On Fri, Apr 14, 2017 at 1:19 AM, Peter Geoghegan <p...@bowt.ie> wrote: > Interestingly enough, I think that Knuth was pretty much spot on with > his "sweet spot" of 7 tapes, even if you have modern hardware. Commit > df700e6 (where the sweet spot of merge order 7 was no longer always > used) was effective because it masked certain overheads that we > experience when doing multiple passes, overheads that Heikki and I > mostly removed. This was confirmed by Robert's testing of my merge > order cap work for commit fc19c18, where he found that using 7 tapes > was only slightly worse than using many hundreds of tapes. If we could > somehow be completely effective in making access to logical tapes > perfectly sequential, then 7 tapes would probably be noticeably > *faster*, due to CPU caching effects.
I don't think there's any one fixed answer, because increasing the number of tapes reduces I/O by adding CPU cost, and visca versa. -- Robert Haas EnterpriseDB: http://www.enterprisedb.com The Enterprise PostgreSQL Company -- Sent via pgsql-hackers mailing list (firstname.lastname@example.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers