On 11/14/2011 03:43 PM, Josh Berkus wrote:
Purpose is to provide consistent WAL writes, even when WALInsertLock contended. Currently no "off" option, thinking is that the overhead of doing this is relatively low and so it can be "always on" - exactly as it is for sync rep.Hmmm, have you had a chance to do any performance tests?
I was planning to run some later this week, but someone else is welcome to take a shot at it. The inspiration for this change was the performance scaling tests I did for sync rep last month. Don't recall if I shared those with this list yet; I've attached the fun graph. Over a slow international link with 100ms ping times, I was only getting the expected 10 TPS doing sync rep with a single client. But as more clients were added, so that a chunk of them were acknowledged in each commit reply, the total throughput among all of them scaled near linearly. With 300 clients, that managed to hit a crazy 2000 TPS.
The best scenario to show this patch working would be a laptop drive spinning at a slow speed (5400 or 4200 RPM) so that individual local commits are slow. That won't be 100ms slow, but close to 10ms is easy to see. When adding clients to a system with a slow local commit, what I've observed is that the scaling levels off between 750 and 1000 TPS, no matter how many clients are involved. The hope is that this alternate implementation will give the higher scaling in the face of slow commits that is seen on sync rep.
-- Greg Smith 2ndQuadrant US g...@2ndquadrant.com Baltimore, MD PostgreSQL Training, Services, and 24x7 Support www.2ndQuadrant.us
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