Re: How to make ResourceOwnerForgetBuffer() O(1), instead of O(N^2) scale

On 2014-10-03 10:35:42 +0300, Heikki Linnakangas wrote:
> On 10/03/2014 07:08 AM, Kouhei Kaigai wrote:
> > Hello,
> > 
> > I recently got a trouble on development of my extension that utilizes
> > the shared buffer when it released each buffer page.
> > 
> > This extension transfers contents of the shared buffers to GPU device
> > using DMA feature, then kicks a device kernel code.
> 
> Wow, that sounds crazy.

Agreed. I doubt that pinning that many buffers is a sane thing to do. At
the very least you'll heavily interfere with vacuum and such.

> > Once backend/extension calls ReadBuffer(), resowner.c tracks which
> > buffer was referenced by the current resource owner, to ensure these
> > buffers being released at end of the transaction.
> > However, it seems to me implementation of resowner.c didn't assume
> > many buffers are referenced by a particular resource owner simultaneously.
> > It manages the buffer index using an expandable array, then looks up
> > the target buffer by sequential walk but from the tail because recently
> > pinned buffer tends to be released first.
> > It made a trouble in my case. My extension pinned multiple thousands
> > buffers, so owner->buffers[] were enlarged and takes expensive cost
> > to walk on.
> > In my measurement, ResourceOwnerForgetBuffer() takes 36 seconds in
> > total during hash-joining 2M rows; even though hash-joining itself
> > takes less than 16 seconds.

> > What is the best way to solve the problem?
> 
> How about creating a separate ResourceOwner for these buffer pins, and
> doing a wholesale ResourceOwnerRelease() on it when you're done?

Or even just unpinning them in reverse order? That should already fix
the performance issues?

Greetings,

Andres Freund

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