Re: Parallel Seq Scan

On Fri, Jan 16, 2015 at 11:27 PM, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> On Fri, Jan 16, 2015 at 11:49 PM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
>> As mentioned downthread, a far bigger consideration is the I/O pattern
>> we create. A sequential scan is so-called because it reads the
>> relation sequentially. If we destroy that property, we will be more
>> than slightly sad. It might be OK to do sequential scans of, say,
>> each 1GB segment separately, but I'm pretty sure it would be a real
>> bad idea to read 8kB at a time at blocks 0, 64, 128, 1, 65, 129, ...
>>
>> What I'm thinking about is that we might have something like this:
>>
>> struct this_lives_in_dynamic_shared_memory
>> {
>> BlockNumber last_block;
>> Size prefetch_distance;
>> Size prefetch_increment;
>> slock_t mutex;
>> BlockNumber next_prefetch_block;
>> BlockNumber next_scan_block;
>> };
>>
>> Each worker takes the mutex and checks whether next_prefetch_block -
>> next_scan_block < prefetch_distance and also whether
>> next_prefetch_block < last_block. If both are true, it prefetches
>> some number of additional blocks, as specified by prefetch_increment.
>> Otherwise, it increments next_scan_block and scans the block
>> corresponding to the old value.
>
> Assuming we will increment next_prefetch_block only after prefetching
> blocks (equivalent to prefetch_increment), won't 2 workers can
> simultaneously see the same value for next_prefetch_block and try to
> perform prefetch for same blocks?

The idea is that you can only examine and modify next_prefetch_block
or next_scan_block while holding the mutex.

> What will be value of prefetch_increment?
> Will it be equal to prefetch_distance or prefetch_distance/2 or
> prefetch_distance/4 or .. or will it be totally unrelated to
> prefetch_distance?

I dunno, that might take some experimentation. prefetch_distance/2
doesn't sound stupid.

--
Robert Haas
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