Re: Parallel Seq Scan

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?

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?

With Regards,
Amit Kapila.
EnterpriseDB: http://www.enterprisedb.com