SPIDS with a value of -2

Any dba looking after a server that takes part in a distributed transaction will probably have come across situations where a SPID that does not show up in master..sysprocesses or sys.dm_exec_sessions (depending on your version of SQL Server) ends up blocking other users.
Looking at sp_lock or the syslockinfo table or sys.dm_tran_locks would show resources being locked by a SPID with a value of -2. SPIDs with a negative cannot be killed in SQL Server.

Often, especially in older (pre version 7) versions of SQL Server the only option was to restart SQL Server to clear those locks. It’s partly because of this (and also because Sybase still requires a restart for orphaned transactions) that the myth persists that SQL Server needs to be restarted.

SPIDs with a value of -2 are orphaned distributed transactions.

What’s an orphaned distributed transaction?

In a nutshell, it’s a distributed transaction for which the transactional state is unknown. A distributed transaction is a database transaction that involves more than one database system (usually) located on different servers.

In order to maintain transactional consistency these transactions need to be coordinated, as we can’t have one database involved in the transaction committing the data it has been sent, whilst another database fails to commit the data it has been sent; either all the data is successfully committed by all the databases involved in the transaction, or the entire transaction is rolled back and no data gets committed by any database.

The process responsible for coordinating these transactions is the Microsoft Distributed Transaction Coordinator, or MSDTC.

You can get to it via Start > Run and typing dcomcnfg.

This brings up the Component Services plugin. Click on Component Services > Computers > My Computer.

Under My Computer, click on the Distributed Transaction Coordinator folder. Under Local DTC you will find the following information about MSDTC transactions:

• Transaction List lists active distributed transactions
• Transaction Statistics reports summary statistics for previous MDTC transactions

Both items provide useful information, but in the context of this blog, it’s the Transaction List that we’re interested in. This lists all the currently active transactions that have not been successfully completed, or been successfully rolled back. The status column lists what MSDTC regards as the state of the transaction, and the Unit of Work ID column with those strange GUID type values displays a unique identifier that is assigned to that transaction; this is a very useful value to have as it will allow any MSDTC transactions executing within SQL Server, including transactions that MSDTC has lost control of and thus have a SPID value of -2 to be killed by specifying the Unit of Work (UoW) ID in place of the SPID number.

If the status of the transaction is ‘Active’ then the transaction is currently executing. Figure 1 below, shows an active distributed transaction when viewed in the MSDTC console (the screenshot was taken from a Windows Vista host):

Figure 1: The MSDTC Console

The problems arises when, for whatever reason, MSDTC loses track of one of these transactions. The transaction will then be marked as in-doubt. If this happens, SQL Server will honour the locks the SPID bound to that transaction has left, but the SPID itself will be assigned a negative value of -2.
This means it will no longer appear as an internal process, but will still cause blocking if the original transaction was accessing tables at the time it went awol, as the locks will have been retained. This is because SQL Server does not know whether it can commit or roll back the transaction as MSDTC has control of distributed transactions.

Sometimes this can happen because the MSDTC service (Distributed Transaction Coordinator) has been terminated, or is in a hung state. When MSDTC is restarted it will go through its own internal transaction log and try and regain control of transactions that were left open in order to commit them or roll them back. Usually, however, it’s because one or more of the applications involved in the distributed transaction has mis-behaved in some way and has hung or crashed, and the only option there is to try and restart the application(s) concerned and hope that it has a recovery mechanism to deal with any MSDTC transactions that were left unresolved.

From time to time, therefore, MSDTC will not be able to regain control of a transaction and it’s status will be marked as ‘in-doubt’ in MSDTC coordinator.

Once upon a time (well, before SQL Server 6.5 SP5 at any rate) the only way to resolve this was to restart SQL Server, and because this was once the only way to deal with orphaned SPIDs, it’s become something of a myth that it remains the only way to deal with the issue.

However, since Service Pack 5 of SQL Server 6.5 (http://support.microsoft.com/default.aspx/kb/195542) it has been possible to kill an MSDTC transaction by specifying something called the UoW ID, as opposed to just the SPID number.

The Unit of Work (UoW) ID

The UoW ID is a 24 character GUID assigned to each transaction issued by MSDTC, and it is the UoW ID that is used to identify and kill orphaned MSDTC transactions in SQL Server.

Killing an MSDTC transaction

So, you’ve come up against an MSDTC transaction that the MSDTC service has been unable to successfully commit or rollback.

