Do not change the SecondLevelDataCache entry (from Microsoft technet article)
Some third-party sources have erroneously reported that modifying the SecondLevelDataCache registry entry in HKEY_LOCAL_MACHINE \System \CurrentControlSet \Control \Session Manager\Memory Management can enhance system performance. The second level (L2) cache is recognized by the operating system and is fully utilized regardless of the setting of this parameter.
(from Microsoft knowledge base item Q183063)This is not related to the hardware; it is only useful for computers with direct-mapped L2 caches. Pentium II and later processors do not have direct- mapped L2 caches. SecondLevelDataCache can increase performance by approximately 2 percent in certain cases for older computers with ample memory (more than 64 MB) by scattering physical pages better in the address space so there are not so many L2 cache collisions. Setting SecondLevelDataCache to 256 KB rather than 2 MB (when the computer has a 2 MB L2 cache) would probably have about a 0.4 percent performance penalty.
(From an arstechnica article) One of the most infamous NT tweaks since the introduction of NT4 has got to be the "L2 cache" tweak, a lone registry entry which stipulates the amount of L2 cache (or secondary cache) that the OS will make use of. Well, as with many things in life, the effects of this tweak are not so black and white. If you're using a processor that implements a direct-mapped L2 cache design (like Pentium I's), then this registry adjustment is indeed for you. However, if you own any Intel processor post-PI, or any modern AMD processor (K6-2 and newer), then your processor is using a set-associative L2 cache design, and thus you need not specify your L2 cache size. Let me explain.
If you choose not to futz with the registry key in question, the OS will call on the HAL (Hardware Abstraction Layer) for retrieval of the L2 cache size when you boot your computer. If this happens to fail, a default value of 256KB is used for all logon sessions on that boot session. This is where the myth that NT can only use 256KB of L2 cache comes from. But it's false. See, the HAL is able to retrieve the processor (L2) cache from any processor using the set-associative cache design, it's only the relatively few processors out there with direct-mapped L2 caches that the HAL won't chat it up with. Thus, this tweak only applies to a select number of people anymore (like people with Pentium I procs between 166-233MHz, as lower rated procs had only 256KB cache).
Labels: Hardware Tips
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