Re: Parallel bitmap heap scan

On Tue, Mar 7, 2017 at 11:27 AM, Dilip Kumar <dilipbalaut(at)gmail(dot)com> wrote:
> On Tue, Mar 7, 2017 at 9:44 PM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
>> I mean, IIUC, the call to PrefetchBuffer() is not done under any lock.
>> And that's the slow part. The tiny amount of time we spend updating
>> the prefetch information under the mutex should be insignificant
>> compared to the cost of actually reading the buffer. Unless I'm
>> missing something.
>
> Okay, but IIUC, the PrefetchBuffer is an async call to load the buffer
> if it's not already in shared buffer? so If instead of one process is
> making multiple async calls to PrefetchBuffer, if we make it by
> multiple processes will it be any faster? Or you are thinking that at
> least we can make BufTableLookup call parallel because that is not an
> async call.

It's not about speed. It's about not forgetting to prefetch. Suppose
that worker 1 becomes the prefetch worker but then doesn't return to
the Bitmap Heap Scan node for a long time because it's busy in some
other part of the plan tree. Now you just stop prefetching; that's
bad. You want prefetching to continue regardless of which workers are
busy doing what; as long as SOME worker is executing the parallel
bitmap heap scan, prefetching should continue as needed.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company