This transaction has now left locks on your data that are associated with an orphaned SPID and you’ve got no option but to kill that MSDTC transaction.

You first need to retrieve the UoW ID. This should be displayed in the MSDTC console, but to verify it is the correct ID, you can interrogate the syslockinfo system view. Specifically the req_transactionUoW column. The req_spid column in syslockinfo hosts the SPID number.
So, to pull out the UoW ID for an orphaned MSDTC transaction you just need to run the following query:


select req_transactionUoW as [UoW ID] from syslockinfo where req_spid = -2


The above will work in SQL Server 2000 onward, but from SQL Server 2005, it’s more politically correct to use the DMVs, so the table and column names have changed slightly:


select request_owner_guid as [UoW ID] from sys.dm_tran_locks where request_session_id = -2

Then to kill it:

kill {'UoW ID'}

where {UoW ID} is the result of the above select statement.

However, this must only be done as an absolute last resort when other methods, such as e.g. restarting the applications/processes involved in that distributed transaction do not resolve the problem. At the end of the day, those locks will have been placed for a reason, so you don’t know if the consistency of the data is now compromised especially if syslockinfo/sys.dm_tran_locks shows the MSDTC transaction(s) concerned have placed exclusive locks on objects.

Useful MSDTC links

Troubleshooting Problems with MSDTC
New DTC Functionality in Windows Server 2003 Service Pack 1
Enable Firewall Exceptions for MS DTC

SQL Server and CPU usage

How many CPUs can SQL Server supports should be a simple question, you might think. But judging by how often this question is asked on the SQL Server forums its clear that there is a lot of confusion about exactly how many CPUs a particular edition of SQL Server can use.

The problem is due mainly to incorrect assumptions about Microsoft’s licencing policy, combined with the prevalence of multi-core CPUs. This has led to some incorrect responses to postings about CPU licencing on the SQL Server forums, so I’ve decided it’s worth blogging about to try and clarify the matter.

This post covers SQL Server 2005 and SQL Server 2008 editions.

Microsoft’s multicore licencing policy is based on the number of sockets on the motherboard, not the number of cores (EDIT 20111118: this is apparently going to change in SQL Server 2012 and will fall in line with the rest of the industry where a formula will be applied based on the number of cores). Therefore, for licencing purposes, a dual-core CPU equates to 1 CPU, not 2. A quad-core CPU equates to 1 CPU, not 4 etc. etc.

What this means is that your free SQL Express edition can use all 4 cores in your quad core CPU and not just one core. You can verify this by interrogating the sys.dm_os_sys_info SQL Server dynamic management view (DMV), particularly. The following query lists how many CPUs a particular instance of SQL Server can see:

select cpu_count from sys.dm_os_sys_info

The sys.dm_os_sys_info DMV returns a whole host of other useful information that used to be very difficult to obtain without using xp_cmdshell or WMI, so it’s well worth exploring further. In fact, most of the DMVs have invaluable information if you know what you’re looking for, but that’s a subject area far too large for a blog; just peruse Books Online as that gives a complete listing and a good rundown of what each DMV offers.
Just remember that, although SQL Server Express will be able to see multiple CPUs and use them to improve concurrency, SQL Server Express edition is prevented from taking advantage of this to parallelise individual queries, which basically means that an individual query will not be executed across multiple CPUs.

Confirming how many CPUs SQL Server is using

Back to counting CPUs. To find out how many CPUs a particular instance is actually using, run the following query which is based on the sys.dm_os_schedulers DMV:

select scheduler_id,cpu_id, status, is_online from sys.dm_os_schedulers where status='VISIBLE ONLINE'

This will return how many CPUs SQL Server is using. Ordinarily, this will always equate to the number of CPUs on the system. However, if CPU affinity is being used to assign specific CPUs to SQL Server, this query will show how many CPUs SQL Server is actively using.

Run the query without the where clause to see how many CPUs are offline and not being used (their status will be VISIBLE OFFLINE). The additional rows returned will show up internally used schedulers such as the scheduler retained for the Dedicated Admin Connection, or DAC.

If this query does not list the number of CPUs you are expecting, and you’re definitely not using CPU affinity, check the edition of SQL Server that you are using against the table below.

Note: To confirm that CPU affinity is indeed being used, the cpu_id column will always be less than 255.

