Re: Steps inside ExecEndGather

> On Mon, Oct 17, 2016 at 6:22 AM, Kouhei Kaigai <kaigai@ak.jp.nec.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?
>
Thanks for the suggestion.
Hmm. Indeed, it is more straightforward way to do, although a new hook
is needed for CSP/FDW.

What I want to collect are: DMA transfer rate between RAM<->GPU, Execution
time of GPU kernels and etc... These are obviously out of the standard
Instrumentation structure, so only CSP/FDW can know its size and format.

If we would have a callback just before the planstate_tree_walker() when
planstate is either CustomScanState or ForeignScanState, it looks to me
the problem can be solved very cleanly.

Best regards,
--
NEC OSS Promotion Center / PG-Strom Project
KaiGai Kohei <kaigai@ak.jp.nec.com>