On Sun, Feb 2, 2014 at 7:13 AM, Andres Freund <and...@2ndquadrant.com> wrote: > Just as reference, we're talking about a performance degradation from > 475963.613865 tps to 197744.913556 in a pgbench -S -cj64 just by setting > max_connections to 90, from 91...
That's pretty terrible. > So, I looked into this, and I am fairly certain it's because of the > (mis-)alignment of the buffer descriptors. With certain max_connections > settings InitBufferPool() happens to get 64byte aligned addresses, with > others not. I checked the alignment with gdb to confirm that. I find your diagnosis to be quite plausible. > 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. I think that is explained here: http://www.agner.org/optimize/blog/read.php?i=142&v=t With Sandy Bridge, "Misaligned memory operands [are] handled efficiently". > Now all this hinges on the fact that by a mere accident > BufferDescriptors are 64byte in size: Are they 64 bytes in size on REL9_*_STABLE? How about on win64? I think we're reasonably disciplined here already, but long is 32-bits in length even on win64. Looks like it would probably be okay, but as you say, it doesn't seem like something to leave to chance. > We could polish up the attached patch and apply it to all the branches, > the costs of memory are minimal. But I wonder if we shouldn't instead > make ShmemInitStruct() always return cacheline aligned addresses. That > will require some fiddling, but it might be a good idea nonetheless? What fiddling are you thinking of? > I think we should also consider some more reliable measures to have > BufferDescriptors cacheline sized, rather than relying on the happy > accident. Debugging alignment issues isn't fun, too much of a guessing > game... +1. Maybe make code that isn't appropriately aligned fail to compile? -- Peter Geoghegan -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
