Re: Synchronized Scan update

On Tue, 2007-03-13 at 10:08 -0700, Jeff Davis wrote:
> On Tue, 2007-03-13 at 12:53 -0400, Tom Lane wrote:
> > Jeff Davis <pgsql@j-davis.com> writes:
> > > I agree that ss_report_loc() doesn't need to report on every call. If
> > > there's any significant overhead I agree that it should report less
> > > often. Do you think that the overhead is significant on such a simple
> > > function?
> > 
> > One extra LWLock cycle per page processed definitely *is* a significant
> > overhead ... can you say "context swap storm"?  I'd think about doing it
> > once every 100 or so pages.
> > 
> 
> No lock is needed to store the hint. If somehow the hint (which is
> stored in a static table, no pointers) gets invalid data due to a race
> condition, the new scan will simply consider the hint invalid and start
> at 0.
> 
> I did this precisely to avoid causing a performance regression for usage
> patterns that don't benefit from sync scans.

Shared memory access is still a performance/scalability concern because
so many people want access to it at the same time. 

There really is no need to do this after each block. 8 CPUs ought to be
able to do 8 scans without tripping over each other. Especially if they
are on separate tables.

--  Simon Riggs              EnterpriseDB   http://www.enterprisedb.com