Xeon w/ L3 1MB cache vs Xeon w/o L3 cache

[Erich Dollansky]

Hi,

Simon wrote:

> Thanks, I read that under higher loads, the L3 cache becomes quite useful, too.
> But, what is considered high-load? what determines if L3 cache is used or not?

The range is very wide. I would define it for your case as soon as 
cache trashing starts.

> is it app to the app to use it? or up to the kernel? or the CPU itself? or a

The CPU decides normally what is stored in the different caches. 
Kernels and applications can help to support the CPU's decision. 
But I do not know of any application supporting this.

> combination? We run a lot of services on our servers and I would say they are
> heavy loaded, but I could be mistaking, I mean, they do a lot of processing but
> not exactly crawling, they are still fast.
> 
This sounds the envirnoment where I did those tests. Without 
knowing what was in the boxes we started wondering some machines 
started crawling for a short period of time while others did not.

A machine with some MB of extra L3 cache starts crawling much later.

The impact to the machine cannot predicted precisely. It depends 
very much on the application. My experience ranges form hardly any 
impact up to 30 times higher throughput.

It sounds like you have a lot of machines. I would consider 
getting one machine with larger L3 caches to test it.

You also should consider this:

http://www.anandtech.com/IT/showdoc.html?i=1982

The cache does not really help if the memory interface becomes the 
bottle-neck.

Erich