On Tue, Feb 25, 2014 at 2:55 PM, Jeremy Harris <j...@wizmail.org> wrote: > On 24/02/14 17:38, Robert Haas wrote: >> >> On Thu, Feb 20, 2014 at 7:27 PM, Jeremy Harris <j...@wizmail.org> wrote: >>> >>> Run under cachegrind, it takes about N/10 last-level cache misses, >>> all for the new item being introduced to the heap. The existing >>> code takes none at all. >> >> >> Can you explain this further? This seems like an important clue that >> could be useful when trying to optimize this code, but I'm a little >> unclear which part of the operation has more cache misses with your >> changes and why. > > > In the patched version, for the heapify operation the outer loop > starts at the last heap-parent tuple and works backward to the > start of the tuples array. A copy is taken of the SortTuple being > operated on for the inner loop to use. This copy suffers cache misses. > > (The inner loop operates on elements between the copy source and the > end of the array). > > > In the original, the outer loop runs the array in increasing index > order. Again a copy is taken of the SortTuple for the inner loop > to use. This copy does not appear to take significant cache misses. > > (The inner loop operates on elements between the copy source and > the start of the array).
Do you have any theory as to why this happens in one case but not the other? -- Robert Haas EnterpriseDB: http://www.enterprisedb.com The Enterprise PostgreSQL Company -- Sent via pgsql-hackers mailing list (email@example.com) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers