Re: [dpdk-users] DPDK TX problems

[Thomas Monjalon <thomas@monjalon.net>]

Thanks for the interesting feedback.
It seems we should test this performance use case in our labs.


18/02/2020 09:36, Hrvoje Habjanic:
> On 08. 04. 2019. 11:52, Hrvoje Habjanić wrote:
> > On 29/03/2019 08:24, Hrvoje Habjanić wrote:
> >>> Hi.
> >>>
> >>> I did write an application using dpdk 17.11 (did try also with 18.11),
> >>> and when doing some performance testing, i'm seeing very odd behavior.
> >>> To verify that this is not because of my app, i did the same test with
> >>> l2fwd example app, and i'm still confused by results.
> >>>
> >>> In short, i'm trying to push a lot of L2 packets through dpdk engine -
> >>> packet processing is minimal. When testing, i'm starting with small
> >>> number of packets-per-second, and then gradually increase it to see
> >>> where is the limit. At some point, i do reach this limit - packets start
> >>> to get dropped. And this is when stuff become weird.
> >>>
> >>> When i reach peek packet rate (at which packets start to get dropped), i
> >>> would expect that reducing packet rate will remove packet drops. But,
> >>> this is not the case. For example, let's assume that peek packet rate is
> >>> 3.5Mpps. At this point everything works ok. Increasing pps to 4.0Mpps,
> >>> makes a lot of dropped packets. When reducing pps back to 3.5Mpps, app
> >>> is still broken - packets are still dropped.
> >>>
> >>> At this point, i need to drastically reduce pps (1.4Mpps) to make
> >>> dropped packets go away. Also, app is unable to successfully forward
> >>> anything beyond this 1.4M, despite the fact that in the beginning it did
> >>> forward 3.5M! Only way to recover is to restart the app.
> >>>
> >>> Also, sometimes, the app just stops forwarding any packets - packets are
> >>> received (as seen by counters), but app is unable to send anything back.
> >>>
> >>> As i did mention, i'm seeing the same behavior with l2fwd example app. I
> >>> did test dpdk 17.11 and also dpdk 18.11 - the results are the same.
> >>>
> >>> My test environment is HP DL380G8, with 82599ES 10Gig (ixgbe) cards,
> >>> connected with Cisco nexus 9300 sw. On the other side is ixia test
> >>> appliance. Application is run in virtual machine (VM), using KVM
> >>> (openstack, with sriov enabled, and numa restrictions). I did check that
> >>> VM is using only cpu's from NUMA node on which network card is
> >>> connected, so there is no cross-numa traffic. Openstack is Queens,
> >>> Ubuntu is Bionic release. Virtual machine is also using ubuntu bionic
> >>> as OS.
> >>>
> >>> I do not know how to debug this? Does someone else have the same
> >>> observations?
> >>>
> >>> Regards,
> >>>
> >>> H.
> >> There are additional findings. It seems that when i reach peak pps
> >> rate, application is not fast enough, and i can see rx missed errors
> >> on card statistics on the host. At the same time, tx side starts to
> >> show problems (tx burst starts to show it did not send all packets).
> >> Shortly after that, tx falls apart completely and top pps rate drops.
> >>
> >> Since i did not disable pause frames, i can see on the switch "RX
> >> pause" frame counter is increasing. On the other hand, if i disable
> >> pause frames (on the nic of server), host driver (ixgbe) reports "TX
> >> unit hang" in dmesg, and issues card reset. Of course, after reset
> >> none of the dpdk apps in VM's on this host does not work.
> >>
> >> Is it possible that at time of congestion DPDK does not release mbufs
> >> back to the pool, and tx ring becomes "filled" with zombie packets
> >> (not send by card and also having ref counter as they are in use)?
> >>
> >> Is there a way to check mempool or tx ring for "left-owers"? Is is
> >> possible to somehow "flush" tx ring and/or mempool?
> >>
> >> H.
> > After few more test, things become even weirder - if i do not free mbufs
> > which are not sent, but resend them again, i can "survive" over-the-peek
> > event! But, then peek rate starts to drop gradually ...
> >
> > I would ask if someone can try this on their platform and report back? I
> > would really like to know if this is problem with my deployment, or
> > there is something wrong with dpdk?
> >
> > Test should be simple - use l2fwd or l3fwd, and determine max pps. Then
> > drive pps 30%over max, and then return back and confirm that you can
> > still get max pps.
> >
> > Thanks in advance.
> >
> > H.
> >
> 
> I did receive few mails from users facing this issue, asking how it was
> resolved.
> 
> Unfortunately, there is no real fix. It seems that this issue is related
> to card and hardware used. I'm still not sure which is more to blame,
> but the combination i had is definitely problematic.
> 
> Anyhow, in the end, i did conclude that card driver have some issues
> when it is saturated with packets. My suspicion is that driver/software
> does not properly free packets, and then DPDK mempool becomes
> fragmented, and this causes performance drops. Restarting software
> releases pools, and restores proper functionality.
> 
> After no luck with ixgbe, we migrated to Mellanox (4LX), and now there
> is no more of this permanent performance drop. With mlx, when limit is
> reached, reducing number of packets restores packet forwarding, and this
> limit seems to be stable.
> 
> Also, we moved to newer servers - DL380G10, and got significant
> performance increase. Also, we moved to newer switch (also cisco), with
> 25G ports, which reduced latency - almost by factor of 2!
> 
> I did not try old ixgbe on newer server, but i did try Intel's XL710,
> and it is not as happy as Mellanox. It gives better PPS, but it is more
> unstable in terms of maximum bw (has similar issues as ixgbe).
> 
> Regards,
> 
> H.