CPUs supported by the various SQL Server Editions

A summary of how many CPUs the different editions of SQL Server support is summarised in the table below:

SQL Server version	Express	Workgroup	Web	Standard	Enterprise
SQL Server 2005	1	2	n/a	4	Unlimited1
SQL Server 2008	1	2	4	4	Unlimited1
1Subject to OS limits

One of the things that I frequently come across when reviewing SQL Server installations is just how many of them have not been set up with appropriate memory configuration settings, or, as in many cases, not set up in the way the administrators of the system had assumed they were; usually the dbas thought the system was set up to use all e.g. 8GB of RAM, but no changes had been made to the OS or SQL Server configuration, so their (32-bit) SQL Server would only be accessing 2 GB, and reporting that it was using 1.6 GB.

The problem is due in part to the fact that on 32-bit systems configuration changes usually have to be made both in SQL Server and at the OS level, and in part to the sprawl of documentation available on configuring SQL Server’s memory settings, as opposed to a single jumping off point which runs through all the settings and considerations that need to be made.
Add to that the black art of establishing exactly how much memory SQL Server is using (most of the obvious options will only show how much memory the buffer pool is using) and it’s easy to see why it’s such a problem area.

In this post I’ll attempt to clear some of the smog and provide what I hope will be one document which answers most of the questions that arise about configuring SQL Server’s memory usage.
This discussion will cover configuring memory for SQL Server 2000, SQL Server 2005 and SQL Server 2008 (with the exception of the Resource Governor). This blog assumes an edition of SQL Server that is not internally limited in its memory usage.

32-bit or 64-bit?

There’s a big difference between the memory configuration settings between 64-bit SQL Server and 32-bit SQL Server, so it’s not possible to start a discussion about SQL Server’s memory management without clarifying whether we are dealing with 32-bit versions or 64-bit versions of the product, as this is key to how much memory SQL Server can address, and (almost as importantly) how it addresses that memory.

Configuring 32-bit SQL Server

Until fairly recently 32-bit software was ubiquitous. The server Windows operating systems were 32-bit, your desktop, usually Windows XP was 32-bit. Therefore, I’ll be focusing a fair bit on 32-bit SQL Server as this is what requires the most configuration, and also where most of the confusion lies.

So, here goes.

The amount of memory a 32-bit process can access is 2^32 or 4294967296 bytes, or 4 GB.
On a 32-bit Windows OS this 4 GB of memory is not all addressable by a single process. Instead, it’s partitioned by the OS into two address spaces. 2 GB is kept by the OS (commonly referred to as the kernel) and the remaining 2 GB is the user mode address space, or the area each application (process) will have access to. Whilst each user mode process gets 2 GB as its addressable memory range, the kernel mode area is shared between all processes. SQL Server runs in the user mode address space and is bound by default to the 2 GB of memory limit on a 32-bit OS.
This directly addressable memory will hereon be referred to by what it is more commonly known as, the virtual address space or VAS.

SQL Server’s default out-of-the-box memory limit is deliberately set to a very high value of 2147483647 , which basically means all available memory, but as you should now know, there’s no way it can actually use anywhere near that much memory, particularly on a 32-bit platform.

64-bit operating systems have a far far bigger address space open to them; 8 TB to be exact. Before you run off to your calculator to evaluate 2^64, the answer won’t be 8TB, but 8TB is what each user mode application gets due to current hardware and OS limitations. The kernel also gets 8TB and this kernel address space is shared by all processes, just as in 32-bit Windows.

Having said all that, I should point out that no current MS Windows OS can address more than 2 TB.

What this means for 64-bit SQL Server is that out of the box, it can address all the memory on a server without any special configuration changes either within SQL Server or at the OS level. The only thing you need to look at is a cap on its memory usage; capping memory usage is covered in the ‘max server memory’ and ‘min server memory’ section which is further down.

To /3GB or not to /3GB

So, as 32-bit applications are natively restricted to a 2 GB VAS, OS configuration tweaks are required to allow access to more than 2 GB, and these are covered next.

The first modification is one that, rather ironically, should be used as a last resort. Ideally, it should be used on the advice of Microsoft Support (PSS).
I’m choosing to get it out of the way now because the /3GB setting is probably the most well known and most misused.
/3GB allows a 32-bit process to increase its VAS to 3 GB by taking away 1 GB of address space from the kernel, and this is why it’s a last resort; there’s no such thing as a free lunch as the removal of 1 GB of addressable memory from the OS can introduce instability. More on that shortly.

To allow a 32-bit process to gain a 3 GB VAS you have to add the /3GB switch to the Windows boot.ini file.

