| From: | Robert Haas <robertmhaas(at)gmail(dot)com> |
|---|---|
| To: | Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> |
| Cc: | Jim Nasby <Jim(dot)Nasby(at)bluetreble(dot)com>, John Gorman <johngorman2(at)gmail(dot)com>, Stephen Frost <sfrost(at)snowman(dot)net>, pgsql-hackers <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: Parallel Seq Scan |
| Date: | 2015-01-22 13:53:16 |
| Message-ID: | CA+TgmobVCb7jnCn60j2KY2UQvkQJ2ECbySKY+MwcC2R0qUq8ag@mail.gmail.com |
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| Lists: | pgsql-hackers |
On Thu, Jan 22, 2015 at 5:57 AM, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> 1. Scanning block-by-block has negative impact on performance and
> I thin it will degrade more if we increase parallel count as that can lead
> to more randomness.
>
> 2. Scanning in fixed chunks improves the performance. Increasing
> parallel count to a very large number might impact the performance,
> but I think we can have a lower bound below which we will not allow
> multiple processes to scan the relation.
I'm confused. Your actual test numbers seem to show that the
performance with the block-by-block approach was slightly higher with
parallelism than without, where as the performance with the
chunk-by-chunk approach was lower with parallelism than without, but
the text quoted above, summarizing those numbers, says the opposite.
Also, I think testing with 2 workers is probably not enough. I think
we should test with 8 or even 16.
--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company
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