Re: [Intel-IOMMU 02/10] Library routine for pre-allocat poolhandling

From: Andrew Morton
Date: Fri Jun 08 2007 - 15:01:41 EST

On Fri, 8 Jun 2007 11:21:57 -0700
"Keshavamurthy, Anil S" <anil.s.keshavamurthy@xxxxxxxxx> wrote:

> On Thu, Jun 07, 2007 at 04:27:26PM -0700, Andrew Morton wrote:
> > On Wed, 06 Jun 2007 11:57:00 -0700
> > anil.s.keshavamurthy@xxxxxxxxx wrote:
> >
> > > Signed-off-by: Anil S Keshavamurthy <anil.s.keshavamurthy@xxxxxxxxx>
> >
> > That was a terse changelog.
> >
> > Obvious question: how does this differ from mempools, and would it be
> > better to fill in any gaps in mempool functionality instead of
> > implementing something similar-looking?
> Very good question. Mempool pre-allocates the elements
> to the required minimum count size during its initilization time.
> However when mempool_alloc() is called it tries to obtain the
> element from OS and if that fails then it looks for the element in
> its pool. If there are no elements in its pool and if the gpf_t
> flags says it can wait then it waits untill someone puts the element
> back to pool, else if gpf_t flag say it can;t wait then it returns NULL.
> In other words, mempool acts as *emergency* pool, i.e only if the OS fails
> to allocate the required memory, then the pool object is used.
> In the IOMMU case, we need exactly opposite of what mempool provides,
> i.e we always want to look for the element in the pool and if the pool
> has no element then go to OS as a worst case. This resource pool
> library routines do the same. Again, this resource pools
> grows and shrinks automatically to maintain the minimum pool
> elements in the background. I am not sure whether this totally
> opposite functionality of mempools and resource pools can be
> merged.


If resource pools are not designed to provide extra robustness via an
emergency pool, then what _are_ they designed for? (Boy this is a hard way
to write a changelog!)

> In fact the very first version of this IOMMU patch used mempools
> and the performance was worse because mempool did not help as
> IOMMU did a very frequent alloc and free of pool objects and
> every call to alloc/free used to go to os. Andi Kleen,
> noticied and told us that mempool usage for IOMMU is wrong and
> hence we came up with resource pool concept.

You _seem_ to be saying that the resource pools are there purely for
alloc/free performance reasons. If so, I'd be skeptical: slab is pretty
darned fast.

> >
> > The changelog very much should describe all this, as well as explaining
> > what the dynamic behaviour of this new thing is, and what applications are
> > envisaged, what problems it solves, etc, etc.
> I can gladly update the changelog if the resource pool concept is
> approved. I will fix all the below minor comments.
> I envision that this might be useful for all vendor's (IBM, AMD, Intel, etc) IOMMU driver
> and for any kernel component which does lots of dynamic alloc/free an object of same size.

That's what kmem_cache_alloc() is for?!?!
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