Re: Process local hint bit cache

On Wed, Mar 30, 2011 at 10:43 AM, Alvaro Herrera
<alvherre(at)commandprompt(dot)com> wrote:
> Excerpts from Merlin Moncure's message of mié mar 30 12:14:20 -0300 2011:
>
>> It is very different -- the slru layer is in shared memory and
>> requires locks to access.   The entire point is trying to avoid
>> accessing this structure in tight code paths.  I'm actually very
>> skeptical the slru layer provides any benefit at all.  I bet it would
>> be cheaper just mmap in the pages directly (as Heikki is also
>> speculating).
>
> Maybe it would be useful to distinguish the last SLRU page(s) (the one
> where clog writing actually takes place) from the older ones (which only
> ever see reading).  You definitely need locks to be able to access the
> active pages, but all the rest could be held as mmapped and accessed
> without locks because they never change (except to be truncated away).
> You know that any page behind RecentXmin is not going to be written
> anymore, so why go through all the locking hoops?

hm, that's a good thought -- cheap and easy -- and good to implement
irrespective of other changes. any improvement helps here, although
from my perspective the largest hobgoblins are shared memory access
generally and the extra i/o.

merlin