Re: [dpdk-dev] [RFC 0/7] changing mbuf pool handler

[Olivier Matz <olivier.matz@6wind.com>]

Hi Hemant,

Thank you for your feedback.

On 09/22/2016 01:52 PM, Hemant Agrawal wrote:
> Hi Olivier
> 
> On 9/19/2016 7:12 PM, Olivier Matz wrote:
>> Hello,
>>
>> Following discussion from [1] ("usages issue with external mempool").
>>
>> This is a tentative to make the mempool_ops feature introduced
>> by David Hunt [2] more widely used by applications.
>>
>> It applies on top of a minor fix in mbuf lib [3].
>>
>> To sumarize the needs (please comment if I did not got it properly):
>>
>> - new hw-assisted mempool handlers will soon be introduced
>> - to make use of it, the new mempool API [4] (rte_mempool_create_empty,
>>   rte_mempool_populate, ...) has to be used
>> - the legacy mempool API (rte_mempool_create) does not allow to change
>>   the mempool ops. The default is "ring_<s|m>p_<s|m>c" depending on
>>   flags.
>> - the mbuf helper (rte_pktmbuf_pool_create) does not allow to change
>>   them either, and the default is RTE_MBUF_DEFAULT_MEMPOOL_OPS
>>   ("ring_mp_mc")
>> - today, most (if not all) applications and examples use either
>>   rte_pktmbuf_pool_create or rte_mempool_create to create the mbuf
>>   pool, making it difficult to take advantage of this feature with
>>   existing apps.
>>
>> My initial idea was to deprecate both rte_pktmbuf_pool_create() and
>> rte_mempool_create(), forcing the applications to use the new API, which
>> is more flexible. But after digging a bit, it appeared that
>> rte_mempool_create() is widely used, and not only for mbufs. Deprecating
>> it would have a big impact on applications, and replacing it with the
>> new API would be overkill in many use-cases.
> 
> I agree with the proposal.
> 
>>
>> So I finally tried the following approach (inspired from a suggestion
>> Jerin [5]):
>>
>> - add a new mempool_ops parameter to rte_pktmbuf_pool_create(). This
>>   unfortunatelly breaks the API, but I implemented an ABI compat layer.
>>   If the patch is accepted, we could discuss how to announce/schedule
>>   the API change.
>> - update the applications and documentation to prefer
>>   rte_pktmbuf_pool_create() as much as possible
>> - update most used examples (testpmd, l2fwd, l3fwd) to add a new command
>>   line argument to select the mempool handler
>>
>> I hope the external applications would then switch to
>> rte_pktmbuf_pool_create(), since it supports most of the use-cases (even
>> priv_size != 0, since we can call rte_mempool_obj_iter() after) .
>>
> 
> I will still prefer if you can add the "rte_mempool_obj_cb_t *obj_cb,
> void *obj_cb_arg" into "rte_pktmbuf_pool_create". This single
> consolidated wrapper will almost make it certain that applications will
> not try to use rte_mempool_create for packet buffers.

The patch changes the example applications. I'm not sure I understand
why adding these arguments would force application to not use
rte_mempool_create() for packet buffers. Do you have a application in mind?

For the mempool_ops parameter, we must pass it at init because we need
to know the mempool handler before populating the pool. For object
initialization, it can be done after, so I thought it was better to
reduce the number of arguments to avoid to fall in the mempool_create()
syndrom :)

Any other opinions?

Regards,
Olivier