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Comments and doc fixes for commit 40d964ec99.

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Reported-by: Justin Pryzby
Author: Justin Pryzby, with few changes by me
Reviewed-by: Amit Kapila and Sawada Masahiko
Discussion: https://postgr.es/m/20200322021801.GB2563@telsasoft.com
This commit is contained in:
Amit Kapila
2020-04-14 08:10:27 +05:30
parent 826ee1a019
commit a6fea120a7
5 changed files with 49 additions and 49 deletions
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20
src/backend/commands/vacuum.c
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@@ -2036,23 +2036,23 @@ vacuum_delay_point(void)
/*
* Computes the vacuum delay for parallel workers.
*
* The basic idea of a cost-based vacuum delay for parallel vacuum is to allow
* each worker to sleep proportional to the work done by it. We achieve this
* The basic idea of a cost-based delay for parallel vacuum is to allow each
* worker to sleep in proportion to the share of work it's done. We achieve this
* by allowing all parallel vacuum workers including the leader process to
* have a shared view of cost related parameters (mainly VacuumCostBalance).
* We allow each worker to update it as and when it has incurred any cost and
* then based on that decide whether it needs to sleep. We compute the time
* to sleep for a worker based on the cost it has incurred
* (VacuumCostBalanceLocal) and then reduce the VacuumSharedCostBalance by
* that amount. This avoids letting the workers sleep who have done less or
* no I/O as compared to other workers and therefore can ensure that workers
* who are doing more I/O got throttled more.
* that amount. This avoids putting to sleep those workers which have done less
* I/O than other workers and therefore ensure that workers
* which are doing more I/O got throttled more.
*
* We allow any worker to sleep only if it has performed the I/O above a
* certain threshold, which is calculated based on the number of active
* workers (VacuumActiveNWorkers), and the overall cost balance is more than
* VacuumCostLimit set by the system. The testing reveals that we achieve
* the required throttling if we allow a worker that has done more than 50%
* We allow a worker to sleep only if it has performed I/O above a certain
* threshold, which is calculated based on the number of active workers
* (VacuumActiveNWorkers), and the overall cost balance is more than
* VacuumCostLimit set by the system. Testing reveals that we achieve
* the required throttling if we force a worker that has done more than 50%
* of its share of work to sleep.
*/
static double