Re: postgresql latency & bgwriter not doing its job

hi

On Thu, Sep 4, 2014 at 7:01 PM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
> On Thu, Sep 4, 2014 at 3:09 AM, Ants Aasma <ants(at)cybertec(dot)at> wrote:
>> On Thu, Sep 4, 2014 at 12:36 AM, Andres Freund <andres(at)2ndquadrant(dot)com> wrote:
>>> It's imo quite clearly better to keep it allocated. For one after
>>> postmaster started the checkpointer successfully you don't need to be
>>> worried about later failures to allocate memory if you allocate it once
>>> (unless the checkpointer FATALs out which should be exceedingly rare -
>>> we're catching ERRORs). It's much much more likely to succeed
>>> initially. Secondly it's not like there's really that much time where no
>>> checkpointer isn't running.
>>
>> In principle you could do the sort with the full sized array and then
>> compress it to a list of buffer IDs that need to be written out. This
>> way most of the time you only need a small array and the large array
>> is only needed for a fraction of a second.
>
> It's not the size of the array that's the problem; it's the size of
> the detonation when the allocation fails.
>
You can use a file backed memory array
Or because it's only a hint and
- keys are in buffers (BufferTag), right?
- transition is only from 'data I care to data I don't care' if a
buffer is concurrently evicted when sorting.

Use a pre allocate buffer index array an read keys from buffers when
sorting, without memory barrier, spinlocks, whatever.