Re: WIP: Avoid creation of the free space map for small tables

On 11/29/18, Amit Kapila <amit.kapila16@gmail.com> wrote:
> On Thu, Nov 29, 2018 at 3:07 PM John Naylor <jcnaylor@gmail.com> wrote:
>> Done. I tried adding it to several schedules, but for some reason
>> vacuuming an empty table failed to truncate the heap to 0 blocks.
>> Putting the test in its own group fixed the problem, but that doesn't
>> seem ideal.
>>
>
> It might be because it fails the should_attempt_truncation() check.
> See below code:
>
> if (should_attempt_truncation(vacrelstats))
> lazy_truncate_heap(onerel, vacrelstats, vac_strategy);

I see. I think truncating the FSM is not essential to show either the
old or new behavior -- I could skip that portion to enable running the
test in a parallel group.

>> Can you please repeat the copy test you have done above with
>> fillfactor as 20 and 30?
>
> I will send the results in a separate email soon.

I ran the attached scripts which populates 100 tables with either 4 or
8 blocks. The test tables were pre-filled with one tuple and vacuumed
so that the FSMs were already created when testing the master branch.
The patch branch was compiled with a threshold of 8, but testing
inserts of 4 pages effectively simulates a threshold of 4. Config was
stock, except for fsync = off. I took the average of 40 runs (2
complete tests of 20 runs each) after removing the 10% highest and
lowest:

fillfactor=20
# blocks	master		patch
4			19.1ms		17.5ms
8			33.4ms		30.9ms

fillfactor=30
# blocks	master		patch
4			20.1ms		19.7ms
8			34.7ms		34.9ms

It seems the patch might be a bit faster with fillfactor=20, but I'm
at a loss as to why that would be. Previous testing with a higher
threshold showed a significant performance penalty starting around 10
blocks [1], but that used truncation rather than deletion, and had a
fill-factor of 10.

--
[1] https://www.postgresql.org/message-id/CAJVSVGWCRMyi8sSqguf6PfFcpM3hwNY5YhPZTt-8Q3ZGv0UGYw%40mail.gmail.com

-John Naylor