On Tue, May 21, 2013 at 10:18 PM, Robert Haas <robertmh...@gmail.com> wrote: > [ MVCC catalog access seems to be pretty cheap ]
In discussions today, Tom Lane suggested testing the time to start up a backend and run a simple query such as "SELECT 2+2" in the absence of a relcache file. I did this and can't measure any overhead as a result of MVCC catalog access. I tried it with no active connections. I tried it with 600 idle active connections (to make taking MVCC snapshots more expensive). I couldn't quite believe it made no difference, so I tried doing it in a tight loop under pgbench. I still can't measure any difference. I haven't tested carefully enough to rule out the possibility of an effect <1/2% at 600 connections, but there certainly isn't anything bigger than that and I don't even think there's that much of a difference. Andres Freund suggested creating a couple of simple tables and having lots of short-lived backends select data from them. rhaas=# create table af1 (x) as select g from generate_series(1,4) g; SELECT 4 rhaas=# create table af2 (x) as select g from generate_series(4,7) g; SELECT 4 Test query: SELECT * FROM af1, af2 WHERE af1.x = af2.x; pgbench command: pgbench -T 10 -c 50 -j 50 -n -f f -C With mvcc_catalog_access=off, I get ~1553 tps; with it on, I get ~1557 tps. Hmm... that could be because of the two-line debugging hunk my patch addes to HeapTupleSatisfiesNow(). After removing that, I get maybe a 1% regression with mvcc_catalog_access=on on this test, but it's not very consistent. If I restart the database server a few times, the overhead bounces around each time, and sometimes it's zero; the highest I saw is 1.4%. But it's not much, and this is a pretty brutal workload for PostgreSQL, since starting up >1500 connections per second is not a workload for which we're well-suited in the first place. All in all, I'm still feeling optimistic. -- Robert Haas EnterpriseDB: http://www.enterprisedb.com The Enterprise PostgreSQL Company -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
