Re: [dpdk-users] Query on handling packets

[Stephen Hemminger <stephen@networkplumber.org>]

On Sat, 24 Nov 2018 16:01:04 +0000
"Wiles, Keith" <keith.wiles@intel.com> wrote:

> > On Nov 22, 2018, at 9:54 AM, Harsh Patel <thadodaharsh10@gmail.com> wrote:
> > 
> > Hi
> > 
> > Thank you so much for the reply and for the solution.
> > 
> > We used the given code. We were amazed by the pointer arithmetic you used, got to learn something new.
> > 
> > But still we are under performing.The same bottleneck of ~2.5Mbps is seen.
> > 
> > We also checked if the raw socket was using any extra (logical) cores than the DPDK. We found that raw socket has 2 logical threads running on 2 logical CPUs. Whereas, the DPDK version has 6 logical threads on 2 logical CPUs. We also ran the 6 threads on 4 logical CPUs, still we see the same bottleneck.
> > 
> > We have updated our code (you can use the same links from previous mail). It would be helpful if you could help us in finding what causes the bottleneck.  
> 
> I looked at the code for a few seconds and noticed your TX_TIMEOUT is macro that calls (rte_get_timer_hz()/2014) just to be safe I would not call rte_get_timer_hz() time, but grab the value and store the hz locally and use that variable instead. This will not improve performance is my guess and I would have to look at the code the that routine to see if it buys you anything to store the value locally. If the getting hz is just a simple read of a variable then good, but still you should should a local variable within the object to hold the (rte_get_timer_hz()/2048) instead of doing the call and divide each time.
> 
> > 
> > Thanks and Regards, 
> > Harsh and Hrishikesh 
> > 
> > 
> > On Mon, Nov 19, 2018, 19:19 Wiles, Keith <keith.wiles@intel.com> wrote:
> > 
> >   
> > > 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
> >   
> 
> Regards,
> Keith
> 

Also rdtsc causes cpu to stop doing any look ahead, so there is a heisenberg effect.
Adding more rdtsc will hurt performance.  It also looks like your code is not doing bursting correctly.
What if multiple packets arrive in one rx_burst?