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

On Mon, Jul 16, 2018 at 3:30 PM Andrew Dunstan
<andrew(dot)dunstan(at)2ndquadrant(dot)com> wrote:
>
>
>
> On 07/16/2018 10:34 AM, Claudio Freire 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
>
>
> That page says:
>
> M_MMAP_MAX
> This parameter specifies the maximum number of allocation
> requests that may be simultaneously serviced using mmap(2).
> This parameter exists because some systems have a limited
> number of internal tables for use by mmap(2), and using more
> than a few of them may degrade performance.
>
> The default value is 65,536, a value which has no special
> significance and which serves only as a safeguard. Setting
> this parameter to 0 disables the use of mmap(2) for servicing
> large allocation requests.
>
>
> I'm confused about the relevance.

It isn't relevant. See my next message, it should have read
DEFAULT_MMAP_THRESHOLD_MAX.