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

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

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
>
>


Regards,
Keith