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

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