Guillaume de Chambrier

Guillaume de Chambrier

“If we would like to get real-time performance on single-CPU systems it is necessary to adapt the entire system, e.g. using the PREEMPT_RT patch or an RTOS. This is not always necessary in a multicore system.”

Architecture layout

Before:

lstopo

After:

sudo gedit /etc/default/grub
GRUB_CMDLINE_LINUX_DEFAULT="quiet splash isolcpus=1,5"

reboot and chneck that the parameter was passed to the kernel

cat /proc/cmdline
BOOT_IMAGE=/boot/vmlinuz-4.15.0-43-generic root=UUID=47226ffd-864a-4f7d-9f15-779cdee4bdf3 ro quiet splash isolcpus=1,5 vt.handoff=1

Processor per CPU before CPU isolation

cpu: 0  pid: 37.0
cpu: 1  pid: 49.0
cpu: 2  pid: 48.0
cpu: 3  pid: 45.0
cpu: 4  pid: 41.0
cpu: 5  pid: 46.0
cpu: 6  pid: 45.0
cpu: 7  pid: 32.0

Processes per CPU after CPU isolation

cpu: 0  pid: 51.0
cpu: 1  pid: 8.0
cpu: 2  pid: 49.0
cpu: 3  pid: 42.0
cpu: 4  pid: 42.0
cpu: 5  pid: 8.0
cpu: 6  pid: 63.0
cpu: 7  pid: 57.0

The remaining 8 process for both cpus are:

  • cpuhp PRI: RT NI 0
  • watchdog PRI: RT NI 0
  • migration PRI: RT NI 0
  • ksoftirqd PRI: 20 NI 0
  • kworker/1:0 PRI 20 NI 0
  • kworker/1:0H PRI 0 NI -20

linux kernel software interrupts

Real-time Linux communications: an evaluation of the Linux communication stack for real-time robotic applications

  stress -d 4 --hdd-bytes 20M -c 4 -i 4 -m 4 --vm-bytes 15M -t 40s

Ethernet Optimization

network questions

  • Is there one transmite queue per interface ?
  • Can I increase the rate at which TCP execution occures.
  • What does increasing the interrupt rate of the NIC driver do ?

rx-tx optimization

/sys/class/net holds all the network interfaces /sys/class/net/eno1/queues holds queues for receiving and sending data over the network interface card. In my case I have:

  • rx-0
  • tx-0

I have one queue for receiving data (rx-0) and one for sending data (tx-0)

low latency ubuntu kernel

sudo apt-get install linux-lowlatency

Low latency vs generic ubuntu Kernel

diff config-4.15.0-50-generic config-4.15.0-50-lowlatency

The low latency has the following additions:

  • CONFIG_IRQ_FORCED_THREADING_DEFAULT=y
  • CONFIG_TREE_RCU (removed)
  • CONFIG_PREEMPT_RCU=y

  • CONFIG_UNINLINE_SPIN_UNLOCK=y

  • CONFIG_PREEMPT_VOLUNTARY=y (removed)
  • CONFIG_PREEMPT=y

  • CONFIG_PREEMPT_COUNT=y

  • CONFIG_HZ_1000=y

  • CONFIG_HZ=1000

  • CONFIG_CEC_PIN=y (removed)

  • CONFIG_CEC_GPIO=m

  • CONFIG_LATENCYTOP=y
  • CONFIG_PREEMPT_TRACER (removed)

Bios settings

  • BIOS settings

  • C-State The C-State represents the processor power state of the core. The C-State is often more commonly known as the processor “idle” state of the core. C-State values range from C0 to Cn, where n is dependent on the specific processor. When the core is active and executing instructions it is in the C0 state. Higher C-States indicate how deep the CPU idle state is.

  • P-State The voltage frequency pair is known as the Device and Processor Performance State (P-State). A P-State of P0 is the highest voltage/frequency pairing. A high P-State will have lower voltage and frequency levels. It takes the processor longer to complete a task in a high P-State, but less energy is consumed.