Re: Wait free LW_SHARED acquisition - v0.2

On Sun, Feb 2, 2014 at 6:00 AM, Andres Freund <andres(at)2ndquadrant(dot)com>wrote:

>
> Some background:
> The setups that triggered me into working on the patchset didn't really
> have a pgbench like workload, the individual queries were/are more
> complicated even though it's still an high throughput OLTP workload. And
> the contention was *much* higher than what I can reproduce with pgbench
> -S, there was often nearly all time spent in the lwlock's spinlock, and
> it was primarily the buffer mapping lwlocks, being locked in shared
> mode. The difference is that instead of locking very few buffers per
> query like pgbench does, they touched much more.
>

Perhaps I should try to argue for this extension to pgbench again:

http://www.postgresql.org/message-id/CAMkU=1w0K3RNhtPuLF8WQoVi6gxgG6mcnpC=-iVjwKJKyDPysw@mail.gmail.com

I think it would go a good job of exercising what you want, provided you
set the scale so that all data fit in RAM but not in shared_buffers.

Or maybe you want it to fit in shared_buffers, since the buffer mapping
lock was contended in shared mode--that suggests the problem is finding the
buffer that already has the page, not making a buffer to have the page.

Cheers,

Jeff