On Mon, Feb 3, 2014 at 3:38 PM, Andres Freund <and...@2ndquadrant.com> wrote: >> > A quick hack (attached) making BufferDescriptor 64byte aligned indeed >> > restored performance across all max_connections settings. It's not >> > surprising that a misaligned buffer descriptor causes problems - >> > there'll be plenty of false sharing of the spinlocks otherwise. Curious >> > that the the intel machine isn't hurt much by this.
>> What fiddling are you thinking of? > > Basically always doing a TYPEALIGN(CACHELINE_SIZE, addr) before > returning from ShmemAlloc() (and thereby ShmemInitStruct). There is something you have not drawn explicit attention to that is very interesting. If we take REL9_3_STABLE tip to be representative (built with full -O2 optimization, no assertions just debugging symbols), setting max_connections to 91 from 90 does not have the effect of making the BufferDescriptors array aligned; it has the effect of making it *misaligned*. You reported that 91 was much better than 90. I think that the problem actually occurs when the array *is* aligned! I suspect that the scenario described in this article accounts for the quite noticeable effect reported: http://danluu.com/3c-conflict -- Peter Geoghegan -- Sent via pgsql-hackers mailing list (firstname.lastname@example.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers