Re: Parallel Seq Scan

On Fri, Mar 13, 2015 at 2:12 PM, Amit Langote <Langote_Amit_f8(at)lab(dot)ntt(dot)co(dot)jp>
wrote:
>
> 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.
>

I think this can happen if funnel->nextqueue is greater
than funnel->nqueues.
Please see if attached patch fixes the issue, else could you share the
scenario in more detail where you hit this issue.

> 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);
>

Okay, will take care of this.

Thanks.

With Regards,
Amit Kapila.
EnterpriseDB: http://www.enterprisedb.com