Re: [PATCH] Rework CACHE to use a FreeList

[Paul Durrant <xadimgnik@xxxxxxxxx>]

On 19/08/2022 11:15, Owen Smith wrote:
> The slab allocation method will allogate about a PAGE worth of objects, and
> every object will be initialized. If the objects initializer allocates any
> resources, this can result in resource starvation. A particular bad example of
> this is the grant table cache, where a page of gnttab objects is 253 objects.
> This is highlighted by xenvif's queues, where the receiver requires 257 grant
> references (1 for the ring, and 256 for the ring slots) which results in 2 
> slabs,
> or 506 gnttab objects, reserving 506 grant references.
>
> Use a FreeList to contain individual objects that are not in use. This trades
> an increase in smaller allocations for reducing the wastage of unused objects.
>
> Signed-off-by: Owen Smith <owen.smith@xxxxxxxxxx>
Sorry I didn't comment on the RFC; I was on PTO and then snowed under 
with mail etc.
I think this is a sledgehammer to crack a nut. I agree that XENVIF is 
being a grant ref hog... but the correct thing to do there is to re-work 
the grant table cache, not the underlying slab allocator; which I think 
is fine. The problem is (ab)using the slab allocator's Ctor to get the 
reference. So a free list implementation is fine... just in the gnttab 
code, rather than the cache code.
  Paul