As I stated, this can introduce system instability by starving the OS of System Page Table Entries (PTEs). A discussion about PTEs is out of the scope of this blog, but its effects can be dramatic and cause blue-screens. The good news is that this mainly affected Windows 2000 so you should be fine if you’re on a later Windows version.

If you’re still looking after a legacy system, there is some scope for manoeuvre here, by adding the /USERVA switch to the boot.ini it is possible to reduce the VAS increase from a straight 3 GB to a lower user-defined amount which will give the OS room to breathe, and thus resolve any instability issues.

The main reason you will be advised by PSS to use /3GB is if you are suffering VAS starvation issues, such as a bloated procedure cache as it can only reside in VAS memory (also see the next section on MemToLeave) because 32-bit version of SQL Server only allow database pages to reside in the part of the SQL Server memory cache (called the buffer pool) that is utilising awe enabled memory.

MemToLeave

(EDIT:[20110630] The correct terminology for this is Virtual Address Space Reservation.)
Because of the inherent address space limitations of a 32-bit process, a certain amount of memory has to be set aside by SQL Server on startup that SQL Server uses for overheads. This memory is set aside in case it all gets used by the buffer pool.

COM objects, extended stored procs, third party backup solutions, some anti-virus apps, memory allocations exceeding 8K and the memory allocated to the threads SQL Server creates to service e.g. user connections come from a section of memory within the VAS but outside the buffer pool which is typically referred to as the MemToLeave area. This is 384 MB by default on an e.g. 2-proc 32-bit SQL. If you want to know more about how it is calculated, check Jonathan Kehayias’s post covering this.

0.5 MB is the default thread stack size for a thread on 32-bit Windows. 64-bit Windows has a default stack size of 2 MB or 4 MB depending on which 64-bit flavour of Windows you are running (AMD x64 or IA64).
SQL Server 2005 and beyond uses a formula to calculate the max worker threads setting which affects the size of the MemToLeave area.
There is a SQL Server startup parameter (-g) which can be used to increase the MemToLeave area, but again, only do this if advised by PSS (it’s ignored on 64-bit as MemToLeave or VAS reservation won’t be an issue on that architecture) as this will reduce the maximum amount of memory the buffer pool can therefore use.

4 GB of RAM and beyond

So, we know 32-bit SQL Server can use 2 GB out of the box, and up to 3 GB (with an OS tweak that is best avoided, if at all possible).
However, 32-bit SQL Server can benefit from much more memory than 3 GB with the help of OS and SQL Server configuration modifications which will be covered next.

/PAE

To address more than 4 GB of RAM on 32-bit Windows, the OS needs to have the /PAE switch added to the boot.ini file, although if your system supports hot-swappable memory you won’t need to add this as Windows should automatically be able to see the additional memory. If you’re not sure, take a look at how much memory the OS can see via System properties; if you have more than 4 GB installed and the OS is only showing 4 GB, review your boot.ini settings. I’m not mentioning specific Windows versions because the /PAE switch applies to all current 32-bit versions of Windows.
(EDIT:[20110630] For Windows Server 2008 you have to run ‘BCDEDIT /SET PAE FORCEENABLE’ from a CMD prompt running under administrator privileges).

Both /PAE and /3GB

Some systems have both /3GB and /PAE enabled.
This is fine as long as the system does not have more than 16 GB of RAM. Add any more memory and Windows will not recognise it because of the overhead required to manage the additional memory.

Clusters

No special configuration settings regarding memory settings are required for a cluster, but I thought I better mention clusters specifically because you won’t believe how many installations there are out there where there are different settings on different nodes within the same cluster.
So, make sure any OS setting changes like /3GB or /PAE are consistently applied across all nodes.

Enable AWE

After configuring the OS, you’ll need to configure SQL Server by enabling AWE (Address Windowing Extensions). AWE is in essence a technique for ‘paging’ in sections of memory beyond the default addressable range.
AWE can be enabled in SQL Server using Query Analyzer/SQL Server Management Studio (SSMS) via the following statements:

sp_configure 'show advanced options', 1
RECONFIGURE
GO
sp_configure 'awe enabled', 1
RECONFIGURE
GO

AWE enablement is not a dynamic option and will require a SQL Server restart, so before you do that make sure the SQL Server service account has the ‘Lock Pages in Memory’ privilege assigned to it.
Once AWE has been enabled within SQL Server and the ‘Lock Pages in Memory’ privilege has been assigned you should be good to go after a restart.

