Win2K Datacenter Server

Microsoft designed Windows 2000 Datacenter Server, the most powerful OS offering in the Win2K family, to meet the needs of customers that require large, mainframelike systems possessing superior availability and outstanding scalability. Until now, Win2K and its predecessor, Windows NT 4.0, couldn't compete with large UNIX implementations. With Datacenter, Microsoft aims to level the playing field by exploiting the largest, most powerful systems ever built for Windows.

One way to describe Datacenter is that it's Win2K Advanced Server plus Service Pack 1 (SP1) plus extra features, available only from OEMs on tested and approved hardware, and offered with optional joint support from the OEMs and Microsoft. The extra features improve on Win2K's scalability, availability, and manageability. And special certification and support requirements in Datacenter further differentiate this OS from the rest of the Win2K server family. Table 1, page 50, compares Datacenter with Win2K AS and Win2K Server.

32 CPUs
With Datacenter, Microsoft has made important strides in advancing Win2K's scalability. Datacenter includes all the scalability tricks in Win2K Server and Win2K AS, plus some new things that you might not have seen yet.

Datacenter marks several "firsts" for Microsoft, one of which is the ability to support up to 32 CPUs in one machine. This capability is the linchpin in Microsoft's scale-up strategy for the Win2K server family.

With NT Server 4.0, Enterprise Edition (NTS/E), you can license one system for as many as eight SMP processors, although systems with four to six processors provide the best performance for the price. Microsoft has dramatically improved processor scalability throughout the Win2K server family, and this improvement is especially apparent in Datacenter. Systems that support 32 processors, such as Unisys' ES7000, are becoming more readily available, so customers can begin to realize the benefits of Win2K's CPU scaling capabilities.

Microsoft made several optimizations in the heart of Win2K to achieve better SMP scaling. These optimizations consist of increased parallelism combined with fewer serializations, and improvements to core functions such as I/O performance, device drivers, and TCP/IP stack performance.

Win2K has more per-processor granularity for system pools and lists than NT 4.0. Each processor has paged and nonpaged look-aside lists for memory allocation, thread pools, and I/O completion ports. Other scaling improvements include more use of fibers (i.e., lightweight threads) to reduce memory and context-switching costs for applications that use fibers, and reduced use of Page Frame Number locks, which was achieved by enlarging the cache virtual address by 50 percent and by implementing a new least recently used (LRU) algorithm.

Microsoft designed Win2K for SMP from the kernel out. The benefits include more linear SMP scalability (i.e., performance improves as you add more CPUs; it doesn't flatten out after four to six CPUs as in NT 4.0), better CPU affinity capabilities (you can associate some processes with a specific CPU—this can help performance by reducing the overhead of context switching as Win2K moves a process from one CPU to another), and better performance for the price when you add CPUs. The four-to-six processor "sweet spot" of NT Server 4.0 is definitely a thing of the past.

Enterprise Memory Architecture
Datacenter supports as much as 64GB of physical memory; Win2K AS supports as much as 8GB. Win2K (like NT) is a 32-bit OS, which means that processes have access to a flat 4GB (2³²) of address space. So how do Datacenter and Win2K AS benefit from more memory? The Win2K Enterprise Memory Architecture (EMA) provides two ways to exploit the larger amounts of memory that the Win2K server family supports: Microsoft's 4GB RAM Tuning (4GT) and Intel's Physical Address Extension (PAE). Applications that take advantage of EMA technologies will scale better than those that don't. Depending on how applications use EMA, they might or might not require modifications to take advantage of the technology.

4GB RAM Tuning. With Microsoft's 4GT (which has been around since NTS/E), the OS typically allocates each process 4GB of virtual memory: 2GB for the application and 2GB for the system. Because the processes don't use all 2GB of their system address space, Microsoft created 4GT to increase the application virtual memory from 2GB to 3GB and reduce the system virtual memory space from 2GB to 1GB, without adding any new APIs. Applications written to take advantage of this extra memory, such as Microsoft SQL Server, can exploit 4GT for better performance.

To enable the 4GT feature when you start Datacenter, you must append /3GB to the Advanced RISC Computing (ARC) path in your system's boot.ini file:

multi(0)disk(0)rdisk(0)
partition(1)\WIN2K="Microsoft Windows 2000 Datacenter Server"

/basevideo /3GB

To use 4GT with an application, you must set the IMAGE_FILE_LARGE_ ADDRESS_AWARE bit in the application's executable file header. You can set the bit with the linker switch / LARGE ADDRESSAWARE or with the Imagecfg utility as follows:

imagecfg ­l <BigApp>.exe

For more information about using 4GT, see the Microsoft article "Information on Application Use of 4GT RAM Tuning" (http://support.microsoft.com/support/ kb/articles/q171/7/93.asp).

In Win2K, 4GT is available only in Win2K AS and Datacenter. Datacenter automatically ignores memory above 16GB when 4GT is enabled because systems using memory above 16GB need 2GB of virtual address space to contain all the necessary page table entries. The administrator who enables 4GT is also choosing not to use memory above 16GB, even if it's present in the system.

PAE. Intel's PAE, the other EMA technology, is new with Win2K and provides access to as much as 64GB of memory in Datacenter (8GB in Win2K AS). In the past, 32-bit Intel processors could address only 4GB of memory. But Intel has extended the addressing capability of its PAE-enabled processors to 64GB (36 bits). PAE requires an x86 Pentium Pro processor or better, more than 4GB of system RAM, and a 450NX-compatible or later chipset. Check with your vendor to ensure your hardware is compatible and tested with PAE.