On Mon, 9 Jul 2007 10:26:08 -0700 (PDT)
Christoph Lameter <[email protected]> wrote:
> > I assume the tradeoff here is better packing versus having a ridiculous
> > number of caches. Is there any other cost?
> > Because even having 1024 caches wouldn't consume a terrible amount of
> > memory and I bet it would result in aggregate savings.
>
> I have tried any number of approaches without too much success. Even one
> slab cache for every 8 bytes. This creates additional admin overhead
> through more control structure (that is pretty minimal but nevertheless
> exists)
>
> The main issue is that kmallocs of different size must use different
> pages. If one allocates one 64 byte item and one 256 byte item and both 64
> byte and 256 byte are empty then SLAB/SLUB will have to allocate 2 pages.
> SLUB can fit them into one. This is basically only relevant early after
> boot. The advantage goes away as the system starts to work and as more
> objects are allocated in the slabs but the power-of-two slab will always
> have to extend its size in page size chunks which leads to some overhead
> that SLOB can avoid by placing entities of multiple size in one slab.
> The tradeoff in SLOB is that is cannot be an O(1) allocator because it
> has to manage these variable sized objects by traversing the lists.
>
> I think the advantage that SLOB generates here is pretty minimal and is
> easily offset by the problems of maintaining SLOB.
Sure. But I wasn't proposing this as a way to make slub cover slob's advantage.
I was wondering what effect it would have on a more typical medium to large sized
system.
Not much, really: if any particular subsystem is using a "lot" of slab memory then
it should create its own cache rather than using kmalloc anyway, so forget it ;)
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