Re: Scaling shared buffer eviction

On Thu, Sep 25, 2014 at 8:51 AM, Robert Haas <robertmhaas@gmail.com> wrote:
> 1. To see the effect of reduce-replacement-locking.patch, compare the
> first TPS number in each line to the third, or the second to the
> fourth.  At scale factor 1000, the patch wins in all of the cases with
> 32 or more clients and exactly half of the cases with 1, 8, or 16
> clients.  The variations at low client counts are quite small, and the
> patch isn't expected to do much at low concurrency levels, so that's
> probably just random variation.  At scale factor 3000, the situation
> is more complicated.  With only 16 bufmappinglocks, the patch gets its
> biggest win at 48 clients, and by 96 clients it's actually losing to
> unpatched master.  But with 128 bufmappinglocks, it wins - often
> massively - on everything but the single-client test, which is a small
> loss, hopefully within experimental variation.
>
> Comments?

Why stop at 128 mapping locks?   Theoretical downsides to having more
mapping locks have been mentioned a few times but has this ever been
measured?  I'm starting to wonder if the # mapping locks should be
dependent on some other value, perhaps the # of shared bufffers...

merlin