Re: [dpdk-dev] [PATCH v3] net/mlx5: relaxed ordering for multi-packet RQ buffer refcnt

[Phil Yang <Phil.Yang@arm.com>]

Alexander Kozyrev <akozyrev@mellanox.com> writes:

<snip>
> >
> > > > > Subject: RE: [dpdk-dev] [PATCH v3] net/mlx5: relaxed ordering for
> > > > > multi- packet RQ buffer refcnt
> > > > >
> > > > > Hi Phil Yang, we noticed that this patch gives us 10% of
> > > > > performance degradation on ARM.
> > > > > x86 seems to be unaffected though. Do you know what may be the
> > > > > reason of this behavior?
> > > >
> > > > Hi Alexander,
> > > >
> > > > Thanks for your feedback.
> > > > This patch removed some expensive memory barriers on aarch64, it
> > > > should get better performance.
> > > > I am not sure the root cause of this degradation now, I will start
> > > > the investigation. We can profiling this issue together.
> > > > Could you share your test case(including your testbed configuration)
> > > > with
> > > us?
<...>
> > >
> > > I'm surprised too, Phil, but looks like it is actually making things
> > > worse. I used Connect-X 6DX on aarch64:
> > > Linux dragon71-bf 5.4.31-mlnx.15.ge938819 #1 SMP PREEMPT Thu Jul 2
> > > 17:01:15 IDT 2020 aarch64 aarch64 aarch64 GNU/Linux Traffic generator
> > > sends 60 bytes packets and DUT executes the following command:
> > > arm64-bluefield-linuxapp-gcc/build/app/test-pmd/testpmd -n 4  -w
> > > 0000:03:00.1,mprq_en=1,rxqs_min_mprq=1  -w
> > > 0000:03:00.0,mprq_en=1,rxqs_min_mprq=1 -c 0xe  -- --burst=64 --
> > > mbcache=512 -i  --nb-cores=1  --rxq=1 --txq=1 --txd=256 --rxd=256
> > > --auto- start --rss-udp Without a patch I'm getting 3.2mpps, and only
> > > 2.9mpps when the patch is applied.
> > You are running on A72 cores, is that correct?
> 
> Correct, cat /proc/cpuinfo
> processor       : 0
> BogoMIPS        : 312.50
> Features        : fp asimd evtstrm crc32 cpuid
> CPU implementer : 0x41
> CPU architecture: 8
> CPU variant     : 0x0
> CPU part        : 0xd08
> CPU revision    : 3

Thanks a lot for your input, Alex.
With your test command line, I remeasured this patch on two different aarch64 machines and both got some performance improvement.

SOC#1. On Thunderx2 (with LSE support), I see 7.6% performance improvement on throughput. 
NIC: ConnectX-6 / driver: mlx5_core version: 5.0-1.0.0.0 / firmware-version: 20.27.1016 (MT_0000000224)

SOC#2. On N1SDP (I disabled LSE to generate A72 likewise instructions), I also see slightly (about 1%~2%) performance improvement on throughput.
NIC: ConnectX-5 / driver: mlx5_core / version: 5.0-2.1.8 / firmware-version: 16.27.2008 (MT_0000000090)

Without LSE (i.e. A72 and SOC#2 case.) it uses the 'Exclusive' mechanism to achieve atomicity.
For example, it generates below instructions for __atomic_add_fetch.
__atomic_add_fetch(&buf->refcnt, 1, __ATOMIC_ACQUIRE);
   70118:       f94037e3        ldr     x3, [sp, #104]
   7011c:       91002060        add     x0, x3, #0x8
   70120:       485ffc02        ldaxrh  w2, [x0]
   70124:       11000442        add     w2, w2, #0x1
   70128:       48057c02        stxrh   w5, w2, [x0]
   7012c:       35ffffa5        cbnz    w5, 70120 <mlx5_rx_burst_mprq+0xb48>

In general, I think this patch will not lead to a sharp decline in performance. 
Maybe you can try other testbeds?

Thanks,
Phil