‘max server memory’ and ‘min server memory’

The more memory you give SQL Server, the greater the need to set an upper limit on how much it uses. When you start SQL Server it’ll ramp up its memory usage until it has used up all the memory it can access, which will either be an internal OS limit or a SQL Server configured limit.
A 32-bit SQL Server instance will therefore grab up to 2 GB if the workload demands it and it is on default settings.
An awe enabled SQL Server instance will go on using up all the memory on the system if the workload is there and an upper limit on its memory usage is not set.

Setting a limit has the double-benefit of not starving the OS of resources and avoiding ‘Out of memory’ errors which can occur on SQL Server systems that may have a lot of memory. The latter (rather contradictory) situation can arise because SQL Server will try and allocate more memory when it is already at the system limit (if no upper limit has been set via the ‘max server memory’ setting) instead of freeing up memory it is already using.

Configuring memory for multiple instances

A third reason to set an upper limit is if you have more than one SQL Server instance installed on a single host, as this will stop the instances competing for memory.
Allocate a high enough ‘max server memory’ limit to each instance to allow it to do its job without running into memory starvation issues, whilst reserving the bulk of the memory for higher priority instances (if any) and the OS.
This is where benchmarking comes in handy.

To set a max server memory limit of 12 GB via Query Analyzer/SSMS:

sp_configure 'max server memory', 12288
RECONFIGURE
GO


SQL Server ramps up its memory usage because by default it is set to use no memory on startup. This is controlled by the ‘min server memory’ setting. Specifying this to a higher value has the benefit of reserving a set amount of memory for SQL Server from the off which can provide a slight performance benefit, especially on busy systems. It’s actually not uncommon to see ‘min server memory’ and ‘max server memory’ set to the same value, to reserve all of SQL Server’s memory straight away. The downside is SQL Server will take slightly longer to start up than if ‘min server memory’ was set to a low value.

SQL does not really release memory

This will probably get me into a bit of trouble, as there are KBs that clearly state that SQL Server releases memory when the OS is under pressure.
True, but only when it’s under a lot of pressure (although this is getting better with each version of SQL Server), and by then it’s often too late as the system is usually in such a degraded state by that stage that a restart is necessary.
That’s why it’s vital to set an upper limit on its memory usage via the ‘max server memory’ setting.
(Edit [20110627): Speaking from experience, this problem manifested itself most noticeably on SQL Server 2000 and earlier but the problem seems to have disappeared from SQL 2005 onwards. Let me know if you disagree (and send me the evidence!)).

Memory corruption

Slightly off-topic, but this is an appropriate place to bring this up.
Certain builds of Windows 2000 and Windows Server 2003 contained a potentially serious memory corruption problem which affected SQL Server more than other applications, mainly because there are few applications that run on Windows that can utilise the amount of memory SQL Server does.
It's difficult to overstate the problems this can cause, so make sure you're on the appropriate Windows Service Packs if you're running SQL Server on a PAE enabled system.
Another issue that arose in SQL Server 2000 SP4 was a bug that meant SQL Server only saw half the memory on awe enabled systems, although it was identified quickly and the hotfix for this was placed alongside the SP4 download.

32-bit SQL Server on 64-bit Windows

If you 32-bit SQL Server on 64-bit Windows the SQL Server process can access the entire 4 GB VAS.

Checking SQL Server's memory usage

This is another area where there is lot of confusion, so below is a run-through of the most common methods for confirming SQL Server's memory usage.

Ignore Task Manager

If you have an awe enabled SQL Server instance, do not rely on Task Manager to display memory usage as it does not show the AWE memory a process is using, so the memory usage figure it presents for the SQL Server process (sqlservr.exe) will be incorrect.

