Performance Tuning :: Queries On PGA Settings And Related Memory Management?

I have few queries on PGA memory management.Since these queries are based on 2-3 examples not exactly same by nature I am summarising it after my understanding for the same

As I understand many workareas can be allocated to a single sql statement and number and sizes of theses workareas is controlled internally by Oracle when Automatic Memory management (PGA_aggregate_target and workarea_size_policy=Auto are set) Since many sessions share the PGA memory, the amount of memory available to each session may vary and if less amount of memory is available for a session for sorting then TEMP tablespace is used

[1] Can we say paging happens and can be checked at this time?

[2] Is there a difference in handling memory while populating pl/sql tables?

As I have encountered ora-04030 while some our developers were populating pl/sql tables but never encountered this error for sorting, hash joins etc Though I don't remember the width of pl/sql table, I am sure the developer used 'LIMIT' clause during bulk collect and still faced the issue.

With a single session on the server, I noticed that the difference in values displayed issuing 'free' command in linux and output values from sesstat did not match at all while there wasn't any heavy OS process involved during the period. I was expecting 'used' and 'free' values displayed by free command (linux) will change and difference would be approximately equals 'before and after values of session pga memory.

[3] Isn't it expected to match?

[4] Can we say in dedicated server, at any moment of time, the SUM of 'session pga memory' represents all the memory used by Oracle SGA, at that point of time?

select sum(value)/1024/1024 "memory in MB" from v$sesstat where statistic#=20;

During one of the tests I got following output (divide value by 10 for my visibility and avoid formatting)

SQL> select a.name, to_char(b.value/10, '999,999,999') value
from v$statname a, v$mystat b
where a.statistic# = b.statistic#
and a.name like '%ga memory%'; 2 3 4

[code]...

The above query is showing above values even when the pl/sql block execution is completed 30 minutes back

[5] Do we call this as 'memory leak' where memory is not released even while some time has passed since session has done something?Of course I am not checking at OS level as mentioned in question [3] above the values won't match!

Still the output of free command for reference(After the pl/sql block executed)

SQL> !free
total used free shared buffers cached
Mem: 3016796 2999660 17136 0 4308 1173260
-/+ buffers/cache: 1822092 1194704
Swap: 1048568 636124 412444

--(After the pl/sql block executed)

SQL> select * from v$pgastat;
NAMEVALUEUNIT
aggregate PGA target parameter 524288000bytes
aggregate PGA auto target 456256512bytes
global memory bound 26214400bytes
total PGA inuse 17328128bytes

[code]...

[6] What could be the significance of negative values of 'session pga memory/max'?

Last We have an OLTP system and in the night we run batch processes in 2-4 sessions

Suppose I have 10 GB RAM and with PGA setting of 3.5 GB Now I want the batch process sessions to use max possible memory during nighttime and toggle the setting back in the morning

[7] With above settings (10 GB RAM and 3.5 GB PGA) how can I divide the memory among 4 sessions?

Shall I set 1) PGA_aggregate_target=0 2)Workarea_size_policy=manual 3) Sort_are_size 4) Hash_area_size

[8] What would be approx values for parameter 3 and 4? will it be straight 3.5 GB/ 4?