DPDK and energy efficiency

Tue Feb 23 21:30:20 UTC 2021

> > No, it is not PMD that runs the processor in a polling loop.
> > It is the application itself, thay may or may not busy loop,
> > depending on application programmers choice.
> 
> From one of my earlier references [2]:
> 
> "we found that a poll mode driver (PMD)
> thread accounted for approximately 99.7 percent
> CPU occupancy (a full core utilization)."
> 
> And further on:
> 
> "we found that the thread kept spinning on the following code block:
> 
> *for ( ; ; ) {for ( i = 0; i < poll_cnt; i ++) {dp_netdev_process_rxq_port
> (pmd, list[i].port, poll_list[i].rx) ;}}*
> This indicates that the thread was continuously
> monitoring and executing the receiving data path."

This comes from OVS code and shows OVS thread spinning, not DPDK PMD.
Blame the OVS application for not using e.g. _mm_pause() and burning
the CPU like crazy.

For comparison, take a look at top+i7z output from DPDK-based 100G DDoS
scrubber currently lifting some low traffic using cores 1-13 on 16 core
host. It uses naive DPDK::rte_pause() throttling to enter C1.

Tasks: 342 total,   1 running, 195 sleeping,   0 stopped,   0 zombie
%Cpu(s):  6.6 us,  0.6 sy,  0.0 ni, 89.7 id,  3.1 wa,  0.0 hi,  0.0 si,  0.0 st

        Core [core-id]  :Actual Freq (Mult.)      C0%   Halt(C1)%  C3 %   C6 %  Temp      VCore
        Core 1 [0]:       1467.73 (14.68x)      2.15    5.35       1    92.3    43      0.6724
        Core 2 [1]:       1201.09 (12.01x)      11.7    93.9       0       0    39      0.6575
        Core 3 [2]:       1200.06 (12.00x)      11.8    93.8       0       0    42      0.6543
        Core 4 [3]:       1200.14 (12.00x)      11.8    93.8       0       0    41      0.6549
        Core 5 [4]:       1200.10 (12.00x)      11.8    93.8       0       0    41      0.6526
        Core 6 [5]:       1200.12 (12.00x)      11.8    93.8       0       0    40      0.6559
        Core 7 [6]:       1201.01 (12.01x)      11.8    93.8       0       0    41      0.6559
        Core 8 [7]:       1201.02 (12.01x)      11.8    93.8       0       0    43      0.6525
        Core 9 [8]:       1201.00 (12.01x)      11.8    93.8       0       0    41      0.6857
        Core 10 [9]:      1201.04 (12.01x)      11.8    93.8       0       0    40      0.6541
        Core 11 [10]:     1201.95 (12.02x)      13.6    92.9       0       0    40      0.6558
        Core 12 [11]:     1201.02 (12.01x)      11.8    93.8       0       0    42      0.6526
        Core 13 [12]:     1204.97 (12.05x)      17.6    90.8       0       0    45      0.6814
        Core 14 [13]:     1248.39 (12.48x)      28.2    84.7       0       0    41      0.6855
        Core 15 [14]:     2790.74 (27.91x)      91.9       0       1       1    41      0.8885 <-- not PMD
        Core 16 [15]:     1262.29 (12.62x)      13.1    34.9     1.7    56.2    43      0.6616

$ dataplanectl stats fcore | grep total
fcore total idle 393788223887 work 860443658 (0.2%) (forced-idle 7458486526622) recv 202201388561 drop 61259353721 (30.3%) limit 269909758 (0.1%) pass 140606076622 (69.6%) ingress 66048460 (0.0%/0.0%) sent 162580376914 (80.4%/100.0%) overflow 0 (0.0%) sampled 628488188/628488188

-- 
Pawel Malachowski
@pawmal80