Re: [RFC PATCH] greatly reduce SLOB external fragmentation

Christoph Lameter
2008-01-10T19:16:50Z

On Thu, 10 Jan 2008, Matt Mackall wrote:

> Here I'm going to differ with you. The premises of the SLAB concept
> (from the original paper) are:
>
> a) fragmentation of conventional allocators gets worse over time

Even fragmentation of SLAB/SLUB gets worses over time. That is why we need
a defrag solution.

> b) grouping objects of the same -type- (not size) together should mean
> they have similar lifetimes and thereby keep fragmentation low

I agree that is crap. The lifetimes argument is mostly only exploitable in
benchmarks. That is why SLUB just groups them by size if possible.

> d) constructors and destructors are cache-friendly

I agree. Crap too. We removed the destructors. The constructors are needed
so that objects in slab pages always have a definite state. That is f.e.
necessary for slab defragmentation because it has to be able to inspect an
object at an arbitrary time and either remove it or move it to another
slab page.

Constructors also make sense because the initialization of a cache object
may be expensive. Initializing list heads and spinlocks can take some code
and that code can be omitted if objects have a definite state when they
are free. We saw that when measuring the buffer_head constructors effect
on performance.