# Prefmem of bus 3

Eric W. Biederman ebiederman at lnxi.com
Tue Mar 9 17:59:01 CET 2004

```Li-Ta Lo <ollie at lanl.gov> writes:

> On Mon, 2004-03-01 at 14:06, Eric W. Biederman wrote:
> >
> > There are many was to assign resources on a bus.  After some
> > experiences with tight memory situations I implemented a near optimal
> > solution.  The solution is optimal if all of your resources are a
> > power of 2 in size.
> >
> > Basically the code is a loop.  For each iteration the
> > code finds the largest unassigned resource.  Then the resource
> > constraints of that resource are considered and padding between
> > the previous resources and the current resources are inserted if
> > necessary.  Then we get into the next iteration.
> >
>
> I still don't understand this. Do you mean that now the resource
> allocation is not sequential (in the order of devices been enumerated)
> and not continuous (there are gaps in allocated address) ?

Correct.

The reason the allocation is not sequential (in the order of devices
been enumerated) is to reduce the number of gaps in the allocated

So a thought problem to help put this in perspective.

First all resources are a power of 2 in size must be that same
power of 2 aligned.

So if I have 3 resources A,B,C with the following sizes:

A: 8K
B: 256M
C: 4K

Allocating them sequentially would use 756M of address space.
After allocating A you need nearly 256M of padding to get B 256M
aligned.  C takes nearly 256M due to padding.

Allocating them largest to smallest (B,A,C) uses just 512M of address
space.  Because the alignment necessary for A is present at the end
of B, and the alignment necessary for A is present at the end of C.

Eric

>
> > The reason this is optimal if all of your resources are a power of
> > two in size is because if your previous resource is a larger or equal
> > power of two no padding will be needed for the current resource.
> >

```