Re: [patch] kmalloc_percpu -- 2 of 2

[Andrew Morton <akpm@digeo.com>]

Dipankar Sarma wrote:
> 
> On Wed, Dec 04, 2002 at 11:34:35AM -0800, Andrew Morton wrote:
> > > +/* Use these with kmalloc_percpu */
> > > +#define get_cpu_ptr(ptr) per_cpu_ptr(ptr, get_cpu())
> > > +#define put_cpu_ptr(ptr) put_cpu()
> >
> > These names sound very much like get_cpu_var() and put_cpu_var(),
> > yet they are using a quite different subsystem.  It would be good
> > to choose something more distinct.  Not that I can think of anything
> > at present ;)
> 
> Well, they are similar, aren't they ? get_cpu_ptr() can just be thought
> of as the dynamic twin of get_cpu_var(). So, in that sense it seems ok
> to me.

hm.  spose so.

> >
> > If we were to remove the percpu_interlaced_alloc() leg here, we'd
> > have a nice, simple per-cpu kmalloc implementation.
> >
> > Could you please explain what all the other code is there for?
> 
> The interlaced allocator allows you to save space when kmalloc_percpu()
> is used to allocate small objects. That is done by interlacing each
> CPU's copy of the objects just like the static per-cpu data area.
> 

Where in the kernel is such a large number of 4-, 8- or 16-byte
objects being used?

The slab allocator will support caches right down to 1024 x 4-byte
objects per page.  Why is that not appropriate?

If it is for locality-of-reference between individual objects then
where in the kernel is this required, and are performance measurements
available?  It is very unusual to have objects which are so small,
and a better design would be to obtain the locality of reference
by aggregating the data into an array or structure.


Sorry, but you have what is basically a brand new allocator in
there, and we need a very good reason for including it.  I'd like
to know what that reason is, please.