Re: [dpdk-dev] [RFC] mempool: implement index-based per core cache

["Morten Brørup" <mb@smartsharesystems.com>]

> From: Morten Brørup
> Sent: Saturday, 30 October 2021 12.24
> 
> > From: dev [mailto:dev-bounces@dpdk.org] On Behalf Of Honnappa
> > Nagarahalli
> > Sent: Monday, 4 October 2021 18.36
> >
> > <snip>
> > >
> > >
> > > > > > Current mempool per core cache implementation is based on
> > pointer
> > > > > > For most architectures, each pointer consumes 64b Replace it
> > with
> > > > > > index-based implementation, where in each buffer is addressed
> > by
> > > > > > (pool address + index)
> 
> I like Dharmik's suggestion very much. CPU cache is a critical and
> limited resource.
> 
> DPDK has a tendency of using pointers where indexes could be used
> instead. I suppose pointers provide the additional flexibility of
> mixing entries from different memory pools, e.g. multiple mbuf pools.
> 
> > > > >
> > > > > I don't think it is going to work:
> > > > > On 64-bit systems difference between pool address and it's elem
> > > > > address could be bigger than 4GB.
> > > > Are you talking about a case where the memory pool size is more
> > than 4GB?
> > >
> > > That is one possible scenario.
> 
> That could be solved by making the index an element index instead of a
> pointer offset: address = (pool address + index * element size).

Or instead of scaling the index with the element size, which is only known at runtime, the index could be more efficiently scaled by a compile time constant such as RTE_MEMPOOL_ALIGN (= RTE_CACHE_LINE_SIZE). With a cache line size of 64 byte, that would allow indexing into mempools up to 256 GB in size.

> 
> > > Another possibility - user populates mempool himself with some
> > external
> > > memory by calling rte_mempool_populate_iova() directly.
> > Is the concern that IOVA might not be contiguous for all the memory
> > used by the mempool?
> >
> > > I suppose such situation can even occur even with normal
> > > rte_mempool_create(), though it should be a really rare one.
> > All in all, this feature needs to be configurable during compile
> time.