[dpdk-users] Optimal number of elements in mempool n = (2^q - 1) vs examples, what is the right thing to do?

[Olivier Matz]

Hi Vlad,

On Wed, 1 Feb 2017 16:54:57 +0000, Vlad.Lazarenko at worldquant.com
(Lazarenko, Vlad (WorldQuant)) wrote:
> Hello,
> 
> I'm new to DPDK and have noticed that documentation for
> rte_mempool_create states that the optimal size for a number of
> elements in the pool is n = (2^q-1). But in many examples it is
> simply set to 2^q (multi_process/simple_mp/main.c uses 2014, for
> example). This is a bit confusing. Is 2^q - 1 really the optimal
> number but examples don't use it, or maybe the documentation for the
> mempool is wrong, or...? If anyone could shed some light on this
> that'd be helpful.

That's true for rte_mempool based on a rte_ring (this is the default,
but since recently, it's possible to use another handler).

The size of a rte_ring is (2^n - 1), because one element in the ring is
reserved to distinguish between a full an an empty ring. So, when a
mempool uses a ring, if we ask for 2^n elements, a ring of size
(2^(n+1) - 1) is created, which can consume additional memory.

On the other hand, the mempool object size is often much larger than
a ring entry (usually 8 bytes, the size of a pointer), especially
knowing that by default, the objects are cache aligned (usually 64
bytes).

So we may remove this note in the future since it's not very relevant.

Regards,
Olivier

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