From: Gregory Stark <stark@enterprisedb.com>
To: Robert Haas <robertmhaas@gmail.com>
Cc: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Alan Stange <stange@rentec.com>
Cc: Jignesh K. Shah <J.K.Shah@sun.com>
Cc: Greg Smith <gsmith@gregsmith.com>
Cc: pgsql-performance\@postgresql.org <pgsql-performance@postgresql.org>
Subject: Re: 8.4 Performance improvements: was Re: Proposal of tunable fix for scalability of 8.4
Date: Sat, 14 Mar 2009 04:02:15 +0000
Message-ID: <878wn84urc.fsf@oxford.xeocode.com> (raw)
In-Reply-To: <603c8f070903131937t1c33fa97k29e7d8d07fa2e83@mail.gmail.com>
References: <C5DDBB25.3333%scott@richrelevance.com>
<49B922C0.6050700@sun.com>
<alpine.GSO.2.01.0903121613270.1925@westnet.com>
<49BA5537.8060002@sun.com>
<87prgl4jbh.fsf@oxford.xeocode.com>
<49BA79A4.9030208@rentec.com>
<12431.1236965004@sss.pgh.pa.us>
<87d4ck50h1.fsf@oxford.xeocode.com>
<26666.1236996376@sss.pgh.pa.us>
<603c8f070903131937t1c33fa97k29e7d8d07fa2e83@mail.gmail.com>
Robert Haas <robertmhaas@gmail.com> writes:
> On Fri, Mar 13, 2009 at 10:06 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>
>> I assume you meant effective_io_concurrency. We'd still need a special
>> case because the default is currently hard-wired at 1, not 0, if
>> configure thinks the function exists. Also there's a posix_fadvise call
>> in xlog.c that that parameter doesn't control anyhow.
>
> I think 1 should mean no prefetching, rather than 0. If the number of
> concurrent I/O requests was 0, that would mean you couldn't perform
> any I/O at all.
That is actually how I had intended it but apparently I messed it up at some
point such that later patches were doing some prefetching at 1 and there was
no way to disable it. When Tom reviewed it he corrected the inability to
disable prefetching by making 0 disable prefetching.
I didn't think it was worth raising as an issue but I didn't realize we were
currently doing prefetching by default? i didn't realize that. Even on a
system with posix_fadvise there's nothing much to be gained unless the data is
on a RAID device, so the original objection holds anyways. We shouldn't do any
prefetching unless the user tells us to.
--
Gregory Stark
EnterpriseDB http://www.enterprisedb.com
Ask me about EnterpriseDB's 24x7 Postgres support!
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To: pgsql-performance@postgresql.org
Cc: stark@enterprisedb.com, robertmhaas@gmail.com, tgl@sss.pgh.pa.us, stange@rentec.com, J.K.Shah@sun.com, gsmith@gregsmith.com
Subject: Re: 8.4 Performance improvements: was Re: Proposal of tunable fix for scalability of 8.4
In-Reply-To: <878wn84urc.fsf@oxford.xeocode.com>
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