Re: Speed up Clog Access by increasing CLOG buffers

On Fri, Sep 23, 2016 at 6:05 PM, Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com>
wrote:

> On 09/23/2016 05:10 AM, Amit Kapila wrote:
>
>> On Fri, Sep 23, 2016 at 5:14 AM, Tomas Vondra
>> <tomas(dot)vondra(at)2ndquadrant(dot)com> wrote:
>>
>>> On 09/21/2016 08:04 AM, Amit Kapila wrote:
>>>
>>>>
>>>>
>>> (c) Although it's not visible in the results, 4.5.5 almost perfectly
>>> eliminated the fluctuations in the results. For example when 3.2.80
>>> produced
>>> this results (10 runs with the same parameters):
>>>
>>> 12118 11610 27939 11771 18065
>>> 12152 14375 10983 13614 11077
>>>
>>> we get this on 4.5.5
>>>
>>> 37354 37650 37371 37190 37233
>>> 38498 37166 36862 37928 38509
>>>
>>> Notice how much more even the 4.5.5 results are, compared to 3.2.80.
>>>
>>>
>> how long each run was? Generally, I do half-hour run to get stable
>> results.
>>
>>
> 10 x 5-minute runs for each client count. The full shell script driving
> the benchmark is here: http://bit.ly/2doY6ID and in short it looks like
> this:
>
> for r in `seq 1 $runs`; do
> for c in 1 8 16 32 64 128 192; do
> psql -c checkpoint
> pgbench -j 8 -c $c ...
> done
> done

I see couple of problems with the tests:

1. You're running regular pgbench, which also updates the small tables. At
scale 300 and higher clients, there is going to heavy contention on the
pgbench_branches table. Why not test with pgbench -N? As far as this patch
is concerned, we are only interested in seeing contention on
ClogControlLock. In fact, how about a test which only consumes an XID, but
does not do any write activity at all? Complete artificial workload, but
good enough to tell us if and how much the patch helps in the best case. We
can probably do that with a simple txid_current() call, right?

2. Each subsequent pgbench run will bloat the tables. Now that may not be
such a big deal given that you're checkpointing between each run. But it
still makes results somewhat hard to compare. If a vacuum kicks in, that
may have some impact too. Given the scale factor you're testing, why not
just start fresh every time?

Thanks,
Pavan

--
Pavan Deolasee http://www.2ndQuadrant.com/
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