Re: Can I run applications run completely from of CPU cache

Hello Vishal

This is the function of the "instruction" cache.  If your
machine has one, and your application's main execution loop
fits within the instruction cache, then you're covered
automatically.  

The slowest part of the application you describe, is
accessing the data.  Does your application access the same
memory address more than once? If you are doing several
different types of tests on the memory (walking 1's,
patterns, etc.). Then design your application so it does
ALL the tests on a block of memory which fits in the "Data"
cache, before moving to another block. 

Data cache can be as much of a bottle neck to processing as
a help, just based upon the way you write your application.
 I do DSP (Digital Signal Processing) and a usual
application is to run an DFT algorithm on a 2 dimensional
matrix.  If the entire matrix will not fit in the cash,
then the typical idea of running the algorithm first on the
rows, and then on the columns is very slow. However if I
write my application to do only the part of the rows which
fit into the cache, then work the columns, that are part of
those rows, then my data is in cache for both parts.  

Since most cache algorithms replace the "oldest" row when a
memory access from the CPU requests a memory address which
has is not cached, when I go back to work the first column,
the CPU gets a "cache miss", and need to wait while the
"cache line" for that memory address is RELOADED.
So, if I do an entire row, and then do the columns, the
first row, would be over written, and there would be a
slight delay while the line I want gets reloaded.

Thus, the general rules are:

(1) Design your application to do every thing it has to do
to the data that resides in cache, before moving onto the
next block of data.

(2) Work on continuous memory address whenever possible. 
The cache only loads an entire cache line at a time.  So if
your application reads bytes from random memory addresses,
your machine is actually loading 32 bytes (or whatever your
machine's cache line size is) into the cache before your
CPU gets to read the single byte.  This true for most PPC
based machines, you'll need to check how your cache works.

(3) Start each data block on a cache boundary. If a cache
line is 32 bytes (8, 32bit words), it loads the cache with
32 bytes, starting at a 32 byte multiple.  Thus, reading
blocks of memory where the starting address is not at a
multiple of 32, causes the cache to load extra data bytes.


I don't know what machine or OS you're using, but the above
are very general rules to think about when working with
caches.  So, I would 

(1) concentrate on how your application access data, and
thus how efficiently it uses the data cache, and 

(2) modularize your application so if the entire
application doesn't fit in the instruction cache, then each
test will.

I believe number 1 above will show you the most
improvement.

I hope this helps, and I hope you'll share some of your
test with the group!

hadz.



--- vsapre80 <[email protected]> wrote:

> Hi,
> 
> is there a way I could get all my code (if I know its
> small enough)
> into the cache, and then run all instructions from the
> CPU cache? The
> data bits could still come from the main memory.
> 
> I am looking for a main memory test program, which has to
> test almost
> the whole memory, except, ofcourse, the resident part of
> the OS etc...
> This program should also be as fast as possible.
> 
> hence the idea.
> 
> Any takers?? Any thoughts, suggestions ??
> 
> Vishal
> 
> 
> 
> 
> 
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HADZ.

The reasonable man adapts himself to the world; 
the unreasonable one persists in trying to adapt 
the world to himself. Therefore all progress depends 
on the unreasonable man. 
    --George Bernard Shaw, Man and Superman (1903) 
"Maxims for Revolutionists" Irish dramatist & socialist (1856 - 1950)


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