ParallelFinish-hook of FDW/CSP (Re: Steps inside ExecEndGather)

Hello,

The attached patch implements the suggestion by Amit before.

What I'm motivated is to collect extra run-time statistics specific
to a particular ForeignScan/CustomScan, not only the standard
Instrumentation; like DMA transfer rate or execution time of GPU
kernels in my case.

Per-node DSM toc is one of the best way to return run-time statistics
to the master backend, because FDW/CSP can assign arbitrary length of
the region according to its needs. It is quite easy to require.
However, one problem is, the per-node DSM toc is already released when
ExecEndNode() is called on the child node of Gather.

This patch allows extensions to get control on the master backend's
context when all the worker node gets finished but prior to release
of the DSM segment. If FDW/CSP has its special statistics on the
segment, it can move to the private memory area for EXPLAIN output
or something other purpose.

One design consideration is whether the hook shall be called from
ExecParallelRetrieveInstrumentation() or ExecParallelFinish().
The former is a function to retrieve the standard Instrumentation
information, thus, it is valid only if EXPLAIN ANALYZE.
On the other hands, if we put entrypoint at ExecParallelFinish(),
extension can get control regardless of EXPLAIN ANALYZE, however,
it also needs an extra planstate_tree_walker().

Right now, we don't assume anything onto the requirement by FDW/CSP.
It may want run-time statistics regardless of EXPLAIN ANALYZE, thus,
hook shall be invoked always when Gather node confirmed termination
of the worker processes.

Thanks,
--
NEC OSS Promotion Center / PG-Strom Project
KaiGai Kohei <kaigai(at)ak(dot)jp(dot)nec(dot)com>

> -----Original Message-----
> From: Amit Kapila [mailto:amit(dot)kapila16(at)gmail(dot)com]
> Sent: Monday, October 17, 2016 11:22 AM
> To: Kaigai Kouhei(海外 浩平)
> Cc: Robert Haas; pgsql-hackers
> Subject: ##freemail## Re: [HACKERS] Steps inside ExecEndGather
>
> On Mon, Oct 17, 2016 at 6:22 AM, Kouhei Kaigai <kaigai(at)ak(dot)jp(dot)nec(dot)com> wrote:
> > Hello,
> >
> > I'm now trying to carry extra performance statistics on CustomScan
> > (like DMA transfer rate, execution time of GPU kernels, etc...)
> > from parallel workers to the leader process using the DSM segment
> > attached by the parallel-context.
> > We can require an arbitrary length of DSM using ExecCustomScanEstimate
> > hook by extension, then it looks leader/worker can share the DSM area.
> > However, we have a problem on this design.
> >
> > Below is the implementation of ExecEndGather().
> >
> > void
> > ExecEndGather(GatherState *node)
> > {
> > ExecShutdownGather(node);
> > ExecFreeExprContext(&node->ps);
> > ExecClearTuple(node->ps.ps_ResultTupleSlot);
> > ExecEndNode(outerPlanState(node));
> > }
> >
> > It calls ExecShutdownGather() prior to the recursive call of ExecEndNode().
> > The DSM segment shall be released on this call, so child node cannot
> > reference the DSM at the time of ExecEndNode().
> >
>
> Before releasing DSM, we do collect all the statistics or
> instrumentation information of each node. Refer
> ExecParallelFinish()->ExecParallelRetrieveInstrumentation(), so I am
> wondering why can't you collect the additional information in the same
> way?
>
>
> --
> With Regards,
> Amit Kapila.
> EnterpriseDB: http://www.enterprisedb.com