Re: Sorting writes during checkpoint

ITAGAKI Takahiro <itagaki(dot)takahiro(at)oss(dot)ntt(dot)co(dot)jp> writes:
> Greg Smith <gsmith(at)gregsmith(dot)com> wrote:
>> If shared_buffers(=NBuffers) is set to something big, this could give some
>> memory churn. And I think it's a bad idea to allocate something this
>> large at checkpoint time, because what happens if that fails? Really not
>> the time you want to discover there's no RAM left.

> Hmm, but I think we need to copy buffer tags into bgwriter's local memory
> in order to avoid locking taga many times in the sorting.

I updated this patch to permanently allocate the working array as Greg
suggests, and to fix a bunch of commenting issues (attached).

However, I am completely unable to measure any performance improvement
from it. Given the possible risk of out-of-memory failures, I think the
patch should not be applied without some direct proof of performance
benefits, and I don't see any.

regards, tom lane