Re: Speed up Clog Access by increasing CLOG buffers

On 09/23/2016 05:10 AM, Amit Kapila wrote:
> On Fri, Sep 23, 2016 at 5:14 AM, Tomas Vondra
> <tomas.vondra@2ndquadrant.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
 

>>
>> It of course begs the question what kernel version is running on
>> the machine used by Dilip (i.e. cthulhu)? Although it's a Power
>> machine, so I'm not sure how much the kernel matters on it.
>>
>
> cthulhu is a x86 m/c and the kernel version is 3.10. Seeing, the
> above results I think kernel version do matter, but does that mean
> we ignore the benefits we are seeing on somewhat older kernel
> version. I think right answer here is to do some experiments which
> can show the actual contention as suggested by Robert and you.
>

Yes, I think it'd be useful to test a new kernel version. Perhaps try 
4.5.x so that we can compare it to my results. Maybe even try using my 
shell script

>>
>> There are results for 64, 128 and 192 clients. Why should we care
>> about numbers in between? How likely (and useful) would it be to
>> get improvement with 96 clients, but no improvement for 64 or 128
>> clients?>>
>
> The only point to take was to see from where we have started seeing
> improvement, saying that the TPS has improved from >=72 client count
> is different from saying that it has improved from >=128.
>

I find the exact client count rather uninteresting - it's going to be 
quite dependent on hardware, workload etc.
>>
>> I don't dare to suggest rejecting the patch, but I don't see how
>> we could commit any of the patches at this point. So perhaps
>> "returned with feedback" and resubmitting in the next CF (along
>> with analysis of improvedworkloads) would be appropriate.
>>
>
> Agreed with your conclusion and changed the status of patch in CF
> accordingly.
>

+1


-- 
Tomas Vondra                  http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services