I've been looking at the way we do vacuums.
The fundamental performance issue is that a vacuum generates
nheapblocks+nindexblocks+ndirtyblocks I/Os. Vacuum cost delay helps to
spread the cost like part payment, but the total is the same. In an I/O
bound system, the extra I/O directly leads to less throughput.
Therefore, we need to do less I/O. Dead space map helps by allowing us
to skip blocks that don't need vacuuming, reducing the # of I/Os to
2*ndirtyblocks+nindexblocks. That's great, but it doesn't help us if the
dead tuples are spread uniformly.
If we could piggyback the vacuum I/Os to the I/Os that we're doing
anyway, vacuum wouldn't ideally have to issue any I/O of its own. I've
tried to figure out a way to do that.
Vacuum is done in 3 phases:
1. Scan heap
2. Vacuum index
3. Vacuum heap
Instead of doing a sequential scan, we could perform the 1st phase by
watching the buffer pool, scanning blocks for dead tuples when they're
in memory and keeping track of which pages we've seen. When all pages
have been seen, the tid list is sorted and 1st phase is done.
In theory, the index vacuum could also be done that way, but let's
assume for now that indexes would be scanned like they are currently.
The 3rd phase can be performed similarly to the 1st phase. Whenever a
page enters the buffer pool, we check the tid list and remove any
matching tuples from the page. When the list is empty, vacuum is complete.
Of course, there's some issues in the design as described above. For
example, the vacuum might take a long time if there's cold spots in the
table. In fact, a block full of dead tuples might never be visited again.
A variation of the scheme would be to keep scanning pages that are in
cache, until the tid list reaches a predefined size, instead of keeping
track of which pages have already been seen. That would deal better with
tables with hot and cold spots, but it couldn't advance the relfrozenid
because there would be no guarantee that all pages are visited. Also, we
could start 1st phase of the next vacuum, while we're still in the 3rd
phase of previous one.
Also, after we've seen 95% of the pages or a timeout expires, we could
fetch the rest of them with random I/O to let the vacuum finish.
I'm not sure how exactly this would be implemented. Perhaps bgwriter or
autovacuum would do it, or a new background process. Presumably the
process would need access to relcache.
One issue is that if we're trying to vacuum every table simultaneously
this way, we'll need more overall memory for the tid lists. I'm hoping
there's a way to implement this without requiring shared memory for the
tid lists, that would make the memory management a nightmare. Also, we'd
need changes to bufmgr API to support this.
This would work nicely with the DSM. The list of pages that need to be
visited in phase 1 could be initialized from the DSM, largely avoiding
the problem with cold spots.
Any thoughts before I start experimenting?
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