Re: Accounting of zero-filled buffers in EXPLAIN (BUFFERS) - Mailing list pgsql-hackers

On Thu, Jul 12, 2018 at 12:46 AM, Haribabu Kommi
<kommi.haribabu@gmail.com> wrote:
>> > On 2018-04-30 14:59:31 +1200, Thomas Munro wrote:
>> >> In EXPLAIN (BUFFERS), there are two kinds of cache misses that show up
>> >> as "reads" when in fact they are not reads at all:
>> >>
>> >> 1.  Relation extension, which in fact writes a zero-filled block.
>> >> 2.  The RBM_ZERO_* modes, which provoke neither read nor write.
>
> I checked the patch and I agree with the change 1). And regarding change 2)
> whether it is  zeroing the contents of the page or not, it does read?
> because if it exists in the buffer pool, we are counting them as hits
> irrespective
> of the mode? Am I missing something?

Further down in the function you can see that there is no read()
system call for the RBM_ZERO_* modes:

                if (mode == RBM_ZERO_AND_LOCK || mode ==
RBM_ZERO_AND_CLEANUP_LOCK)
                        MemSet((char *) bufBlock, 0, BLCKSZ);
                else
                {
                        ...
                        smgrread(smgr, forkNum, blockNum, (char *) bufBlock);
                        ...
                }

I suppose someone might argue that even when it's not a hit and it's
not a read, we might still want to count this buffer interaction in
some other way.  Perhaps there should be a separate counter?  It may
technically be a kind of cache miss, but it's nowhere near as
expensive as a synchronous system call like read() so I didn't propose
that.

Some more on my motivation:  In our zheap prototype, when the system
is working well and we have enough space, we constantly allocate
zeroed buffer pages at the insert point (= head) of an undo log and
drop pages at the discard point (= tail) in the background;
effectively a few pages just go round and round via the freelist and
no read() or write() syscalls happen.  That's something I'm very happy
about and it's one of our claimed advantages over the traditional heap
(which tends to read and dirty more pages), but EXPLAIN (BUFFERS)
hides this virtuous behaviour when comparing with the traditional
heap: it falsely and slanderously reports that zheap is reading undo
pages when it is not.  Of course I don't intent to litigate zheap
design in this thread, I just I figured that since this accounting is
wrong on principle and affects current PostgreSQL too (at least in
theory) I would propose this little patch independently.  It's subtle
enough that I wouldn't bother to back-patch it though.