Re: effective_cache_size

On Mon, Nov 5, 2018 at 12:50:01PM +0100, Peter Eisentraut wrote:
> On 16/10/2018 17:38, Bruce Momjian wrote:
> > diff --git a/src/backend/utils/misc/guc.c b/src/backend/utils/misc/guc.c
> > index 2317e8b..e471d7f 100644
> > --- a/src/backend/utils/misc/guc.c
> > +++ b/src/backend/utils/misc/guc.c
> > @@ -2987,10 +2987,9 @@ static struct config_int ConfigureNamesInt[] =
> >
> > {
> > {"effective_cache_size", PGC_USERSET, QUERY_TUNING_COST,
> > - gettext_noop("Sets the planner's assumption about the size of the disk cache."),
> > - gettext_noop("That is, the portion of the kernel's disk cache that "
> > - "will be used for PostgreSQL data files. This is measured in disk "
> > - "pages, which are normally 8 kB each."),
> > + gettext_noop("Sets the planner's assumption about the size of the data cache."),
> > + gettext_noop("That is, the size of the cache used for PostgreSQL data files. "
> > + "This is measured in disk pages, which are normally 8 kB each."),
> > GUC_UNIT_BLOCKS,
> > },
> > &effective_cache_size,
>
> This change completely loses the context that this is the kernel's/host
> system's memory size. What is "data cache"? I think this is a bad
> change. I know it's confusing, but the old description at least had
> some basis in terms that are known to the user.

Well, the change as outlined in the email is that effective_cache_size
is a combination of shared_buffers and kernel cache size, which I think
the docs now make clear. Do you have better wording for the GUC?

--
Bruce Momjian <bruce(at)momjian(dot)us> http://momjian.us
EnterpriseDB http://enterprisedb.com

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