DBCC MEMORYSTATUS

Running the above command outputs the memory usage of SQL Server including how that memory is allocated, so unless you need to know how and where that memory is being used, the output it generates can be a bit bewildering. The important bits of this output pertaining to SQL Server's total memory usage are as follows:

Buffer Counts                  Buffers
------------------------------ --------------------
Committed                      3872
Target                         65536
Hashed                         2485
Stolen Potential               60972
External Reservation           0
Min Free                       64
Visible                        65536
Available Paging File          702099

The key figures in the above output are committed, target and hashed.
Committed is the amount of memory in use by the buffer pool and includes AWE pages.
Target is how big SQL Server wants the buffer to grow, so you can infer from this whether SQL Server wants more memory or is releasing memory.
There's an excellent KB on interpreting all the output INF: Using DBCC MEMORYSTATUS to Monitor SQL Server Memory Usage for SQL Server 2000 and How to use the DBCC MEMORYSTATUS command to monitor memory usage on SQL Server 2005.
Edit (05/02/09): Remember the buffer count numbers refer to pages of memory which are 8K in SQL Server

System Monitor (perfmon)

Perfect way to get a quick reference on exactly how much memory SQL Server is using at that moment. Start System Monitor and add the SQL Server: Memory Manager: Total Server Memory (KB) counter.

Replace "SQL Server" with MSSQL$ and the name of the named instance if it's not a default instance, e.g. MSSQL$INSTANCE1.

'Total' memory usage

When trying to establish exactly how much memory SQL Server is using it's not just the buffer pool memory you have look at, but the MemToLeave area as well. The key point to bear in mind here is that it's not only SQL Server that can make allocations from this latter area of memory but third party processes as well, which can make it impossible to precisely account for SQL Server's absolute memory usage, contrary to some myths out there about calculating SQL Server's memory usage via e.g. DBCC MEMORYSTATUS, as such methods can only account for SQL Server's own memory allocations and not allocations by foreign processes.

Edit [20110627]: Soft NUMA section removed.

64-bit

I mentioned at the start of this post that all you have to worry about for 64-bit SQL Server is setting a max memory limit as SQL Server can access all the memory current Windows operating systems can support, and 8 TB in total. That’s mostly true, with the exception of a certain privilege that the SQL Server service account needs, and that’s the ‘Lock Pages in Memory’ privilege.
This privilege is vital as it prevents the OS from paging out SQL Server memory to the swap file.
With the introduction of SQL Server 2005, this right was restricted on 64-bit Windows to only take effect on Enterprise Editions of SQL Server, so if you’re wondering why your huge new multi-gigabyte multi-core 64-bit system is paging like crazy, this might be why. [Edit: This has finally been reversed for both SQL Server 2005 and SQL Server 2008 Standard Editions: http://blogs.msdn.com/psssql/archive/2009/04/24/sql-server-locked-pages-and-standard-sku.aspx
Whilst we’re on the subject of paging on 64-bit SQL Server systems, take a look at the following KB:
How to reduce paging of buffer pool memory in the 64-bit version of SQL Server 2005 which covers issues a number of issues that cause SQL Server’s (Standard or Enterprise editions) memory to be paged out.

In summary…

The table below describes how much memory SQL Server can use, and assumes an edition of SQL Server that has no internal limitations as to how much memory it can use, e.g. Express and Workgroup editions are limited to 1 GB and 3GB respectively.

SQL Server type	Installed physical memory
	Up to 4GB		More than 4GB (/PAE enabled 1)
32-bit SQL Server	Default memory usage	With /3GB 2	All available RAM3
	2 GB	3 GB
64-bit SQL Server	All available RAM 3
1 Not all 32-bit systems now need to have /PAE explicitly set in boot.ini for the OS to see more than 4 GB of RAM 2. Assuming /USERVA switch has not been used to tune memory usage to between 2 GB and 3 GB 3. Assuming 'max server memory' is left on defaults, otherwise SQL Server will use no more memory than that stipulated by the 'max server memory'setting.

When I started this post I wanted to keep it as short and succinct as possible, but there’s a lot more to configuring SQL Server’s memory usage than simply setting a ‘max server memory’ limit. Configuring SQL Server’s memory settings can be quite a complex undertaking, especially in a 32-bit environment. It’s not easy to cover all the pertinent points without branching off and describing the different areas of its memory architecture, although I’ve tried to provide the relevant information without going into too much detail.
Hopefully, this blog has clarified how to configure SQL Server’s memory usage and provided enough information to answer most memory configuration related questions, although, as you might have guessed, there’s no black-and-white way of precisely determining SQL Server’s memory usage as there are so many external processes that can make allocations from within SQL Server’s address space…

Thanks also to Jonathan Keheyias for additional information and comments on this post. Please read his post on this topic if you want to delve a bit deeper.

Useful links

How It Works: DBCC MemoryStatus Locked Pages Allocated and SinglePageAllocator Values

INF: Using DBCC MEMORYSTATUS to Monitor SQL Server Memory Usage

MemToLeave and 64-bit SQL Server