Re: optimizing vacuum truncation scans

On Mon, Jul 13, 2015 at 12:06 PM, Haribabu Kommi
<kommi(dot)haribabu(at)gmail(dot)com> wrote:
> On Thu, Jul 9, 2015 at 5:36 PM, Haribabu Kommi <kommi(dot)haribabu(at)gmail(dot)com> wrote:
>>
>> I will do some performance tests and send you the results.
>
> Here are the performance results tested on my machine.
>
>
> Head vm patch vm+prefetch patch
>
> First vacuum 120sec <1sec <1sec
> second vacuum 180 sec 180 sec 30 sec
>
> I did some modifications in the code to skip the vacuum truncation by
> the first vacuum command.
> This way I collected the second vacuum time taken performance.
>
> I just combined your vm and prefetch patch into a single patch
> vm+prefetch patch without a GUC.
> I kept the prefetch as 32 and did the performance test. I chosen
> prefetch based on the current
> buffer access strategy, which is 32 for vacuum presently instead of an
> user option.
> Here I attached the modified patch with both vm+prefetch logic.
>
> I will do some tests on a machine with SSD and let you know the
> results. Based on these results,
> we can decide whether we need a GUC or not? based on the impact of
> prefetch on ssd machines.

Following are the performance readings on a machine with SSD.
I increased the pgbench scale factor to 1000 in the test instead of 500
to show a better performance numbers.

Head vm patch vm+prefetch patch

First vacuum 6.24 sec 2.91 sec 2.91 sec
second vacuum 6.66 sec 6.66 sec 7.19 sec

There is a small performance impact on SSD with prefetch.

Regards,
Hari Babu
Fujitsu Australia