f065da42931@postgresql.org>
<20181122015409.i4jd6e2ypwmbkyu5@alap3.anarazel.de>
>>>>> "Andres" == Andres Freund <andres@anarazel.de> writes:
Andres> This actually isn't great outside of JIT compilation as well -
Andres> we waste a substantial portion of time rebuilding the
Andres> expression everytime, and the hashtable grows every time as
Andres> well.
Why would the hashtable grow? All of BuildTupleHashTable's callers
follow this pattern: the hashtable is allocated in a memory context that
will be reset on every rescan. Everything the hash table initialization
does is assumed to be completely discardable.
Which means that if you want to add a ResetTupleHashTable, it'll mean
significant reorganization of the callers' use of memory contexts, since
the hashtable will have to go into per-query memory (and presumably
create a child context of its own).
(There's an implicit assumption by all of BuildTupleHashTable's callers
that it will not allocate anything outside of the passed-in tablecxt,
which will be reset. Having BuildTupleHashTable allocate anything in its
caller's context is effectively an API break. But I notice this
_already_ got broken in pg11: the ExprContext that now gets allocated in
BuildTupleHashTable will be leaked into per-query memory on each cycle.)
--
Andrew (irc:RhodiumToad)
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To: pgsql-bugs@postgresql.org
Cc: andrew@tao11.riddles.org.uk, andres@anarazel.de, pgsql-bugs@lists.postgresql.org, jakub.janecek@comgate.cz
Subject: Re: BUG #15486: PG11 jit on 50x slower than jit off
In-Reply-To: <87k1l5sl7l.fsf@news-spur.riddles.org.uk>
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