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

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

On 12/19/2015 1:32 AM, Stephen Hemminger 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()
>
Stephen:
How about we move while a bit:
    switch(count % 4) {
    case 0: while (idx != count) {
            ... reset ...
    case 3:
            ... reset ...
    case 2:
            ... reset ...
    case 1:
            ... reset ...
     }
     }

With do {} while, we probably need one more extra check on if count is
zero. Duff's initial implementation assumes that count isn't zero. With
while loop, we save one line of code.