Processor and I/O affinity

In SQL Server 2005, when does manually assigning processor or I/O affinity (in SSMS, server properties) actually improve performance?
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Aneesh RetnakaranDatabase AdministratorCommented:
Use the affinity mask option when multiple SQL Server instances run on consolidated hardware like multiple instances of SQL Server run on a single server.
The affinity mask option is represented as a binary pattern that is expressed by a decimal number. The affinity mask option settings are given below .

First processor only                      1
Second processor only      2
First and second processor only      3
First four processors                     15
First eight processors                     255


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vanya1967Author Commented:
I'm increasing the question value. I don't understand why using the affinity mask option would be useful when consolidating instances of SQL Server onto one server. Why would that help performance?
Aneesh RetnakaranDatabase AdministratorCommented:
To carry out multitasking, Microsoft Windows 2000 and Windows Server 2003 sometimes move process threads among different processors. Although efficient from an operating system point of view, this activity can reduce Microsoft SQL Server performance under heavy system loads, as each processor cache is repeatedly reloaded with data. Assigning processors to specific threads can improve performance under these conditions by eliminating processor reloads; such an association between a thread and a processor is called processor affinity.
Aneesh RetnakaranDatabase AdministratorCommented:
Hope i answered it correctly.
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