Re: Parallel Seq Scan

From: Amit Langote <Langote_Amit_f8(at)lab(dot)ntt(dot)co(dot)jp>
To: Amit Kapila <amit(dot)kapila16(at)gmail(dot)com>, Robert Haas <robertmhaas(at)gmail(dot)com>
Cc: Andres Freund <andres(at)2ndquadrant(dot)com>, Kouhei Kaigai <kaigai(at)ak(dot)jp(dot)nec(dot)com>, Amit Langote <amitlangote09(at)gmail(dot)com>, Fabrízio Mello <fabriziomello(at)gmail(dot)com>, Thom Brown <thom(at)linux(dot)com>, Stephen Frost <sfrost(at)snowman(dot)net>, pgsql-hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Parallel Seq Scan
Date: 2015-03-13 08:42:08
Message-ID: 5502A2E0.2080900@lab.ntt.co.jp
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On 13-03-2015 PM 05:32, Amit Langote wrote:
> On 12-03-2015 PM 11:46, Amit Kapila wrote:
>> [parallel_seqscan_v10.patch]
>
> There may be a bug in TupleQueueFunnelNext().
>
> 1) I observed a hang with stack looking like:
>
> #0 0x00000039696df098 in poll () from /lib64/libc.so.6
> #1 0x00000000006f1c6a in WaitLatchOrSocket (latch=0x7f29dc3c73b4,
> wakeEvents=1, sock=-1, timeout=0) at pg_latch.c:333
> #2 0x00000000006f1aca in WaitLatch (latch=0x7f29dc3c73b4, wakeEvents=1,
> timeout=0) at pg_latch.c:197
> #3 0x000000000065088b in TupleQueueFunnelNext (funnel=0x17b4a20, nowait=0
> '\000', done=0x17ad481 "") at tqueue.c:269
> #4 0x0000000000636cab in funnel_getnext (funnelstate=0x17ad3d0) at
> nodeFunnel.c:347
> ...
> <snip>
>
> 2) In some cases, there can be a segmentation fault with stack looking like:
>
> #0 0x000000396968990a in memcpy () from /lib64/libc.so.6
> #1 0x00000000006507e7 in TupleQueueFunnelNext (funnel=0x263c800, nowait=0
> '\000', done=0x2633461 "") at tqueue.c:233
> #2 0x0000000000636cab in funnel_getnext (funnelstate=0x26333b0) at
> nodeFunnel.c:347
> #3 0x0000000000636901 in ExecFunnel (node=0x26333b0) at nodeFunnel.c:179
> ...
> <snip>
>
> I could get rid of (1) and (2) with the attached fix.

Hit send too soon!

By the way, the bug seems to be exposed only with a certain pattern/sequence
of workers being detached (perhaps in immediate successive) whereby the
funnel->nextqueue remains incorrectly set.

The patch attached this time.

By the way, when I have asserts enabled, I hit this compilation error:

createplan.c: In function ‘create_partialseqscan_plan’:
createplan.c:1180: error: ‘Path’ has no member named ‘path’

I see following line there:

Assert(best_path->path.parent->rtekind == RTE_RELATION);

Thanks,
Amit

Attachment Content-Type Size
TupleQueueFunnelNext-bugfix.patch text/x-diff 878 bytes

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