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().

  ExecEndGather(GatherState *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().

Is there some technical reason why parallel context needs to be released
prior to ExecEndNode() of the child nodes? Or, just convention of coding?

I think I'm not an only person who wants to use DSM of CustomScan to write
back something extra status of parallel workers.
How about an idea to move ExecShutdownGather() after the ExecEndNode()?

To avoid this problem, right now, I allocate an another DSM then inform
its handle to the parallel workers. This segment can be survived until
ExecEndCustomScan(), but not best effective way, of course.

NEC OSS Promotion Center / PG-Strom Project
KaiGai Kohei <kai...@ak.jp.nec.com>

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