Re: [dpdk-dev] [PATCH v2 1/2] mbuf: provide rte_pktmbuf_alloc_bulk API

["Wiles, Keith" <keith.wiles@intel.com>]

On 12/21/15, 9:21 AM, "Xie, Huawei" <huawei.xie@intel.com> wrote:

>On 12/19/2015 3:27 AM, Wiles, Keith wrote:
>> On 12/18/15, 11:32 AM, "dev on behalf of Stephen Hemminger" <dev-bounces@dpdk.org on behalf of stephen@networkplumber.org> wrote:
>>
>>> On Fri, 18 Dec 2015 10:44:02 +0000
>>> "Ananyev, Konstantin" <konstantin.ananyev@intel.com> wrote:
>>>
>>>>
>>>>> -----Original Message-----
>>>>> From: dev [mailto:dev-bounces@dpdk.org] On Behalf Of Stephen Hemminger
>>>>> Sent: Friday, December 18, 2015 5:01 AM
>>>>> To: Xie, Huawei
>>>>> Cc: dev@dpdk.org
>>>>> Subject: Re: [dpdk-dev] [PATCH v2 1/2] mbuf: provide rte_pktmbuf_alloc_bulk API
>>>>>
>>>>> On Mon, 14 Dec 2015 09:14:41 +0800
>>>>> Huawei Xie <huawei.xie@intel.com> wrote:
>>>>>
>>>>>> v2 changes:
>>>>>>  unroll the loop a bit to help the performance
>>>>>>
>>>>>> rte_pktmbuf_alloc_bulk allocates a bulk of packet mbufs.
>>>>>>
>>>>>> There is related thread about this bulk API.
>>>>>> http://dpdk.org/dev/patchwork/patch/4718/
>>>>>> Thanks to Konstantin's loop unrolling.
>>>>>>
>>>>>> Signed-off-by: Gerald Rogers <gerald.rogers@intel.com>
>>>>>> Signed-off-by: Huawei Xie <huawei.xie@intel.com>
>>>>>> Acked-by: Konstantin Ananyev <konstantin.ananyev@intel.com>
>>>>>> ---
>>>>>>  lib/librte_mbuf/rte_mbuf.h | 50 ++++++++++++++++++++++++++++++++++++++++++++++
>>>>>>  1 file changed, 50 insertions(+)
>>>>>>
>>>>>> diff --git a/lib/librte_mbuf/rte_mbuf.h b/lib/librte_mbuf/rte_mbuf.h
>>>>>> index f234ac9..4e209e0 100644
>>>>>> --- a/lib/librte_mbuf/rte_mbuf.h
>>>>>> +++ b/lib/librte_mbuf/rte_mbuf.h
>>>>>> @@ -1336,6 +1336,56 @@ static inline struct rte_mbuf *rte_pktmbuf_alloc(struct rte_mempool *mp)
>>>>>>  }
>>>>>>
>>>>>>  /**
>>>>>> + * Allocate a bulk of mbufs, initialize refcnt and reset the fields to default
>>>>>> + * values.
>>>>>> + *
>>>>>> + *  @param pool
>>>>>> + *    The mempool from which mbufs are allocated.
>>>>>> + *  @param mbufs
>>>>>> + *    Array of pointers to mbufs
>>>>>> + *  @param count
>>>>>> + *    Array size
>>>>>> + *  @return
>>>>>> + *   - 0: Success
>>>>>> + */
>>>>>> +static inline int rte_pktmbuf_alloc_bulk(struct rte_mempool *pool,
>>>>>> +	 struct rte_mbuf **mbufs, unsigned count)
>>>>>> +{
>>>>>> +	unsigned idx = 0;
>>>>>> +	int rc;
>>>>>> +
>>>>>> +	rc = rte_mempool_get_bulk(pool, (void **)mbufs, count);
>>>>>> +	if (unlikely(rc))
>>>>>> +		return rc;
>>>>>> +
>>>>>> +	switch (count % 4) {
>>>>>> +	while (idx != count) {
>>>>>> +		case 0:
>>>>>> +			RTE_MBUF_ASSERT(rte_mbuf_refcnt_read(mbufs[idx]) == 0);
>>>>>> +			rte_mbuf_refcnt_set(mbufs[idx], 1);
>>>>>> +			rte_pktmbuf_reset(mbufs[idx]);
>>>>>> +			idx++;
>>>>>> +		case 3:
>>>>>> +			RTE_MBUF_ASSERT(rte_mbuf_refcnt_read(mbufs[idx]) == 0);
>>>>>> +			rte_mbuf_refcnt_set(mbufs[idx], 1);
>>>>>> +			rte_pktmbuf_reset(mbufs[idx]);
>>>>>> +			idx++;
>>>>>> +		case 2:
>>>>>> +			RTE_MBUF_ASSERT(rte_mbuf_refcnt_read(mbufs[idx]) == 0);
>>>>>> +			rte_mbuf_refcnt_set(mbufs[idx], 1);
>>>>>> +			rte_pktmbuf_reset(mbufs[idx]);
>>>>>> +			idx++;
>>>>>> +		case 1:
>>>>>> +			RTE_MBUF_ASSERT(rte_mbuf_refcnt_read(mbufs[idx]) == 0);
>>>>>> +			rte_mbuf_refcnt_set(mbufs[idx], 1);
>>>>>> +			rte_pktmbuf_reset(mbufs[idx]);
>>>>>> +			idx++;
>>>>>> +	}
>>>>>> +	}
>>>>>> +	return 0;
>>>>>> +}
>>>>> This is weird. Why not just use Duff's device in a more normal manner.
>>>> But it is a sort of Duff's method.
>>>> Not sure what looks weird to you here?
>>>> while () {} instead of do {} while();?
>>>> Konstantin
>>>>
>>>>
>>>>
>>> It is unusual to have cases not associated with block of the switch.
>>> Unusual to me means, "not used commonly in most code".
>>>
>>> Since you are jumping into the loop, might make more sense as a do { } while()
>> I find this a very odd coding practice and I would suggest we not do this, unless it gives us some great performance gain.
>>
>> Keith
>The loop unwinding could give performance gain. The only problem is the
>switch/loop combination makes people feel weird at the first glance but
>soon they will grasp this style. Since this is inherited from old famous
>duff's device, i prefer to keep this style which saves lines of code.

Please add a comment to the code to reflex where this style came from and why you are using it, would be very handy here.

>>>
>>
>> Regards,
>> Keith
>>
>>
>>
>>
>
>


Regards,
Keith