Re: sinval synchronization considered harmful

On Wed, Jul 20, 2011 at 09:46:33PM -0400, Robert Haas wrote:
> For the last week or so, in between various other tasks, I've been
> trying to understand why performance drops off when you run pgbench -n
> -S -c $CLIENTS -j $CLIENTS -T $A_FEW_MINUTES at very high client
> counts. The answer, in a word, is SIGetDataEntries(). I believe we
> need to bite the bullet and rewrite this using a lock-free algorithm,
> using memory barriers on processors with weak memory ordering.
> Perhaps there is another way to do it, but nothing I've tried has
> really worked so far, and I've tried quite a few things. Here's the
> data.
>
> On unpatched master, performance scales pretty much linearly out to 32
> clients. As you add more clients, it drops off:

> [80 clients]
> tps = 132518.586371 (including connections establishing)
> tps = 130968.749747 (including connections establishing)
> tps = 132574.338942 (including connections establishing)

> [80 clients, with lazy vxid locks and sinval-unlocked]
> tps = 203256.701227 (including connections establishing)
> tps = 190637.957571 (including connections establishing)
> tps = 190228.617178 (including connections establishing)

Nice numbers. The sinval-unlocked.patch implementation looks like it's taking a
sound direction.

In
http://archives.postgresql.org/message-id/CA+TgmobbxMh_9zjudheSWO6m8sBMb5hdZt+3ChCLuv5eztv8Ug@mail.gmail.com,
you quoted 210k TPS when you stubbed out AcceptInvalidationMessages(). Is it
correct to conclude that AcceptInvalidationMessages() still reduces the
transaction rate by 5-10% with this stack of patches?

--
Noah Misch http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services