Re: HOT patch - version 15

Pavan Deolasee wrote:
> Until patch version 14, defragmentation and pruning were separate
> operations, though we used to invoke both at the same time. The only
> difference was that pruning can work with a simple exclusive lock
> whereas defragmentation needs vacuum-strength lock. Tom argued
> that its not worth the complexity, so I changed that and clubbed
> both the operations together. What it means: pruning also now needs
> vacuum-strength lock and is always followed by defragmentation.

Interesting.  See my previous post to Heikki stating we might need to
change that based on Heikki test results.

> So as the patch stands (version 15), we call heap_page_prune_defrag()
> inside heap_release_fetch() and heapgetpage(), apart from the vacuum
> loop. It checks for PageIsPrunable() before proceeding. PageIsPrunable
> is set when a tuple is updated/deleted. So for read-mostly workloads
> heap_page_prune_defrag will mostly be no-op.
>
> If PageIsPrunable is set, cleanup lock is available (exclusive lock is tried
> conditionally, so we don't wait if there is contention) and we are running
> low on free space (right now if free space is less than 1.2 times the
> average
> tuple size or less than BLCKSZ/8), the entire page is pruned and
> fragmentation
> is repaired.

Looking at the patch I see:

+       /*
+        * Mark the page as clear of prunable tuples. If we find a tuple which
+        * may become prunable, we shall set the hint again.
+        */
+       PageClearPrunable(page);

I like the idea of the page hint bit, but my question is if there is a
long-running transaction, isn't each SELECT going to try to defragment a
page over and over again because there is still something prunable on
the page?

I think we need to find out how hard it would be to try the
defragmentation only on INSERT or UPDATE.  The hint bit might still be
useful.

> We also prune-defrag is vacuum lazy and vacuum full. But I assume we
> are not worried about these maintenance activities.

Right.

--
  Bruce Momjian  <bruce@momjian.us>          http://momjian.us
  EnterpriseDB                               http://www.enterprisedb.com

  + If your life is a hard drive, Christ can be your backup. +