Comments on: Tuning Oracle to Make a Query Slower

By: DanyC

DanyC — Wed, 31 Oct 2012 23:25:23 +0000

One word buddy: Many thanks!!

Dany

By: osborne

osborne — Tue, 30 Oct 2012 20:27:23 +0000

$ cat table_SIZE.SQL
set lines 155
compute sum of totalsize_megs on report
break on report
col owner for a20
col segment_name for a30
col segment_type for a10
col totalsize_megs for 999,999.9
select * from (
select owner, segment_name, segment_type type,
sum(bytes/1024/1024) as totalsize_megs,
tablespace_name
from dba_segments
where owner like nvl(‘&owner’,owner)
and segment_name like nvl(‘&table_name’,segment_name)
and segment_type like nvl(‘&type’,segment_type)
group by owner, segment_name, tablespace_name, segment_type
order by 4 desc
)
where rownum < 30
order by 4;

$ cat realfreemem.sh
#!/bin/ksh

FREEMEM=`cat /proc/meminfo | grep MemFree: | sed -e 's/ */ /g' | cut -f2 -d' '`
CACHED=`cat /proc/meminfo | grep Cached: | grep -v SwapCached: | sed -e 's/ */ /g' | cut -f2 -d' '`
TOTALMEM=`cat /proc/meminfo | grep MemTotal: | sed -e 's/ */ /g' | cut -f2 -d' '`

((FREEMEM=${FREEMEM}/1024))
((CACHED=${CACHED}/1024))
((TOTALMEM=${TOTALMEM}/1024))

((TOTALFREE=${FREEMEM}+${CACHED}))
((REALFREE=$TOTALFREE*100))
((REALFREEPCT=$REALFREE/$TOTALMEM))

if [ "$1" = "-a" ]; then
echo 'Free Memory:' ${FREEMEM}'M'
echo 'Cached Memory:' ${CACHED}'M'
echo 'Total Free Memory:' ${TOTALFREE}'M'
echo 'Total Memory:' ${TOTALMEM}'M'
echo 'Percent Memory Free (including cache):' ${REALFREEPCT}'%'
else
echo 'Percent Memory Free (including cache):' ${REALFREEPCT}'%'
fi

return $REALFREEPC

By: Danyc

Danyc — Tue, 23 Oct 2012 09:18:35 +0000

Hi Kerry,

By any chance, can you show the output of the fss/ realfreemem.sh and table_size scripts?

Many thanks,
Dani

By: Alex

Alex — Mon, 05 Dec 2011 11:32:10 +0000

an alternative to flushing caches in linux : echo 3 > /proc/sys/vm/drop_caches

By: osborne

osborne — Wed, 23 Nov 2011 20:52:12 +0000

Gwen,

Absolutely, if we wanted to make this query run fast, then making the buffer cache bigger would do it. But I was trying to make it go slower ;) – that’s why it was interesting (to me anyway).

Frits,

Yes, in 32 bit days there were distinct advantages to having big file system cache in some cases (like yours).

By: Frits Hoogland

Frits Hoogland — Wed, 23 Nov 2011 20:06:56 +0000

More or less exactly what I encountered some 5-7 years ago. Large solaris machine (at that time; 32GB memory), database user-data size 1-2GB. In order to make the processing go faster, I routinely advised to mount the data filesystems with ‘forcedirectio’….resulting in the database performance getting much times slower, because now it needed to be read from disk, instead from the operating system cache.

By: Gwen Shapira

Gwen Shapira — Wed, 23 Nov 2011 19:29:39 +0000

In short, if you can read 10g table into the OS cache and not the SGA buffer cache, you are not sizing your SGA right.