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Date: Fri, 4 Mar 2016 10:17:05 +0800
From: Yuanhan Liu <yuanhan.liu@linux.intel.com>
To: "Xie, Huawei" <huawei.xie@intel.com>
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Cc: "Michael S. Tsirkin" <mst@redhat.com>, "dev@dpdk.org" <dev@dpdk.org>,
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Subject: Re: [dpdk-dev] [PATCH v2 1/7] vhost: refactor
	rte_vhost_dequeue_burst
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On Thu, Mar 03, 2016 at 04:30:42PM +0000, Xie, Huawei wrote:
> On 2/18/2016 9:48 PM, Yuanhan Liu wrote:
> > +	mbuf_avail  = 0;
> > +	mbuf_offset = 0;
> > +	while (desc_avail || (desc->flags & VRING_DESC_F_NEXT) != 0) {
> > +		/* This desc reachs to its end, get the next one */
> > +		if (desc_avail == 0) {
> > +			desc = &vq->desc[desc->next];
> > +
> > +			desc_addr = gpa_to_vva(dev, desc->addr);
> > +			rte_prefetch0((void *)(uintptr_t)desc_addr);
> > +
> > +			desc_offset = 0;
> > +			desc_avail  = desc->len;
> > +
> > +			PRINT_PACKET(dev, (uintptr_t)desc_addr, desc->len, 0);
> > +		}
> > +
> > +		/*
> > +		 * This mbuf reachs to its end, get a new one
> > +		 * to hold more data.
> > +		 */
> > +		if (mbuf_avail == 0) {
> > +			cur = rte_pktmbuf_alloc(mbuf_pool);
> > +			if (unlikely(!cur)) {
> > +				RTE_LOG(ERR, VHOST_DATA, "Failed to "
> > +					"allocate memory for mbuf.\n");
> > +				if (head)
> > +					rte_pktmbuf_free(head);
> > +				return NULL;
> > +			}
> 
> We could always allocate the head mbuf before the loop, then we save the
> following branch and make the code more streamlined.
> It reminds me that this change prevents the possibility of mbuf bulk
> allocation, one solution is we pass the head mbuf from an additional
> parameter.

Yep, that's also something I have thought of.

> Btw, put unlikely before the check of mbuf_avail and checks elsewhere.

I don't think so. It would benifit for the small packets. What if,
however, when TSO or jumbo frame is enabled that we have big packets?

	--yliu