Re: [dpdk-users] Peformance troubleshouting of TCP/IP stack over DPDK.

[Pavel Vajarov <freakpv@gmail.com>]

On Wed, May 6, 2020 at 5:55 PM Stephen Hemminger <stephen@networkplumber.org>
wrote:

> On Wed, 6 May 2020 08:14:20 +0300
> Pavel Vajarov <freakpv@gmail.com> wrote:
>
> > Hi there,
> >
> > We are trying to compare the performance of DPDK+FreeBSD networking stack
> > vs standard Linux kernel and we have problems finding out why the former
> is
> > slower. The details are below.
> >
> > There is a project called F-Stack <https://github.com/F-Stack/f-stack>.
> > It glues the networking stack from
> > FreeBSD 11.01 over DPDK. We made a setup to test the performance of
> > transparent
> > TCP proxy based on F-Stack and another one running on Standard Linux
> > kernel.
> > We did the tests on KVM with 2 cores (Intel(R) Xeon(R) Gold 6139 CPU @
> > 2.30GHz)
> > and 32GB RAM. 10Gbs NIC was attached in passthrough mode.
> > The application level code, the one which handles epoll notifications and
> > memcpy data between the sockets, of the both proxy applications is 100%
> the
> > same. Both proxy applications are single threaded and in all tests we
> > pinned the applications on core 1. The interrupts from the network card
> > were pinned to the same core 1 for the test with the standard Linux
> > application.
> >
> > Here are the test results:
> > 1. The Linux based proxy was able to handle about 1.7-1.8 Gbps before it
> > started to throttle the traffic. No visible CPU usage was observed on
> core
> > 0 during the tests, only core 1, where the application and the IRQs were
> > pinned, took the load.
> > 2. The DPDK+FreeBSD proxy was able to thandle 700-800 Mbps before it
> > started to throttle the traffic. No visible CPU usage was observed on
> core
> > 0 during the tests only core 1, where the application was pinned, took
> the
> > load. In some of the latter tests I did some changes to the number of
> read
> > packets in one call from the network card and the number of handled
> events
> > in one call to epoll. With these changes I was able to increase the
> > throughput
> > to 900-1000 Mbps but couldn't increase it more.
> > 3. We did another test with the DPDK+FreeBSD proxy just to give us some
> > more info about the problem. We disabled the TCP proxy functionality and
> > let the packets be simply ip forwarded by the FreeBSD stack. In this test
> > we reached up to 5Gbps without being able to throttle the traffic. We
> just
> > don't have more traffic to redirect there at the moment. So the bottlneck
> > seem to be either in the upper level of the network stack or in the
> > application
> > code.
> >
> > There is a huawei switch which redirects the traffic to this server. It
> > regularly
> > sends arping and if the server doesn't respond it stops the redirection.
> > So we assumed that when the redirection stops it's because the server
> > throttles the traffic and drops packets and can't respond to the arping
> > because
> > of the packets drop.
> >
> > The whole application can be very roughly represented in the following
> way:
> >  - Write pending outgoing packets to the network card
> > - Read incoming packets from the network card
> >  - Push the incoming packets to the FreeBSD stack
> >  - Call epoll_wait/kevent without waiting
> >  - Handle the events
> >  - loop from the beginning
> > According to the performance profiling that we did, aside from packet
> > processing,
> >  about 25-30% of the application time seems to be spent in the
> > epoll_wait/kevent
> > even though the `timeout` parameter of this call is set to 0 i.e.
> > it shouldn't block waiting for events if there is none.
> >
> > I can give you much more details and code for everything, if needed.
> >
> > My questions are:
> > 1. Does somebody have observations or educated guesses about what amount
> of
> > traffic should I expect the DPDK + FreeBSD stack + kevent to process in
> the
> > above
> > scenario? Are the numbers low or expected?
> > We've expected to see better performance than the standard Linux kernel
> one
> > but
> > so far we can't get this performance.
> > 2. Do you think the diffrence comes because of the time spending handling
> > packets
> > and handling epoll in both of the tests? What do I mean. For the standard
> > Linux tests
> > the interrupts handling has higher priority than the epoll handling and
> > thus the application
> > can spend much more time handling packets and processing them in the
> kernel
> > than
> > handling epoll events in the user space. For the DPDK+FreeBSD case the
> time
> > for
> > handling packets and the time for processing epolls is kind of equal. I
> > think, that this was
> > the reason why we were able to get more performance increasing the number
> > of read
> > packets at one go and decreasing the epoll events. However, we couldn't
> > increase the
> > throughput enough with these tweaks.
> > 3. Can you suggest something else that we can test/measure/profile to get
> > better idea
> > what exactly is happening here and to improve the performance more?
> >
> > Any help is appreciated!
> >
> > Thanks in advance,
> > Pavel.
>
> First off, if you are testing on KVM, are you using PCI pass thru or SR-IOV
> to make the device available to the guest directly. The default mode uses
> a Linux bridge, and this results in multiple copies and context switches.
> You end up testing Linux bridge and virtio performance, not TCP.
>
> To get full speed with TCP and most software stacks you need TCP
> segmentation
> offload.
>
> Also software queue discipline, kernel version, and TCP congestion control
> can have a big role in your result.
>

Hi,

Thanks for the response.

We did the tests on Ubuntu 18.04.4 LTS (GNU/Linux 4.15.0-96-generic x86_64).
The NIC was given to the guest using SR-IOV.
The TCP segmentation offload was enabled for both tests (standard Linux and
DPDK+FreeBSD).
The congestion control algorithm for both tests was 'cubic'.

What do you mean by 'software queue discipline'?

Regards,
Pavel.