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SubjectRe: [RFC PATCH] greatly reduce SLOB external fragmentation
On Thu, 10 Jan 2008, Matt Mackall wrote:

> > 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.
> Are you saying that the state of -freed- objects matters for your active
> defragmentation? That's odd.

The state of the object immediately after it is allocated matters for a
defrag solution. A kmalloc leads to an object in a undetermined state if
you have no constructor. Code will then initialize the object but defrag
f.e. must be able to inspect the object before. This means either that the
freed object has a defined state or that kmalloc establishes that state
before the object is marked as allocated.

> > 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.
> Hmm. SLOB proves that you don't need to segregate objects based on
> constructors, so you could combine even slabs that have constructors and
> just delay construction until allocation. I'm surprised constructors
> have measurable advantage..

That is not working if you need to inspect allocated objects at any time
for a defrag solution. All objects in a defragmentable slab need to have a
consistent object state if allocated. If you have some without
constructors then these object have no defined state and may contain
arbitrary bytes.

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