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Display SQL Server RAM memory buffer cache

SQL Server Tips by Donald Burleson


This is one of the many SQL Server scripts to display SQL Server RAM memory from the book "High Performance SQL Server DBA".

SQL Server has offered dynamic memory management since version 7.0.  This means the DBA can turn loose the reins and let SQL Server determine workload demands and adjust memory accordingly, with one eye always on overall server memory utilization, in order to keep performance high.  The basic memory regions SQL Server uses are the database or buffer cache , the plan cache , referred to in older versions as the procedure cache , and workspace memory . 

The database cache holds 8KB pages that contain database information.  SQL Server attempts to eliminate seldom-used pages from the database cache so room is left for often-referenced data. 

As has already been mentioned, the plan cache holds compiled and executable plans for both code objects (procedures, etc.) and ad-hoc queries. 

Workspace memory is sometimes required for database requests that require hashing or sorting operations.  There are also miscellaneous areas of memory used for locking and such. 

Before looking at the various memory efficiency ratios, it is a good idea to understand how much memory SQL Server has allocated across the various memory regions.  The up_memory_status procedure below works for SQL Server 2000 and 2005 and gives a quick overview of all primary memory allotments:

<    up_memory_status

IF OBJECT_ID('up_memory_status') IS NOT NULL
BEGIN
    DROP PROCEDURE up_memory_status
    IF OBJECT_ID('up_memory_status') IS NOT NULL
        PRINT '<<< FAILED DROPPING PROCEDURE up_memory_status >>>'
    ELSE
        PRINT '<<< DROPPED PROCEDURE up_memory_status >>>'
END
go
create procedure up_memory_status
AS
   set nocount on

declare     @total_memory_kb          float,
            @low                      int,
            @proc_cache_kb            float,
            @buffer_cache_used_kb     float,
            @proc_cache_used_kb       float,
            @buffer_cache_free_kb     float
 
    select
        @total_memory_kb = cntr_value
    from  
        master..sysperfinfo
    where 
        object_name like '%Memory Manager%' and
        counter_name = 'Total Server Memory (KB)'
 
    select
        @low = low
    from
        master..spt_values
    where
        number = 1 and
        type = 'E'
 
    select
        @proc_cache_kb = (cntr_value * @low) / 1024
    from  
        master..sysperfinfo
    where 
        object_name like '%Buffer Manager%' and
        counter_name = 'Procedure cache pages'
 
    select
        @buffer_cache_free_kb = (cntr_value * @low) / 1024
    from  
        master..sysperfinfo
    where 
        object_name like '%Buffer Manager%' and
        counter_name = 'Free pages'
 
    create table #proccache (
       label varchar(50),
       counter float)
    insert into #proccache
    exec ('DBCC MEMORYSTATUS')
   
    select
        @proc_cache_used_kb = (isnull(counter,0) * @low) / 1024
    from
        #proccache
See code depot for full script
        @buffer_cache_used_kb = @total_memory_kb -
        @proc_cache_kb -
        @buffer_cache_free_kb -
        (select
            sum(cntr_value)
         from
            master..sysperfinfo
         where
            object_name like '%Memory Manager%' and
            counter_name in ('Connection Memory (KB)','Granted Workspace Memory (KB)',
                             'Lock Memory (KB)','Optimizer Memory (KB)',
                             'SQL Cache Memory (KB)'))
 
    drop table #proccache
 
    select          
           total_sql_server_memory_kb =
            @total_memory_kb,
           buffer_cache_kb =
            @buffer_cache_used_kb + @buffer_cache_free_kb,
           procedure_cache_kb =
            @proc_cache_kb, 
           buffer_cache_used_kb =
            @buffer_cache_used_kb,
           procedure_cache_used_kb =
            @proc_cache_used_kb,
           total_cache_used_kb =
            @buffer_cache_used_kb + @proc_cache_used_kb,
           percent_procedure_cache_used =
            convert(decimal(5,2),100 * @proc_cache_used_kb / @proc_cache_kb),
           percent_buffer_cache_used =
            convert(decimal(5,2),100 * (@buffer_cache_used_kb) /
            (@buffer_cache_used_kb + @buffer_cache_free_kb))

-- show miscellaneous memory regions
select
    counter_name,
    cntr_value
from
    master..sysperfinfo
where
    object_name like '%Memory Manager%' and
    counter_name in ('Connection Memory (KB)','Granted Workspace Memory (KB)',
                     'Lock Memory (KB)','Optimizer Memory (KB)',
                     'SQL Cache Memory (KB)')
order by
    1
         
go
IF OBJECT_ID('up_memory_status') IS NOT NULL
    PRINT '<<< CREATED PROCEDURE up_memory_status >>>'
ELSE
    PRINT '<<< FAILED CREATING PROCEDURE up_memory_status >>>'
go

