Re: Fix parallel vacuum buffer usage reporting

On Fri, Apr 26, 2024 at 9:12 PM Alena Rybakina <lena.ribackina@yandex.ru> wrote:
>
> Hi!
>>
>> The same script was run, but using vacuum verbose analyze, and I saw the difference again in the fifth step:
>> with your patch: buffer usage: 32312 hits, 607 misses, 1566 dirtied
>> master: buffer usage: 32346 hits, 573 misses, 1360 dirtied
>
> Isn't there a chance for the checkpointer to run during this time? That could make the conditions between the two
runsslightly different and explain the change in buffer report. 
>
> [0]
https://github.com/postgres/postgres/blob/8a1b31e6e59631807a08a4e9465134c343bbdf5e/src/backend/access/heap/vacuumlazy.c#L2826-L2831
>
> Looking at the script, you won't trigger the problem.
>
> Thank you for the link I accounted it in my next experiments.
>
> I repeated the test without processing checkpoints with a single index, and the number of pages in the buffer used
almostmatched: 
>
> master branch: buffer usage: 32315 hits, 606 misses, 4486 dirtied
>
> with applied patch v4 version: buffer usage: 32315 hits, 606 misses, 4489 dirtied
>
> I think you are right - the problem was interfering with the checkpoint process, by the way I checked the first
versionpatch. To cut a long story short, everything is fine now with one index. 
>
> Just in case, I'll explain: I considered this case because your patch could have impact influenced it too.
>
> On 25.04.2024 10:17, Anthonin Bonnefoy wrote:
>
>
> On Wed, Apr 24, 2024 at 4:01 PM Alena Rybakina <lena.ribackina@yandex.ru> wrote:
>>
>> I tested the main postgres branch with and without your fix using a script that was written by me. It consists of
fivescenarios and I made a comparison in the logs between the original version of the master branch and the master
branchwith your patch: 
>
>  Hi! Thanks for the tests.
>
>> I have attached a test file (vacuum_check_logs.sql)
>
> The reporting issue will only happen if there's a parallel index vacuum and it will only happen if there's at least 2
indexes[0]. You will need to create an additional index. 
>
> Speaking of the problem, I added another index and repeated the test and found a significant difference:
>
> I found it when I commited the transaction (3):
>
> master: 2964 hits, 0 misses, 0 dirtied
>
> with applied patch v4 version: buffer usage: 33013 hits, 0 misses, 3 dirtied
>
> When I deleted all the data from the table and later started vacuum verbose again (4):
>
> master: buffer usage: 51486 hits, 0 misses, 0 dirtied
>
> with applied patch v4 version:buffer usage: 77924 hits, 0 misses, 0 dirtied
>
> when I inserted 1 million data into the table and updated it (5):
>
> master:buffer usage: 27904 hits, 5021 misses, 1777 dirtied
>
> with applied patch v4 version:buffer usage: 41051 hits, 9973 misses, 2564 dirtied
>
> As I see, the number of pages is significantly more than it was in the master branch and ,frankly, I couldn't fully
figureout if it was a mistake or not. 

I think that the patch fixes the problem correctly.

I've run pgindent and updated the commit message. I realized that
parallel vacuum was introduced in pg13 but buffer usage reporting in
VACUUM command was implemented in pg15. Therefore, in pg13 and pg14,
VACUUM (PARALLEL) is available but VACUUM (PARALLEL, VERBOSE) doesn't
show the buffer usage report. Autovacuum does show the buffer usage
report but parallel autovacuum is not supported. Therefore, we should
backpatch it down to 15, not 13.

I'm going to push the patch down to pg15, barring any objections.

Regards,

--
Masahiko Sawada
Amazon Web Services: https://aws.amazon.com