On 2017-07-27 11:30, Masahiko Sawada wrote:
On Tue, Jul 25, 2017 at 2:27 AM, Claudio Freire <klaussfre...@gmail.com> wrote:
On Mon, Jul 24, 2017 at 2:20 PM, Claudio Freire <klaussfre...@gmail.com> wrote:
On Mon, Jul 24, 2017 at 2:10 PM, Sokolov Yura
<funny.fal...@postgrespro.ru> wrote:
On 2017-07-24 19:11, Claudio Freire wrote:
I was mostly thinking about something like the attached patch.

Simple, unintrusive, and shouldn't cause any noticeable slowdown.


Your change is small, clear, and currently useful for huge tables under high update load (until "allowing vacuum to use more than 1GB memory"
is merged).

In high-bloat conditions, it doesn't take long to accumulate 1GB of
dead tuples (which is about 178M tuples, btw).

The index scan takes way longer than the heap scan in that case.

But it still delays updating fsm until whole first batch of dead tuples cleared (ie all indices scanned, and all heap pages cleared), and on such
huge table it will be hours.

So, true, it will get delayed considerably. But as you realized,
there's not much point in trying to vacuum the FSM sooner, since it
won't be accurate shortly afterwards anyway. Dead line pointers do use
up a fair bit of space, especially on narrow tables.

In a particular table I have that exhibits this problem, most of the
time is spent scanning the index. It performs dozens of index scans
before it's done, so it would vacuum the FSM quite often enough, even
if I were to increase the mwm setting n-fold.

I hate to reply to myself, but I wanted to add: in any case, the case
I'm trying to avoid is the case where the FSM *never* gets vacuumed.
That's bad. But it may not be the phenomenon you're experiencing in
your tests.


I think the frequently vacuuming the FSM during long-time vacuum would
be worth to have in order to avoid a table bloating. The patch
triggers to vacuum the FSM after vacuumed the table and indexes but I
think that we can have a similar mechanism for a table with no index.

Regards,

--
Masahiko Sawada
NIPPON TELEGRAPH AND TELEPHONE CORPORATION
NTT Open Source Software Center

I could be wrong, but it looks like table without index doesn't
suffer that much. Since there is no indices, there is only one stage -
scanning heap, and no quadratic behavior cause of full dead-tuple array
and repeating index vacuuming.

--
Sokolov Yura aka funny_falcon
Postgres Professional: https://postgrespro.ru
The Russian Postgres Company


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