Anyone know CUP cache and java array?

Thread view:

Kevin - 03 May 2006 17:21 GMT

I think the CUP needs to load the data structure to its CPU cache in
order to operate them, right?
So when we have a java array, for example:

int[] myArray = new int[2000];

When I want to update one of its values, for example:

myArray[1000] = myArray[1000] + 1;

In order to process this line, will the CPU load the whole array into
its CPU cache to process? Or it can just load that particular item to
its cache to update it?

Thanks.

Reply to this Message

Oliver Wong - 03 May 2006 17:36 GMT

>I think the CUP needs to load the data structure to its CPU cache in
> order to operate them, right?
[quoted text clipped - 9 lines]
> its CPU cache to process? Or it can just load that particular item to
> its cache to update it?

Probably the latter. You can make arrays that, as a whole, are too big
to fit into cache.

- Oliver

Reply to this Message

Dimitri Ognibene - 03 May 2006 17:52 GMT

The cpu, usually, doesn't know anything about arrays so it works only
on elements.
However the cpu or the memory interface can load a whole segment of
data near to the memory element you are accessing..but you can't have
any control on this behavior, which however is enough fast, it is
usefull for cicles on array..
Dimitri

Reply to this Message

Eric Sosman - 03 May 2006 17:50 GMT

Kevin wrote On 05/03/06 12:21,:

> I think the CUP needs to load the data structure to its CPU cache in
> order to operate them, right?

Usually, yes. Note that most modern computers have
multiple levels of cache with different characteristics.
Also, it may be possible to do some kinds of manipulation
(e.g., block copies) without involving some caches.

> So when we have a java array, for example:
>
[quoted text clipped - 7 lines]
> its CPU cache to process? Or it can just load that particular item to
> its cache to update it?

In one sense you don't know. You write this line of
Java, javac compiles it, and the JVM executes it ... somehow.
The details of the execution are (mercifully -- take my word
for it) hidden from you.

In another sense, it's easy to get at least a partial
answer. Choose whichever cache level interests you and
consider its size. Then write a Java program with an array
of, say, five times that size. Do you think your program
will run, or do you think the CPU cache will explode like
Mr. Creosote after the "waafer-thin" mint?

Signature

Eric.Sosman@sun.com

Reply to this Message

Anyone know CUP cache and java array?

Free Magazines