Re: Parallel bitmap heap scan

On Sat, Feb 18, 2017 at 10:45 PM, Dilip Kumar <dilipbalaut(at)gmail(dot)com> wrote:
> in 0002:
> - Improved comments.
> - Code refactoring in BitmapHeapNext.
> - Removed local tbm creation in BitmapHeapNext : as per new tidbitmap
> it's of no use.

I have observed one problem with 0002 and I though of sharing that
before fixing the same because we might have encountered the same
problem in some other patches i.e parallel shared hash and there might
be already a way to handle that.

The problem is that In ExecEndBitmapHeapScan we do tbm_free. Wherein,
it frees local pagetable memory and the shared memory using callback
routine and if other than the Backend (actual backend for the client
which is executing gather node) node any other worker become the
leader (worker which is responsible for creating the shared TBM) then
it will finish it work and free the shared memory by calling
ExecEndBitmapHeapScan, and it's possible that other worker are still
operating on the shared memory.

So far I never observed this issue in our test may be because either
Backend become the leader or by the time leader (non backend) free the
TBM others also finishes there work.

Solution:
- One solution to this problem can be that leader should not complete
the scan unless all other worker have finished the work.
- We can also think of that we don't make anyone wait but we make a
arrangement that last worker to finish the bitmapscan only free the
memory, but one problem with this solution is that last worker can be
non-leader also, which will have access to shared memory but how to
free the pagetable local memory (it's local to the leader).

Any suggestion on this ?

--
Regards,
Dilip Kumar
EnterpriseDB: http://www.enterprisedb.com