Re: [HACKERS] Block level parallel vacuum

On Wed, Dec 4, 2019 at 1:58 AM Masahiko Sawada
<masahiko.sawada@2ndquadrant.com> wrote:
>
> On Tue, 3 Dec 2019 at 11:55, Amit Kapila <amit.kapila16@gmail.com> wrote:
> >
> In your code, I think if two workers enter to compute_parallel_delay
> function at the same time, they add their local balance to
> VacuumSharedCostBalance and both workers sleep because both values
> reach the VacuumCostLimit.
>

True, but isn't it more appropriate because the local cost of any
worker should be ideally added to shared cost as soon as it occurred?
I mean to say that we are not adding any cost in shared balance
without actually incurring it.   Then we also consider the individual
worker's local balance as well and sleep according to local balance.

>
> > 2. I think if we cam somehow disallow very small indexes to use parallel workers, then it will be better.   Can we
use min_parallel_index_scan_size to decide whether a particular index can participate in a parallel vacuum?
 
>
> I think it's a good idea but I'm concerned that the default value of
> min_parallel_index_scan_size, 512kB, is too small for parallel vacuum
> purpose. Given that people who want to use parallel vacuum are likely
> to have a big table the indexes that can be skipped by the default
> value would be only brin indexes, I think.
>

Yeah or probably hash indexes in some cases.

> Also I guess that the
> reason why the default value is small is that
> min_parallel_index_scan_size compares to the number of blocks being
> scanned during index scan, not whole index. On the other hand in
> parallel vacuum we will compare it to the whole index blocks because
> the index vacuuming is always full scan. So I'm also concerned that
> user will get confused about reasonable setting.
>

This setting is about how much of index we are going to scan, so I am
not sure if it matters whether it is part or full index scan.  Also,
in an index scan, we will launch multiple workers to scan that index
and here we will consider launching just one worker.

> As another idea how about using min_parallel_table_scan_size instead?
>

Hmm, yeah, that can be another option, but it might not be a good idea
for partial indexes.

> That is, we cannot do parallel vacuum on the table smaller than that
> value.
>

Yeah, that makes sense, but I feel if we can directly target index
scan size that may be a better option.  If we can't use
min_parallel_index_scan_size, then we can consider this.

-- 
With Regards,
Amit Kapila.
EnterpriseDB: http://www.enterprisedb.com