Re: Clock sweep not caching enough B-Tree leaf pages?

On Thu, Apr 17, 2014 at 9:21 AM, Stephen Frost <sfrost@snowman.net> wrote:
> * Robert Haas (robertmhaas@gmail.com) wrote:
>> several orders of magnitude more often.  That's clearly bad.  On
>> systems that are not too heavily loaded it doesn't matter too much
>> because we just fault the page right back in from the OS pagecache.
>
> Ehhh.  No.  If it's a hot page that we've been holding in *our* cache
> long enough, the kernel will happily evict it as 'cold' from *its*
> cache, leading to...
>
>> But I've done pgbench runs where such decisions lead to long stalls,
>> because the page has to be brought back in from disk, and there's a
>> long I/O queue; or maybe just because the kernel thinks PostgreSQL is
>> issuing too many I/O requests and makes some of them wait to cool
>> things down.
>
> Exactly this.

Yes, I believe that's why this is so effective.


-- 
Peter Geoghegan