Re: RFC: Allow EXPLAIN to Output Page Fault Information

On Mon Dec 30, 2024 at 5:39 PM CET, Tom Lane wrote:
> Bruce Momjian <bruce(at)momjian(dot)us> writes:
>> I certainly would love to see storage I/O numbers as distinct from
>> kernel read I/O numbers.
>
> Me too, but I think it is 100% wishful thinking to imagine that
> page fault counts match up with that.

Okay I played around with this patch a bit, in hopes of proving you
wrong. But I now agree with you. I cannot seem to get any numbers out of
this that make sense.

The major page fault numbers are always zero, even after running:

echo 1 > /proc/sys/vm/drop_caches

If Takahori has a way to get some more useful insights from this patch,
I'm quite interested in the steps he took (I might very well have missed
something obvious).

**However, I think the general direction has merit**: Changing this patch to
use `ru_inblock`/`ru_oublock` gives very useful insights. `ru_inblock`
is 0 when everything is in page cache, and it is very high when stuff is
not. I was only hacking around and basically did this:

s/ru_minflt/ru_inblock/g
s/ru_majflt/ru_oublock/g

Obviously more is needed. We'd probably want to show these numbers in
useful units like MB or something. Also, maybe there's some better way
of getting read/write numbers for the current process than
ru_inblock/ru_oublock (but this one seems to work at least reasonably
well).

One other thing that I noticed when playing around with this, which
would need to be addressed: Parallel workers need to pass these values
to the main process somehow, otherwise the IO from those processes gets lost.

For the record, the queries I used to test this patch were:

create table t_big(a int, b text);
insert into t_big SELECT i, repeat(i::text, 200) FROM generate_series(1, 3000000) i;
explain (ANALYZE, PAGEFAULTS) select max(a), max(b) from t_big;
explain (analyze, PAGEFAULTS) insert into t_big SELECT i, repeat(i::text, 200) FROM generate_series(1, 3000000) i;

And then seeing if there was any difference in the explain analyze
output after running the following (as root):

echo 1 > /proc/sys/vm/drop_caches