[dpdk-users] Query on handling packets

[Harsh Patel]

Hello,
The data mentioned in the previous mails are observations and the number of
threads mentioned are what the system is creating and not given by us to
the system. I'm not sure how to explain this by a picture but I will
provide a text explanation.

First, we ran the Linux kernel code which uses raw sockets and we gave 2
cores. That example used 2 threads on its own.
Secondly, we ran our DPDK in ns-3 code and we the same number of cores i.e.
2 cores. That example spawned 6 threads on its own.
(Note:- These are observations)
All of the above statistics were provided to answer the question if both
the simulations might be given different number of cores and may be that
was the reason of the performance bottleneck. Clearly they are both using
same no. of cores (2) and the results are what I have sent earlier. (Raw
socket ~ 10 Mbps and DPDK ~ 2.5 Mbps)

Now we thought that we might give more cores to DPDK in ns-3 code, which
might improve its performance.
This is where we gave 4 cores to our DPDK in ns-3 code which still spawned
the same 6 threads. And it gave the same results as 2 cores for DPDK in
ns-3.
This was the observation.

>From this, we assume that the number of cores is not a reason for the less
performance. This is not a problem we need to look somewhere else.
So, the problem due to which we are getting less performance and a
bottleneck around 2.5Mbps is somewhere else and we need to figure that out.

Ask again if not clear. If clear, we need to see where the problem is and
can you help in finding the reason why this happennig?

Thanks & Regards,
Harsh & Hrishikesh


On Fri, 30 Nov 2018 at 21:24, Wiles, Keith <keith.wiles at intel.com> wrote:

>
>
> > On Nov 30, 2018, at 3:02 AM, Harsh Patel <thadodaharsh10 at gmail.com>
> wrote:
> >
> > Hello,
> > Sorry for the long delay, we were busy with some exams.
> >
> > 1) About the NUMA sockets
> > This is the result of the command you mentioned :-
> > ======================================================================
> > Core and Socket Information (as reported by '/sys/devices/system/cpu')
> > ======================================================================
> >
> > cores =  [0, 1, 2, 3]
> > sockets =  [0]
> >
> >        Socket 0
> >        --------
> > Core 0 [0]
> > Core 1 [1]
> > Core 2 [2]
> > Core 3 [3]
> >
> > We don't know much about this and would like your input on what else to
> be checked or what do we need to do.
> >
> > 2) The part where you asked for a graph
> > We used `ps` to analyse which CPU cores are being utilized.
> > The raw socket version had two logical threads which used cores 0 and 1.
> > The DPDK version had 6 logical threads, which also used cores 0 and 1.
> This is the case for which we showed you the results.
> > As the previous case had 2 cores and was not giving desired results, we
> tried to give more cores to see if the DPDK in ns-3 code can achieve the
> desired throughput and pps. (We thought giving more cores might improve the
> performance.)
> > For this new case, we provided 4 total cores using  EAL arguments, upon
> which, it used cores 0-3. And still we got the same results as the one sent
> earlier.
> > We think this means that the bottleneck is a different problem unrelated
> to number of cores as of now. (This whole section is an answer to the
> question in the last paragraph raised by Kyle to which Keith asked for a
> graph)
>
> In the CPU output above you are running a four core system with no
> hyper-threads. This means you only have four core and four threads in the
> terms of DPDK. Using 6 logical threads will not improve performance in the
> DPDK case. DPDK normally uses a single thread per core. You can have more
> than one pthread per core, but having more than one thread per code
> requires the software to switch threads. Having context switch is not a
> good performance win in most cases.
>
> Not sure how your system is setup and a picture could help.
>
> I will be traveling all next week and responses will be slow.
>
> >
> > 3) About updating the TX_TIMEOUT and storing rte_get_timer_hz()
> > We have not tried this and will try it by today and will send you the
> status after that in some time.
> >
> > 4) For the suggestion by Stephen
> > We are not clear on what you suggested and it would be nice if you
> elaborate your suggestion.
> >
> > Thanks and Regards,
> > Harsh and Hrishikesh
> >
> > PS :- We are done with our exams and would be working now on this
> regularly.
> >
> > On Sun, 25 Nov 2018 at 10:05, Stephen Hemminger <
> stephen at networkplumber.org> wrote:
> > On Sat, 24 Nov 2018 16:01:04 +0000
> > "Wiles, Keith" <keith.wiles at intel.com> wrote:
> >
> > > > On Nov 22, 2018, at 9:54 AM, Harsh Patel <thadodaharsh10 at 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 at intel.com>
> wrote:
> > > >
> > > >
> > > > > On Nov 17, 2018, at 4:05 PM, Kyle Larose <eomereadig at gmail.com>
> wrote:
> > > > >
> > > > > On Sat, Nov 17, 2018 at 5:22 AM Harsh Patel <
> thadodaharsh10 at 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?
>
> Regards,
> Keith
>
>