Help: Strange PGA Behavior

Question

hi,
during R/3 4.7 Export, PGA is very stranger. Could somebody give me help?
DB version, oracle 9.2.0.4 Enterprise, OS, SunOS 5.8（machine is old, disk has poor performance）
key parameters:
pgamax_size=1073741824
db_cache_size=1000000000
pga_aggregate_target=2000000000
workarea_size_policy=auto    
PGA statistics information:
SQL> select * from v$pgastat;
NAME                                                                  VALUE UNIT
aggregate PGA target parameter                                   2000000000 bytes
aggregate PGA auto target                                                 0 bytes
global memory bound                                                       0 bytes
total PGA inuse                                                    23502848 bytes
total PGA allocated                                                58076160 bytes
maximum PGA allocated                                              59555840 bytes
total freeable PGA memory                                           2883584 bytes
PGA memory freed back to OS                                          327680 bytes
total PGA used for auto workareas                                   1324032 bytes
maximum PGA used for auto workareas                                 1726464 bytes
total PGA used for manual workareas                                       0 bytes
NAME                                                                  VALUE UNIT
maximum PGA used for manual workareas                                     0 bytes
over allocation count                                                     0
bytes processed                                                  1.6069E+10 bytes
extra bytes read/written                                         6.3215E+10 bytes
cache hit percentage                                                  20.26 percent
16 rows selected.
Sort Location:
SQL> select name, value from v$sysstat where name in ('sorts (memory)', 'sorts (disk)');
NAME                                                                  VALUE
sorts (memory)                                                        13958
sorts (disk)                                                             22
Qestions:
1. why "aggregate PGA auto target"  is  0 bytes?
2. why only a small part of PGA is used, but some sorts are performed in disk?
Any tip is appreciated.
Best Regard,
Rongfeng
Edited by: Rongfeng Shi on Aug 22, 2008 10:39 AM

Answer 1

Hello Rongfeng,
you have selected a very hard topic but let's start.
At first the sapnote #936441 which is pointing you to the parameter pgamax_size is telling the "half-truth".
If everyone asks something about that topic i always refer to the "ADVANCED MANAGEMENT OF WORKING AREAS IN ORACLE 9I/10G" from Joze Senegacnik. This document is very detailed and has many explanations.
http://tonguc.yilmaz.googlepages.com/JozeSenegacnik-PGAMemoryManagementvO.zip
> 1. why "aggregate PGA auto target" is 0 bytes?
Check the statement from the paper: "aggregate PGA auto target u2013 SQL memory target u2013 P_A_T minus used memory for other categories in PGAs"
> 2. why only a small part of PGA is used, but some sorts are performed in disk?
Because your PGA is "only" 1.8 GB and you have the following limitations:
> SMMMAX_SIZE
> This parameter defines the maximum work area size in auto mode (serial) and defaults to 50% of pgamax_size. In Oracle 9i/10gR1 the maximum value is 0.1GB (100MB), while in 10gR2 it can go up to 0.25GB as we will see later.After one of the instance restarts I found that the pgamax_size and smmmax_size had even bigger values with P_A_T set to 4GB.
Regards
Stefan