Monday, December 12, 2005
NT AND LEVEL TWO CACHE
Note: this myth is likely to surface often and in many places. Help spread the word.
A few days ago, Microsoft's "Windows Platform News" e-mail newsletter sent out a tip:
A quick check of all the machines at the Em/Pournelle Graphics Lab, which I run with David Em, shows that, no, none of them are set for anything other than "0" in this position. We decided (since fiddling with computers is so much more fun than actually writing about them) to experiment. What would happen on ourIntergraph dual-processor PII-300, which of course has 512K of cache per processor, if we set this flag to 512 Decimal (0x200 Hex)? This machine runs NT 4 Service Pack 4 already.
We tested by constructing a series of PhotoShop 5 filters, using 5's new "action palette" (sort of like DeBab's batch mode, it's great for doing the same thing to multiple files). Ran them both before and after the update. If there was a difference, it was less than 1 second on a 1:30 series of filters, well within measurement error.
Meanwhile, I decided to email someone who had a real reason for an opinion. (One of the great things about a lab is that you can crash one system while writing on another.) To this end, I askedJamie Hanrahan of Kernel Mode Systems his opinion. Jamie teaches (and writes books about) NT internals and device-driver issues, so he knows NT from the inside quite well.
email@example.com (JH645) CompuServe: 74140,2055
A quick sanity check says that, yes, Jamie is of course right; all that cache would be used automatically. (The print Chaos Manor column in Byte used to have a semi-regular feature called "Things that Make You Feel Stupid", and I think this is in that spirit.) That NT could calculate operation strategies based on available cache makes sense, but it should automatically use all the cache that's there; it's either used, or it isn't.
So, conclusion: Any of you out there with customers or who are in support can tell the Great Unwashed to stay the heck away from Regedt32. Theyre not going to suddenly get 25% more performance out of their expensive new boxes. (Yes, yesdesktops should have such functionality denied to mere users and others who can hurt themselves playing with fire. Thats what NT security is for.)
And big nasty harrumphs to MS for putting out their so-called "tips" without even running them by their own engineers first. Editors are important.
Re: your discussion about changing the size of the Windows NT L2 cache. My grandmother used to say, "A little knowledge is a dangerous thing." In this case its not so much dangerous as time consuming. The "Windows Platform News" tip also appeared in at least one consumer magazine recently. In both cases, the information is correct, but incomplete. Heres the missing parts [emphasis mine]