 

Once the amount of memory SQL Server has allocated for use has been revealed, the DBA can then move on to looking at various memory efficiency ratios.  The up_ratios_memory procedure should work on all versions of SQL Server. It returns the buffer cache , procedure plan, ad-hoc SQL, and log cache hit ratios:

<    up_ratios_memory

 

IF OBJECT_ID('up_ratio_memory') IS NOT NULL
BEGIN
    DROP PROCEDURE up_ratio_memory
    IF OBJECT_ID('up_ratio_memory') IS NOT NULL
        PRINT '<<< FAILED DROPPING PROCEDURE up_ratio_memory >>>'
    ELSE
        PRINT '<<< DROPPED PROCEDURE up_ratio_memory >>>'
END
go
create procedure up_ratio_memory
AS
   set nocount on

-- buffer cache hit ratio

select
    buffer_cache_hit_ratio =
    convert(decimal(15,2),
    (t1.cntr_value*1.0/t2.cntr_value*1.0 *100.0))
from
    master.dbo.sysperfinfo t1,
    master.dbo.sysperfinfo t2
where
    t1.object_name = 'SQLServer:Buffer Manager' and
    t2.object_name = 'SQLServer:Buffer Manager' and
See code depot for full script

-- procedure plan hit ratio

select
    procedure_plan_hit_ratio =
    convert(decimal(15,2),(t1.cntr_value*1.0/t2.cntr_value*1.0)*100.0)
from
    master.dbo.sysperfinfo t1,
    master.dbo.sysperfinfo t2
where
    t1.object_name = 'SQLServer:Cache Manager' and
    t2.object_name = 'SQLServer:Cache Manager' and
    t1.counter_name = 'Cache Hit Ratio' and
    t2.counter_name = 'Cache Hit Ratio Base' and
    t1.instance_name = t2.instance_name and
    t2.cntr_value > 0 and
    t1.instance_name in ('Procedure Plans')

-- ad hoc sql hit ratio

select
    ad_hoc_sql_hit_ratio =
    convert(decimal(15,2),(t1.cntr_value*1.0/t2.cntr_value*1.0)*100.0)
from
    master.dbo.sysperfinfo t1,
    master.dbo.sysperfinfo t2
where
    t1.object_name = 'SQLServer:Cache Manager' and
    t2.object_name = 'SQLServer:Cache Manager' and
    t1.counter_name = 'Cache Hit Ratio' and
    t2.counter_name = 'Cache Hit Ratio Base' and
    t1.instance_name = t2.instance_name and
    t2.cntr_value > 0 and
    t1.instance_name in ('Adhoc Sql Plans')

-- log cache hit ratio

select
    log_cache_hit_ratio =
    isnull(convert(decimal(15,2),100 * (sum(t1.cntr_value)*1.0/sum(t2.cntr_value)*1.0)),0)
from
    master.dbo.sysperfinfo t1,
    master.dbo.sysperfinfo t2
where
See code depot for full script
         
go
IF OBJECT_ID('up_ratio_memory') IS NOT NULL
    PRINT '<<< CREATED PROCEDURE up_ratio_memory >>>'
ELSE
    PRINT '<<< FAILED CREATING PROCEDURE up_ratio_memory >>>'
go

SQL Server Buffer Cache Performance

To help ensure excellent performance, the buffer cache hit ratio should be maintained in the neighborhood of 90% or higher. However, one should be aware that every server has its own personality and might exhibit excellent performance with below average readings for the cache hit ratio. One should also be aware that excessive logical I/O activity can produce a very high cache hit ratio while actually degrading overall database performance, so a high buffer cache hit ratio is not the silver bullet for overall high performance in SQL Server.

If the DBA is seeing low readings for the buffer cache hit ratio, the Page Life Expectancy statistic should be checked.  This statistic indicates the length of time SQL Server estimates a page will remain in the buffer cache.  Obviously, pages served from memory result in much shorter response times than pages that must be read from disk and then into the cache.  So, it is wise for often used data to be pinned in the buffer cache.  The page_life   query easily provides the DBA with this measure:

This is one of the many SQL Server scripts to display SQL Server RAM memory from the book "High Performance SQL Server DBA".


 

 

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Note: This Oracle documentation was created as a support and Oracle training reference for use by our DBA performance tuning consulting professionals.  Feel free to ask questions on our Oracle forum.

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