On Tue, Sep 19, 2017 at 8:55 PM, Peter Geoghegan <pg@bowt.ie> wrote:
> On Tue, Sep 19, 2017 at 4:47 PM, Claudio Freire <klaussfreire@gmail.com> wrote:
>> Maybe this is looking at the problem from the wrong direction.
>>
>> Why can't the page be added to the FSM immediately and the check be
>> done at runtime when looking for a reusable page?
>>
>> Index FSMs currently store only 0 or 255, couldn't they store 128 for
>> half-recyclable pages and make the caller re-check reusability before
>> using it?
>
> No, because it's impossible for them to know whether or not the page
> that their index scan just landed on recycled just a second ago, or
> was like this since before their xact began/snapshot was acquired.
>
> For your reference, this RecentGlobalXmin interlock stuff is what
> Lanin & Shasha call "The Drain Technique" within "2.5 Freeing Empty
> Nodes". Seems pretty hard to do it any other way.
I don't see the difference between a vacuum run and distributed
maintainance at _bt_getbuf time. In fact, the code seems to be in
place already.
_bt_page_recyclable seems to prevent old transactions from treating
those pages as recyclable already, and the description of the
technique in 2.5 doesn't seem to preclude doing the drain while doing
other operations. In fact, Lehman even considers the possibility of
multiple concurrent garbage collectors.
It's only a matter of making the page visible in the FSM in a way that
can be efficiently skipped if we want to go directly to a page that
actually CAN be recycled to avoid looping forever looking for a
recyclable page in _bt_getbuf. In fact, that's pretty much Lehman's
drain technique right there. FSM entries with 128 would be "the
queue", and FSM entries with 255 would be "the freelist". _bt_getbuf
can be the GC getting a buffer to try and recycle, give up after a few
tries, and get an actual recycleable buffer instead (or extend the
relationship). In essence, microvacuum.
Unless I'm missing something and RecentGlobalXmin somehow doesn't
exclude all old transactions, I don't see a problem.
Lanin & Shasha use reference counting to do GC wait during the drain,
and most of the ordering of operations needed there is because of
that, but using the xmin seems to make all those considerations moot.
An xact earlier than RecentGlobalXmin cannot have active transactions
able to follow links to that page AFAIK.
TBH, I didn't read the whole papers, though I probably will.
But, in essence, what's the difference of vacuum doing
if (_bt_page_recyclable(page)) { /* Okay to recycle this page */
RecordFreeIndexPage(rel,blkno); vstate->totFreePages++; stats->pages_deleted++; }
VS doing it in _bt_getbuf?
What am I missing?
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