From: Mark Mielke <mark@mark.mielke.cc>
To: Scott Marlowe <scott.marlowe@gmail.com>
Cc: Gregory Stark <stark@enterprisedb.com>
Cc: Luke Lonergan <llonergan@greenplum.com>
Cc: Carlo Stonebanks <stonec.register@sympatico.ca>
Cc: pgsql-performance@postgresql.org
Subject: Re: random_page_costs - are defaults of 4.0 realistic for SCSI RAID 1
Date: Mon, 10 Sep 2007 18:22:06 -0400
Message-ID: <46E5C38E.8030806@mark.mielke.cc> (raw)
In-Reply-To: <dcc563d10709101459o11451c33y8cab7dcf1e00a93f@mail.gmail.com>
References: <C30AF4C7.4230B%llonergan@greenplum.com>
<87y7fekxga.fsf@oxford.xeocode.com>
<dcc563d10709101459o11451c33y8cab7dcf1e00a93f@mail.gmail.com>
Scott Marlowe wrote:
> On 9/10/07, Gregory Stark <stark@enterprisedb.com> wrote:
>
>> "Luke Lonergan" <llonergan@greenplum.com> writes:
>>
>>> Should be a lot higher, something like 10-15 is approximating accurate.
>>>
>> Most people's experience is that due to Postgres underestimating the benefits
>> of caching lowering the random_page_cost is helpful.
>>
> Quite often the real problem is that they have effective_cache_size
> too small, and they use random_page_cost to get the planner to switch
> to index scans on small tables. With a large effective_cache_size and
> small to moderate table (i.e. it fits in memory pretty handily) the
> planner seems much better in the last few major releases about picking
> an index over a sequential scan
In my case, I set effective_cache_size to 25% of the RAM available to
the system (256 Mbytes), for a database that was about 100 Mbytes or
less. I found performance to increase when reducing random_page_cost
from 4.0 to 3.0.
For a database that truly fits entirely in memory, I assume
random_page_cost is closer to 1.0. The planner should know that there is
no significant seek cost for RAM.
I will try to compare results tonight using 8.2. The last time I checked
may have been 8.1. I am also curious to see what the current algorithm
is with regard to effective_cache_size.
Cheers,
mark
--
Mark Mielke <mark@mielke.cc>
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To: pgsql-performance@postgresql.org
Cc: mark@mark.mielke.cc, scott.marlowe@gmail.com, stark@enterprisedb.com, llonergan@greenplum.com, stonec.register@sympatico.ca
Subject: Re: random_page_costs - are defaults of 4.0 realistic for SCSI RAID 1
In-Reply-To: <46E5C38E.8030806@mark.mielke.cc>
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