On Wed, Oct 20, 2010 at 1:13 PM, Robert Haas <robertmh...@gmail.com> wrote: > On Wed, Oct 20, 2010 at 3:47 PM, daveg <da...@sonic.net> wrote: >> On Wed, Oct 20, 2010 at 12:28:25PM -0700, Greg Stark wrote: >>> On Wed, Oct 20, 2010 at 12:17 PM, Greg Stark <gsst...@mit.edu> wrote: >>> > I don't think it's a big cost once all the processes >>> > have been forked if you're reusing them beyond perhaps slightly more >>> > efficient cache usage. >>> >>> Hm, this site claims to get a 13% win just from the reduced tlb misses >>> using a preload hack with Pg 8.2. That would be pretty substantial. >>> >>> http://oss.linbit.com/hugetlb/ >> >> That was my motivation in trying a patch. TLB misses can be a substantial >> overhead. I'm not current on the state of play, but working at Sun's >> benchmark lab on a DB TPC-B benchmark something for the first generation >> of MP systems, something like 30% of all bus traffic was TLB misses. The >> next iteration of the hardward had a much larger TLB. >> >> I have a client with 512GB memory systems, currently with 128GB configured >> as postgresql buffer cache. Which is 32M TLB entires trying to fit in the >> few dozed cpu TLB slots. I suspect there may be some contention. >> >> I'll benchmark of course. > > Do you mean 128GB shared buffers, or shared buffers + OS cache? I > think that the general wisdom is that performance tails off beyond > 8-10GB of shared buffers anyway, so a performance improvement on 128GB > shared buffers might not mean much unless you can also show that 128GB > shared buffers actually performs better than some smaller amount.
I'm sure someone will correct me if I'm wrong, but when I looked at this a couple years ago I believe a side effect of using hugetlbs is that these segments are never swapped out. I made a weak attempt to patch postgres to use hugetlbs when allocating shared memory. If I can find that patch I'll send it out.. Mark -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
