Re: pgsql: Allow vacuum command to process indexes in parallel.

Hi,

On 2020-01-20 02:33:34 +0000, Amit Kapila wrote:
> Allow vacuum command to process indexes in parallel.
>
> This feature allows the vacuum to leverage multiple CPUs in order to
> process indexes. This enables us to perform index vacuuming and index
> cleanup with background workers. This adds a PARALLEL option to VACUUM
> command where the user can specify the number of workers that can be used
> to perform the command which is limited by the number of indexes on a
> table. Specifying zero as a number of workers will disable parallelism.
> This option can't be used with the FULL option.
>
> Each index is processed by at most one vacuum process. Therefore parallel
> vacuum can be used when the table has at least two indexes.
>
> The parallel degree is either specified by the user or determined based on
> the number of indexes that the table has, and further limited by
> max_parallel_maintenance_workers. The index can participate in parallel
> vacuum iff it's size is greater than min_parallel_index_scan_size.
>
> Author: Masahiko Sawada and Amit Kapila
> Reviewed-by: Dilip Kumar, Amit Kapila, Robert Haas, Tomas Vondra,
> Mahendra Singh and Sergei Kornilov
> Tested-by: Mahendra Singh and Prabhat Sahu
> Discussion:
> https://postgr.es/m/CAD21AoDTPMgzSkV4E3SFo1CH_x50bf5PqZFQf4jmqjk-C03BWg@mail.gmail.com
> https://postgr.es/m/CAA4eK1J-VoR9gzS5E75pcD-OH0mEyCdp8RihcwKrcuw7J-Q0+w@mail.gmail.com

Coverity is complaining that:
> ** CID ...: Incorrect expression (UNINTENDED_INTEGER_DIVISION)
> /srv/coverity/git/pgsql-git/postgresql/src/backend/commands/vacuum.c: 2078 in compute_parallel_delay()
>
>
> ________________________________________________________________________________________________________
> *** CID ...: Incorrect expression (UNINTENDED_INTEGER_DIVISION)
> /srv/coverity/git/pgsql-git/postgresql/src/backend/commands/vacuum.c: 2078 in compute_parallel_delay()
> 2072 shared_balance = pg_atomic_add_fetch_u32(VacuumSharedCostBalance, VacuumCostBalance);
> 2073
> 2074 /* Compute the total local balance for the current worker */
> 2075 VacuumCostBalanceLocal += VacuumCostBalance;
> 2076
> 2077 if ((shared_balance >= VacuumCostLimit) &&
> >>> CID ...: Incorrect expression (UNINTENDED_INTEGER_DIVISION)
> >>> Dividing integer expressions "VacuumCostLimit" and "nworkers", and then converting the integer quotient to type "double". Any remainder, or fractional part of the quotient, is ignored.
> 2078 (VacuumCostBalanceLocal > 0.5 * (VacuumCostLimit / nworkers)))
> 2079 {
> 2080 /* Compute sleep time based on the local cost balance */
> 2081 msec = VacuumCostDelay * VacuumCostBalanceLocal / VacuumCostLimit;
> 2082 pg_atomic_sub_fetch_u32(VacuumSharedCostBalance, VacuumCostBalanceLocal);
> 2083 VacuumCostBalanceLocal = 0;

Which seems like a fair enough complaint?

Greetings,

Andres Freund