Luke Lonergan wrote:
What do you mean with using readahead inside the heapscan? Starting an async read request?


Nope - just reading N buffers ahead for seqscans.  Subsequent calls use
previously read pages.  The objective is to issue contiguous reads to
the OS in sizes greater than the PG page size (which is much smaller
than what is needed for fast sequential I/O).

Are you filling multiple buffers in the buffer cache with a single read-call? The OS should be doing readahead for us anyway, so I don't see how just issuing multiple ReadBuffers one after each other helps.

Yes, I think the ring buffer strategy should be used when the table size
is > 1 x bufcache and the ring buffer should be of a fixed size smaller
than L2 cache (32KB - 128KB seems to work well).

I think we want to let the ring grow larger than that for updating transactions and vacuums, though, to avoid the WAL flush problem.

--
  Heikki Linnakangas
  EnterpriseDB   http://www.enterprisedb.com

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