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

["Xie, Huawei" <huawei.xie@intel.com>]

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