Without DB Restart:

show parameter sga_target;
alter system set sga_target = 10G;

With DB Restart:

show parameter sga_max_size;
alter system set sga_max_size = 12G scope=spfile;

Memory recommendation

SQL> select * from v$memory_target_advice order by memory_size;
MEMORY_SIZE MEMORY_SIZE_FACTOR ESTD_DB_TIME ESTD_DB_TIME_FACTOR VERSION
180 .5 458 1.344 0
270 .75 367 1.0761 0
360 1 341 1 0
450 1.25 335 .9817 0
540 1.5 335 .9817 0
630 1.75 335 .9817 0
720 2 335 .9817 0

Backup

 set linesize 212
 set pagesize 1024
 
 col filename format a64
 
 select
     filename,
     round(bytes/1024/1024/1024, 2) "GByte",
     open_time,
     close_time,
     round(elapsed_time/100/60, 2) "Minuten",
     round(short_wait_time_total/100/60, 2) "Short Waits gesamt (Min)",
     round(long_wait_time_total/100/60, 2) "Long Waits gesamt (Min)",
     round(effective_bytes_per_second/1024/1024, 2) "MB/Sekunde"
 from
     v$backup_async_io
 where
     open_time > sysdate - 1
     and filename not like '%archivelog%'
 order by 3;
 
 exit;