high memory usage of DBWR processes in Redhat

There is no active sessions in the database ,logfile didn't switch for many hours. Why my dbwr process consume  so much memory in linux top display?  Is there any impact to the system or database?  many thanks for your help...

SYS@HMDS>select version from v$instance;

VERSION
-----------------
11.2.0.1.0

SYS@HMDS>show parameter db_wr

NAME                                 TYPE        VALUE
------------------------------------ ----------- ------------------------------
db_writer_processes                  integer     4

SYS@HMDS>show parameter mem

NAME                                 TYPE        VALUE
------------------------------------ ----------- ------------------------------
hi_shared_memory_address             integer     0
memory_max_target                    big integer 16G
memory_target                        big integer 16G
shared_memory_address                integer     0

[root@hmds etc]# cat redhat-release