Re: Turning off HOT/Cleanup sometimes

On 04/15/2015 07:11 PM, Alvaro Herrera wrote:
> Heikki Linnakangas wrote:
>> On 04/15/2015 05:44 PM, Alvaro Herrera wrote:
>
>>> Robert's proposal is "when reading a page, if dirty HOT-clean it; if not
>>> dirty, also HOT-clean it but only 5 times in each scan".  This runs
>>> HOT-cleanup some number of times (as many as there are dirty), and
>>> causes at most 5 pages to become dirty.
>>>
>>>
>>> Am I right in thinking that HOT-clean in a dirty page is something that
>>> runs completely within CPU cache?  If so, it would be damn fast and
>>> would have benefits for future readers, for very little cost.
>>
>> If there are many tuples on the page, it takes some CPU effort to scan all
>> the HOT chains and move tuples around. Also, it creates a WAL record, which
>> isn't free.
>
> But if the page is in CPU cache, the CPU effort shouldn't be all that
> noticeable, should it?  That's my point, but then maybe I'm wrong.  Now,
> the WAL logging is annoying, so let's limit that too -- do it at most
> for, say, 20 dirty pages and at most 5 clean pages.

There isn't much difference between that and just doing it on first 5 
pages. Both of those numbers were pulled out of thin air, anyway. I'd 
rather just keep it simple.

- Heikki