Re: [dpdk-ci] [dpdk-users] DPDK TX problems

[Lincoln Lavoie <lylavoie@iol.unh.edu>]

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Hi Thomas,

I've captured this as https://bugs.dpdk.org/show_bug.cgi?id=429, so we can
add this to the list of development items for the testing, etc.

Cheers,
Lincoln

On Thu, Mar 26, 2020 at 4:54 PM Thomas Monjalon <thomas@monjalon.net> wrote:

> 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.
>
>
>
>
>

-- 
*Lincoln Lavoie*
Senior Engineer, Broadband Technologies
21 Madbury Rd., Ste. 100, Durham, NH 03824
lylavoie@iol.unh.edu
https://www.iol.unh.edu
+1-603-674-2755 (m)
<https://www.iol.unh.edu/>

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