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

On 07/16/2018 11:35 AM, Claudio Freire wrote:
> On Mon, Jul 16, 2018 at 11:34 AM Claudio Freire <klaussfreire@gmail.com> wrote:
>> On Fri, Jul 13, 2018 at 5:43 PM Andrew Dunstan
>> <andrew.dunstan@2ndquadrant.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.


Ah, ok. Thanks. ignore the email I just sent about that.

cheers

andrew

-- 
Andrew Dunstan                https://www.2ndQuadrant.com
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