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
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