Re: Vacuum: allow usage of more than 1GB of work mem

On Mon, Jul 16, 2018 at 11:34 AM Claudio Freire <klaussfreire(at)gmail(dot)com> wrote:
>
> On Fri, Jul 13, 2018 at 5:43 PM Andrew Dunstan
> <andrew(dot)dunstan(at)2ndquadrant(dot)com> wrote:
> >
> >
> >
> > On 07/13/2018 09:44 AM, Heikki Linnakangas wrote:
> > > On 13/07/18 01:39, Andrew Dunstan wrote:
> > >> On 07/12/2018 06:34 PM, Alvaro Herrera wrote:
> > >>> On 2018-Jul-12, Andrew Dunstan wrote:
> > >>>
> > >>>> I fully understand. I think this needs to go back to "Waiting on
> > >>>> Author".
> > >>> Why? Heikki's patch applies fine and passes the regression tests.
> > >>
> > >> Well, I understood Claudio was going to do some more work (see
> > >> upthread).
> > >
> > > Claudio raised a good point, that doing small pallocs leads to
> > > fragmentation, and in particular, it might mean that we can't give
> > > back the memory to the OS. The default glibc malloc() implementation
> > > has a threshold of 4 or 32 MB or something like that - allocations
> > > larger than the threshold are mmap()'d, and can always be returned to
> > > the OS. I think a simple solution to that is to allocate larger
> > > chunks, something like 32-64 MB at a time, and carve out the
> > > allocations for the nodes from those chunks. That's pretty
> > > straightforward, because we don't need to worry about freeing the
> > > nodes in retail. Keep track of the current half-filled chunk, and
> > > allocate a new one when it fills up.
> >
> >
> > Google seems to suggest the default threshold is much lower, like 128K.
> > Still, making larger allocations seems sensible. Are you going to work
> > on that?
>
> Below a few MB the threshold is dynamic, and if a block bigger than
> 128K but smaller than the higher threshold (32-64MB IIRC) is freed,
> the dynamic threshold is set to the size of the freed block.
>
> See M_MMAP_MAX and M_MMAP_THRESHOLD in the man page for mallopt[1]
>
> So I'd suggest allocating blocks bigger than M_MMAP_MAX.
>
> [1] http://man7.org/linux/man-pages/man3/mallopt.3.html

Sorry, substitute M_MMAP_MAX with DEFAULT_MMAP_THRESHOLD_MAX, the
former is something else.