On Wed, Jan 19, 2011 at 7:57 AM, Merlin Moncure <mmonc...@gmail.com> wrote: > On Tue, Jan 18, 2011 at 12:44 PM, Robert Haas <robertmh...@gmail.com> wrote: >> On Tue, Jan 18, 2011 at 9:24 AM, Merlin Moncure <mmonc...@gmail.com> wrote: >>> a few weeks back I hacked an experimental patch that removed the hint >>> bit action completely. the results were very premature and/or >>> incorrect, but my initial findings suggested that hint bits might not >>> be worth the cost from performance standpoint. i'd like to see some >>> more investigation in this direction before going with a complex >>> application mechanism (although that would be beneficial vs the status >>> quo). >> >> I think it's not very responsible to allege that hint bits aren't >> providing a benefit without providing the patch that you used and the >> tests that you ran. This is a topic that needs careful analysis, and >> I think that saying "hint bits don't provide a benefit... maybe..." >> doesn't do anything but confuse the issue. How about doing some tests >> with the patch from my OP and posting the results? If removing hint >> bits entirely doesn't degrade performance, then surely the >> less-drastic approach I've taken here ought to be OK too. But in my >> testing, it didn't look too good. > > hm. well, I would have to agree on the performance hit -- I figure 5% > scan penalty should be about the maximum you'd want to pay to get the > i/o reduction. Odds are you're correct and I blew something...I'd be > happy to test your patch.
Ah, I tested your patch vs stock postgres vs my patch, basically your results are unhappily correct (mine was just a hair faster than yours which you'd expect). The differential was even wider on my laptop class hardware, maybe 26%. I also agree that even if the penalty was reduced or determined to be worth it anyways, your approach to move the setting/i/o around to appropriate places is the way to go vs wholesale removal, unless some way is found to reduce clog lookup penalty to a fraction of what it is now (not likely, I didn't profile but I bet a lot of the problem is the lw lock). Interesting I didn't notice this on my original test :(. merlin -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers