Re: Parallel Seq Scan vs kernel read ahead

Hi Thomas,

Some more data points:

create table t_heap as select generate_series(1, 100000000) i;

Query: select count(*) from t_heap;
shared_buffers=32MB (so that I don't have to clear buffers, OS page
cache)
OS: FreeBSD 12.1 with UFS on GCP
4 vCPUs, 4GB RAM Intel Skylake
22G Google PersistentDisk
Time is measured with \timing on.

Without your patch:

max_parallel_workers_per_gather    Time(seconds)
                              0           33.88s
                              1           57.62s
                              2           62.01s
                              6          222.94s

With your patch:

max_parallel_workers_per_gather    Time(seconds)
                              0           29.04s
                              1           29.17s
                              2           28.78s
                              6          291.27s

I checked with explain analyze to ensure that the number of workers
planned = max_parallel_workers_per_gather

Apart from the last result (max_parallel_workers_per_gather=6), all
the other results seem favorable.
Could the last result be down to the fact that the number of workers
planned exceeded the number of vCPUs?

I also wanted to evaluate Zedstore with your patch.
I used the same setup as above.
No discernible difference though, maybe I'm missing something:

Without your patch:

max_parallel_workers_per_gather    Time(seconds)
                              0           25.86s
                              1           15.70s
                              2           12.60s
                              6           12.41s


With your patch:

max_parallel_workers_per_gather    Time(seconds)
                              0           26.96s
                              1           15.73s
                              2           12.46s
                              6           12.10s

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