PGA usage

To find the total PGA memory used by processes:

SELECT ROUND(SUM(pga_used_mem)/(1024*1024),2) PGA_USED_MB FROM v$process;

To find PGA usage for a specific session

SELECT SID, b.NAME, ROUND(a.VALUE/(1024*1024),2) MB FROM 
v$sesstat a,  v$statname b
WHERE (NAME LIKE '%session uga memory%' OR NAME LIKE '%session pga memory%')
AND a.statistic# = b.statistic# 
AND SID = 80;

To calculate the amount of memory that you gone need for PGA, estimate the number of maximum connected sessions and run:

SELECT :MAX_CONNECTED_SESSIONS*(2048576+P1.VALUE+P2.VALUE)/(1024*1024) YOU_NEED_PGA_MB 
FROM V$PARAMETER P1, V$PARAMETER P2
WHERE P1.NAME = 'sort_area_size'
AND P2.NAME = 'hash_area_size';

To change PGA memory parameter

ALTER SYSTEM SET pga_aggregate_target = 3500M SCOPE=BOTH;