From: Luke Lonergan <llonergan@greenplum.com>
To: PFC <lists@peufeu.com>
To: Jeffrey W. Baker <jwbaker@acm.org>
Cc: pgsql-performance@postgresql.org
Subject: Re: Huge Data sets, simple queries
Date: Wed, 01 Feb 2006 09:42:12 -0800
Message-ID: <C00632F4.1B71F%llonergan@greenplum.com> (raw)
In-Reply-To: <op.s4ad01lhcigqcu@apollo13>
PFC,
On 2/1/06 1:01 AM, "PFC" <lists@peufeu.com> wrote:
> 3- Read one block of A, then one block of B, then one block of A, etc.
> Essentially this is the same as the threaded case, except there's only one
> thread.
> 53 seconds total (with heavy seeking noise from the hdd).
>
> I was half expecting 3 to take the same as 2. It simulates, for
> instance,
> scanning a table and its index, or scanning 2 sort bins. Well, maybe one
> day...
This is actually interesting overall - I think what this might be showing is
that the Linux SW RAID1 is alternating I/Os to the mirror disks from
different processes (LWP or HWP both maybe?), but not within one process.
> It would be nice if the Kernel had an API for applications to tell it
> "I'm gonna need these blocks in the next seconds, can you read them in the
> order you like (fastest), from whatever disk you like, and cache them for
> me please; so that I can read them in the order I like, but very fast ?"
More control is always good IMO, but for now there's I/O reordering in the
SCSI layer and readahead tuning. There is POSIX fadvise() also to tell the
underlying I/O layer what the access pattern looks like.
- Luke
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Reply to all the recipients using the --to and --cc options:
reply via email
To: pgsql-performance@postgresql.org
Cc: llonergan@greenplum.com, lists@peufeu.com, jwbaker@acm.org
Subject: Re: Huge Data sets, simple queries
In-Reply-To: <C00632F4.1B71F%llonergan@greenplum.com>
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
This inbox is served by DDX for PostgreSQL; see mirroring instructions
for how to clone and mirror all data and code used for this inbox