Re: [dpdk-users] Query on handling packets

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

> On Nov 17, 2018, at 4:05 PM, Kyle Larose <eomereadig@gmail.com> wrote:
> 
> On Sat, Nov 17, 2018 at 5:22 AM Harsh Patel <thadodaharsh10@gmail.com> wrote:
>> 
>> Hello,
>> Thanks a lot for going through the code and providing us with so much
>> information.
>> We removed all the memcpy/malloc from the data path as you suggested and
> ...
>> After removing this, we are able to see a performance gain but not as good
>> as raw socket.
>> 
> 
> You're using an unordered_map to map your buffer pointers back to the
> mbufs. While it may not do a memcpy all the time, It will likely end
> up doing a malloc arbitrarily when you insert or remove entries from
> the map. If it needs to resize the table, it'll be even worse. You may
> want to consider using librte_hash:
> https://doc.dpdk.org/api/rte__hash_8h.html instead. Or, even better,
> see if you can design the system to avoid needing to do a lookup like
> this. Can you return a handle with the mbuf pointer and the data
> together?
> 
> You're also using floating point math where it's unnecessary (the
> timing check). Just multiply the numerator by 1000000 prior to doing
> the division. I doubt you'll overflow a uint64_t with that. It's not
> as efficient as integer math, though I'm not sure offhand it'd cause a
> major perf problem.
> 
> One final thing: using a raw socket, the kernel will take over
> transmitting and receiving to the NIC itself. that means it is free to
> use multiple CPUs for the rx and tx. I notice that you only have one
> rx/tx queue, meaning at most one CPU can send and receive packets.
> When running your performance test with the raw socket, you may want
> to see how busy the system is doing packet sends and receives. Is it
> using more than one CPU's worth of processing? Is it using less, but
> when combined with your main application's usage, the overall system
> is still using more than one?

Along with the floating point math, I would remove all floating point math and use the rte_rdtsc() function to use cycles. Using something like:

uint64_t cur_tsc, next_tsc, timo = (rte_timer_get_hz() / 16);	/* One 16th of a second use 2/4/8/16/32 power of two numbers to make the math simple divide */

cur_tsc = rte_rdtsc();

next_tsc = cur_tsc + timo; /* Now next_tsc the next time to flush */

while(1) {
	cur_tsc = rte_rdtsc();
	if (cur_tsc >= next_tsc) {
		flush();
		next_tsc += timo;
	}
	/* Do other stuff */
}

For the m_bufPktMap I would use the rte_hash or do not use a hash at all by grabbing the buffer address and subtract the
mbuf = (struct rte_mbuf *)RTE_PTR_SUB(buf, sizeof(struct rte_mbuf) + RTE_MAX_HEADROOM);


DpdkNetDevice:Write(uint8_t *buffer, size_t length)
{
	struct rte_mbuf *pkt;
	uint64_t cur_tsc;

	pkt = (struct rte_mbuf *)RTE_PTR_SUB(buffer, sizeof(struct rte_mbuf) + RTE_MAX_HEADROOM);

	/* No need to test pkt, but buffer maybe tested to make sure it is not null above the math above */

	pkt->pk_len = length;
	pkt->data_len = length;

	rte_eth_tx_buffer(m_portId, 0, m_txBuffer, pkt);

	cur_tsc = rte_rdtsc();

	/* next_tsc is a private variable */
	if (cur_tsc >= next_tsc) {
		rte_eth_tx_buffer_flush(m_portId, 0, m_txBuffer);	/* hardcoded the queue id, should be fixed */
		next_tsc = cur_tsc + timo; /* timo is a fixed number of cycles to wait */
	}
	return length;
}

DpdkNetDevice::Read()
{
	struct rte_mbuf *pkt;

	if (m_rxBuffer->length == 0) {
		m_rxBuffer->next = 0;
		m_rxBuffer->length = rte_eth_rx_burst(m_portId, 0, m_rxBuffer->pmts, MAX_PKT_BURST);

		if (m_rxBuffer->length == 0)
			return std::make_pair(NULL, -1);
	}

	pkt = m_rxBuffer->pkts[m_rxBuffer->next++];

	/* do not use rte_pktmbuf_read() as it does a copy for the complete packet */

	return std:make_pair(rte_pktmbuf_mtod(pkt, char *), pkt->pkt_len);
}

void
DpdkNetDevice::FreeBuf(uint8_t *buf)
{
	struct rte_mbuf *pkt;

	if (!buf)
		return;
	pkt = (struct rte_mbuf *)RTE_PKT_SUB(buf, sizeof(rte_mbuf) + RTE_MAX_HEADROOM);

	rte_pktmbuf_free(pkt);
}

When your code is done with the buffer, then convert the buffer address back to a rte_mbuf pointer and call rte_pktmbuf_free(pkt); This should eliminate the copy and floating point code. Converting my C code to C++ priceless :-)

Hopefully the buffer address passed is the original buffer address and has not be adjusted.


Regards,
Keith