Re: Parallel Seq Scan

On Sun, Jan 11, 2015 at 6:01 AM, Stephen Frost <sfrost(at)snowman(dot)net> wrote:
> So, for my 2c, I've long expected us to parallelize at the relation-file
> level for these kinds of operations. This goes back to my other
> thoughts on how we should be thinking about parallelizing inbound data
> for bulk data loads but it seems appropriate to consider it here also.
> One of the issues there is that 1G still feels like an awful lot for a
> minimum work size for each worker and it would mean we don't parallelize
> for relations less than that size.

Yes, I think that's a killer objection.

> [ .. ] and
> how this thinking is an utter violation of the modularity we currently
> have there.

As is that.

My thinking is more along the lines that we might need to issue
explicit prefetch requests when doing a parallel sequential scan, to
make up for any failure of the OS to do that for us.

>> So, if the workers have been started but aren't keeping up, the master
>> should do nothing until they produce tuples rather than participating?
>> That doesn't seem right.
>
> Having the master jump in and start working could screw things up also
> though.

I don't think there's any reason why that should screw things up.
There's no reason why the master's participation should look any
different from one more worker. Look at my parallel_count code on the
other thread to see what I mean: the master and all the workers are
running the same code, and if fewer worker show up than expected, or
run unduly slowly, it's easily tolerated.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
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