Re: parallel select
- From: <genegurevich@xxxxxxxxxxxx>
- To: greg@xxxxxxxxxxxxxxxxxx
- Date: Tue, 14 Jul 2009 10:08:43 -0500
Hi. Greg:
Thank you for your Email. I never knew that PX granule size is 100M. How
can I see that? Is there a DB parameter that determites that?
I have looked at the waits for the master and slave sessions. For the
master session I see SQL*Net message to and from client waits
For the slaves I see PX Deq Credit: send blkd and PX qref latch . Wait
time for the latter is 0. I read that the qref latch means that consumers
are not keeping up with the producers, but I am not sure how to correct
that.
The multiblock read count is currently 32 , The array size is 15, trimspool
is off.
The filesystems are not local, they are on SAN. The data is spooled to a
remote server. These will affect the timing, but there is
not much I can do about them
I will try increasing the MBRC and see if that helps the performance. But
my concern is that most of the transactions against this
database are quick small reads and I don't want to read extra data and slow
things down for the online users.
thank you
Gene Gurevich
Greg Rahn
<greg@structuredd
ata.org> To
Sent by: genegurevich@xxxxxxxxxxxx
oracle-l-bounce@f cc
reelists.org oracle-l@xxxxxxxxxxxxx
Subject
Re: parallel select
07/13/2009 06:51
PM
Please respond to
greg@structuredda
ta.org
On Mon, Jul 13, 2009 at 2:42 PM, <genegurevich@xxxxxxxxxxxx> wrote:
> I am watching a very long select * from a tables with about 500mil rows.
It
> runs under oracle 10.2.0.3 for hours how. The table is about 35-40G
> and the app team needs to dump it into a flat file. My DB block size is
8K,
> the extent size (this is a LMTS) is 300M.
That is quite a large extent size for a 40GB table. Generally I would
recommend auto allocate and just use a large initial/next if you want
to bypass the 64KB/1MB/... extent ranges and jump to the larger
extents. Using such a large extent size could result in nearly 300MB
being wasted from jumping above the HWM with a direct path load.
> I have been watching the v$session_longops and the message that I have
been
> seeing looks something like this:
>
> Rowid Range Scan: TABLE_NAME : 2899 out of 12647 Blocks done
>
> The table has about 5mil blocks. 12647 blocks is about one third of one
> extent. I run it with DOP=4. So it looks like the parallel slaves are
> each reading from one third of one extent at a time.
The PX granule size is 100MB, which happens to be 1/3 of the table
extent size of 300MB. A granule is the unit of work for PX severs.
> This operation seems
> to take about 40 minutes. Extrapolating to 5mil blocks yields a
> huge number with will not be satisfactory to the application. Why would
it
> take 40 minutes to read 100M worth of data? What could
> be the bottleneck?
I would recommend that you:
1) look at the ASH report see what the session waits are or
2) look at the wait events for the PX servers in v$session and see what
they are
No need to guess what the bottleneck would be when the wait interface
will tell you.
Things to consider:
- what is your MBRC? For FTS operations like this, 1MB is ideal (8KB
block * 128 MBRC). This should be the default if left unset.
- the speed of the filesystem you are writing to (hopefully a local FS)
- sqlplus settings (arraysize, trimspool, etc)
--
Regards,
Greg Rahn
http://structureddata.org
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