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

On Mon, Sep 5, 2016 at 11:50 AM, Claudio Freire <klaussfreire@gmail.com> wrote:
> On Sun, Sep 4, 2016 at 3:46 AM, Simon Riggs <simon@2ndquadrant.com> wrote:
>> On 3 September 2016 at 04:25, Claudio Freire <klaussfreire@gmail.com> wrote:
>>> The patch also makes vacuum free the dead_tuples before starting
>>> truncation. It didn't seem necessary to hold onto it beyond that
>>> point, and it might help give the OS more cache, especially if work
>>> mem is configured very high to avoid multiple index scans.
>>
>> How long does that part ever take? Is there any substantial gain from this?
>>
>> Lets discuss that as a potential second patch.
>
> In the test case I mentioned, it takes longer than the vacuum part itself.
>
> Other than freeing RAM there's no gain. I didn't measure any speed
> difference while testing, but that's probably because the backward
> scan doesn't benefit from the cache, but other activity on the system
> might. So, depending on the workload on the server, extra available
> RAM may be a significant gain on its own or not. It just didn't seem
> there was a reason to keep that RAM reserved, especially after making
> it a huge allocation.
>
> I'm fine either way. I can remove that from the patch or leave it
> as-is. It just seemed like a good idea at the time.


Rebased and split versions attached