Re: Deferring some AtStart* allocations?

On Sun, Jun 29, 2014 at 9:12 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> Meh.  Even "SELECT 1" is going to be doing *far* more pallocs than that to
> get through raw parsing, parse analysis, planning, and execution startup.
> If you can find a few hundred pallocs we can avoid in trivial queries,
> it would get interesting; but I'll be astonished if saving 4 is measurable.

I got nerd-sniped by this problem today, probably after staring at the
profiling data very similar to what led Andres to ask the question in
the first place.  The gains are indeed not measurable on a
macrobenchmark, but I had to write the patch to figure that out, so
here it is.

Although the gain isn't a measurable percentage of total runtime, it
does appear to be a significant percentage of the palloc traffic on
trivial queries. I did 30-second SELECT-only pgbench runs at scale
factor 10, using prepared queries.  According to perf, on unpatched
master, StartTransactionCommand accounts for 11.46% of the calls to
AllocSetAlloc.  (I imagine this is by runtime, not by call count, but
it probably doesn't matter much either way.)  But with this patch,
StartTransactionCommand's share drops to 4.43%.  Most of that is
coming from AtStart_Inval(), which wouldn't be hard to fix either.

I'm inclined to think that tamping down palloc traffic is worthwhile
even if we can't directly measure the savings on a macrobenchmark.
AllocSetAlloc is often at or near the top of profiling results, but
there's rarely a single caller responsible for enough of those calls
for to make it worth optimizing. But that means that if we refuse to
take patches that save just a few pallocs, we're basically giving up
on ever improving anything in this area, and that doesn't seem like a
good idea either; it's only going to get slowly worse over time as we
add more features.

BTW, if anyone's curious what the biggest source of AllocSetAlloc
calls is on this workload, the answer is ExecInitIndexScan(), which
looks to be indirectly responsible for almost half the total (or just
over half, with the patch applied).  That apparently calls a lot of
different things that allocate small amounts of memory, and it
accounts for nearly all of the AllocSetAlloc traffic that originates
from PortalStart().  I haven't poked around in there enough to know
whether there's anything worth optimizing, but given the degree to
which this patch shifts the profile, I bet the number that it takes to
make a material savings is more like a few dozen than a few hundred.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
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