Re: [RFC] Structure in Compressed Cache

["Rodrigo S. de Castro" <rodsc@bigfoot.com>]

On Tue, Oct 31, 2000 at 02:06:08PM -0200, Marcelo Tosatti wrote:
> On Mon, 30 Oct 2000, Rodrigo S. de Castro wrote:
> > 	In my implementation of compressed cache (kernel 2.2.16), I
> > started the project having my cache as a slab cache, structure
> > provided by kernel. I have all step 1 (a cache with no compression)
> > done, but I had a problem with marking pages in my cache. After an
> > email sent to the list about this subject, I started looking at shared
> > memory mechanism (mainly ipc/shm.c), and I saw that there's another
> > way of making it: with a page table allocation and memory mapping. I
> > could go on with my initial idea (with slab cache) but I think that
> > doing the latter way (with page table and memory mapping) would be
> > more complete (and, of course, harder). I will have a pool of
> > (compressed) pages that gotta be always in memory and will be
> > "between" physical memory and swap. As the project is growing I would
> > like to define now which path to follow, taking in account
> > completeness and upgradeability (to future versions of kernel). Which
> > way do you think that is better? Please, I also ask you to tell me in
> > case you know if there's another way, maybe better, of doing it.
> 
> Slab cache memory is physically contiguous and non swappable, so it may be
> a waste to use it to cache userspace data. 

	Today I reread Bonwick's paper (The Slab Allocator: An
Object-Caching Kernel Memory Allocator) and I saw that a slab of
objects 'consists of one or more pages of virtually contiguous memory'
and the whole cache (that has many slabs) doesn't, necessarily. I am
storing on a slab cache only a small structure that holds some
information about and a pointer to a physical page that is allocated
in its constructor. In that way, a slab will certainly have a size of
a page (since its structure is smaller than 1/8 of page), and thus
there's no problem for us at all, because there won't have actually
any continguous data. I hope you could get it. I am wondering if I
missing something, because I can't see a problem that this approach
would waste. :-) I did tried to understand some code of slab.c and
even then I couldn't see contiguous memory problem in our whole
cache. Well, I ask anyone who knows something about slab cache to join
this discussion. :-)

[]'s
-- 
Rodrigo S. de Castro   <rcastro@linux.ime.usp.br>
University of Sao Paulo - Brazil
Compressed Caching - http://tutu.ime.usp.br



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