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

[Pavel Shirshov]

Hi Olivier,

It's a good explanation of rte_mempool internals. I think it would be
good to have your comment in the rte_mempool documentation. Could we
add it there?

Thanks

On Mon, Feb 6, 2017 at 5:51 AM, Olivier Matz <olivier.matz at 6wind.com> wrote:
> 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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