Level2 Cache and 2K+XP

[kissfan]

Hi Guys,

I have read somewhere that 2K=XP don't use the level2 cache on processors correctly, and that there is a registry hack that, solves the problem.

Anybody know what it is?

[reboot]

Can you find where you read that?
Sounds like a big steaming pile of bullshirt to me

[Blakhart]

No this is not true. Do a regedit search for "secondlevel".
It is normaly at hex0, wich is xpese for 256. If one has 512 of second level, one might want to set it to hex200 wich is 512. 256 set manualy would be decimal 256 or hex100.
Something like that.

One can always use decimal rather than hex.
You will not notice any improvements unless say xp didn't see the 512 cache and set it to 256....



For your own safety, double check whatever I say with competent advisors, or suffer the consequences...

[reboot]

OK, I have done some of my own research, and here's MY answer to it all:
Note: this myth is likely to surface often and in many places. Help spread the word.

USE ALL OF YOUR L2 CACHE If you have more than 256KB of L2 cache, Windows NT might not be using all of it. To correct this, make an emergency repair disk (rdisk /s). Run Regedt32.exe. Under HKEY_LOCAL_MACHINE, select System\CurrentControlSet\Control\SessionManager\MemoryManagement. On the right side of the window you will find SecondLevelDataCache. This defaults to 0, which is the correct value for 256KB of L2 cache. Double-click SecondLevelDataCache to bring up the D_WORD Editor. Click the Decimal radio button, enter the amount of L2 cache you have, and click OK. Exit RegEdt32.

There is nothing you can do from the CPU to enable or disable part of the L2 cache; you can turn it off completely but you can’t enable only 256K of it.

I think that registry value is only "informational". Perhaps they change some other sizing policies based on the amount of cache they think the system has, but I don’t for a second believe that they’re somehow completely ignoring part of it.

But this is something I’m only reasonably certain of and that I’ve been meaning to nail down.

"0" in registry entries, esp. under the MemoryManagement key, usually tells the system "do your own default calculations based on actual hardware", not "force to a special value for one type of hardware".

"You should only consider editing the default value if you are running computers with direct-mapped L2 caches. Pentium II and later processors do not have direct-mapped L2 caches. A slight performance improvement (two percent) can be detected in older computers (486 and earlier) with memory of 64 MB and more due to the fact that physical pages are distributed better in the address space, reducing L2-cache collisions. However, defeating the operating system’s default behavior and setting the value of the SecondLevelDataCache to 256 KB rather than 2 MB when the computer has a 2 MB L2 cache reduces performance slightly."

AND STILL MORE:

Do not change the SecondLevelDataCache entry (from Microsoft technet article) http://www.microsoft.com/technet/tre...t2/sopch06.asp
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).



Another urban myth...

[kissfan]

The forum where i found this is
here , but as a good sensible idea I thought I would post about it here. And as always the guys+gals at PCmech have come up trumps.

If MS say you don't need to do it then I supose you don't need to do it.

Thanks for the info Reboot