why does MEMORYCLERK_SQLOPTIMIZER take lot of memory and how to clear it?

Hi guys,

The following is my issue:
1.Microsoft SQL Server 2008 R2 (RTM) - 10.50.1600.1 (X64)   Apr  2 2010 15:48:46   Copyright (c) Microsoft Corporation  Enterprise Edition (64-bit) on Windows NT 6.1 <X64> (Build 7600: )
2.16CPU and 32RAM
3.Lock pages in memory is enabled and max server memory for Sql is 27GB
4.select SUM(single_pages_kb)*1.0/1024 as total_single_pages_MB,
    SUM(multi_pages_kb)*1.0/1024 as total_multi_pages_MB,
    SUM(virtual_memory_reserved_kb)*1.0/1024 as total_virtual_memory_reserved_MB,
    SUM(virtual_memory_committed_kb)*1.0/1024 as total_virtual_memory_committed_MB,
    SUM(awe_allocated_kb)*1.0/1024 as total_awe_allocated_MB
from sys.dm_os_memory_clerks

total_single_pages_MB  total_multi_pages_MB   total_virtual_memory_reserved_MB     total_virtual_memory_committed_MB         total_awe_allocated_MB
24468.945312        47.726562        32964.976562                172.726562            27717.062500

5.select top 10
    type,
    SUM(single_pages_kb)*1.0/1024 as total_single_pages_MB
from sys.dm_os_memory_clerks
group by type
order by SUM(single_pages_kb) desc 
type                                                         total_single_pages_MB                
------------------------------------------------------------ ---------------------------------------
MEMORYCLERK_SQLOPTIMIZER                                     24736.351562                         
MEMORYCLERK_SOSNODE                                          39.218750                            
OBJECTSTORE_LOCK_MANAGER                                     19.671875                            
USERSTORE_SCHEMAMGR                                          18.734375                            
.........

Does anyone tell me why does MEMORYCLERK_SQLOPTIMIZER take so much memory and how to clear it ?
I have tried the following script,but it does not work.

DBCC FREEPROCCACHE
GO
DBCC DROPCLEANBUFFERS
GO
DBCC FREESYSTEMCACHE ('ALL')
GO



Thanks in Advance.
Regards,
nicofer