Re: Removing PD_ALL_VISIBLE

On Sun, 2013-01-20 at 22:19 -0500, Tom Lane wrote:
> Robert Haas <robertmhaas@gmail.com> writes:
> > On Fri, Jan 18, 2013 at 3:31 AM, Jeff Davis <pgsql@j-davis.com> wrote:
> >> So, I attached a new version of the patch that doesn't look at the VM
> >> for tables with fewer than 32 pages. That's the only change.
> 
> > That certainly seems worthwhile, but I still don't want to get rid of
> > this code.  I'm just not seeing a reason why that's something that
> > desperately needs to be done.
> 
> Yeah, I'm having the same problem.  Despite Jeff's test results, I can't
> help thinking that lack of PD_ALL_VISIBLE *will* hurt us under some
> workloads, and it's not obvious to me what benefit we get from dropping
> it.

OK. I respectfully disagree with the arguments I've seen so far, but we
can all move on.

I already had some more test results (again, thanks to Nathan Boley), so
I finished them up and attached them to the bottom of this email for the
archives (there's always hope, right?).

Regards,Jeff Davis


Test goal: is 32 is an appropriate threshold for using the VM after we
remove PD_ALL_VISIBLE?

Test setup: 500 31-page tables and 500 33-page tables. Test recent build
against patched version, with varying shared_buffers. The first test is
500 connections each doing 20 iterations of COUNT(*) against the 500
31-page tables. The next test is the same, except against the 33-page
tables.

Test results:
 (There were a few outliers I discarded as being too fast. It always
happened in the first run, and I strongly suspect the connections began
unevenly, leading to lower concurrency. They didn't seem to favor one
build over another.)

master, 31-page (first column is shared_buffers, second is range of
seconds):   32MB:  5.8 -  6.1   64MB:  3.1 -  3.7   96MB:  1.7 -  2.2  112MB:  0.6 -  1.1  128MB:  0.4 -  0.4  256MB:
0.4-  0.4
 

patch, 31-page (doesn't use VM because 31 is below threshold):   32MB:  5.1 -  5.9     64MB:  3.4 -  3.8   96MB:  1.7 -
2.0  112MB:  0.7 -  1.1  128MB:  0.4 -  0.5  256MB:  0.4 -  0.5
 

master, 33-page:   32MB:  5.9 -  7.0   64MB:  4.2 -  4.7   96MB:  2.4 -  2.8  112MB:  1.2 -  1.6  128MB:  0.5 -  0.5
256MB: 0.4 -  0.5
 

patch, 33-page (does use VM because 33 is above threshold):   32MB:  6.2 -  7.2   64MB:  4.4 -  4.7   96MB:  2.8 -  3.0
112MB:  1.7 -  1.8  128MB:  0.5 -  1.0  256MB:  0.4 -  0.5
 

Conclusion:

32 pages is a low enough threshold that skipping the VM is
insignificant.

When the 500 tables are 33 pages, and it does use the VM, we do see a
measurable cost under cache pressure. The penalty is fairly small (10%
ballpark), and this is a worst-case, so I don't think it's a problem.
From the last test results, we know it gets back to even by the time the
tables are 128 pages (1MB). So it could be that there is a slightly
higher threshold (between 32 and 128) that would be slightly better. But
given how specific this case is and how small the penalty is, I think 32
is a fine threshold.

Also, to reiterate, I focused my tests almost entirely on scans, though
some of the concern was around inserts/updates/deletes. I tried, but was
unable to show any difference on those tests. I suspect that the
bottleneck is elsewhere.