Latency tuning guidelines

Many of our tutorials and applications depend on reliabile, low latency processing.

This is a rough checklist of steps for creating an image (e.g. baseline_1804_lowlatency) that has minimized deterministic latency.

factors affecting this:

• maximum stable processor clock speed
  • we cannot depend on boost states (intel turbo boost), as they can't be maintained under all load conditions, or for many cores.
  • it is possible to fix a small number of cores to a high boost by disabling others, but this usually requires bios support
• number of cpus
  • systems with multiple cpus can introduce latency with communication between them. Single cpu systems are much simpler to optimize
  • NUMA issues: with multiple cpus, you must pay close attention to which memory is allocated to which cpu, as well as where pcie devices are attached.

1. Choosing node to use:
   • from above, pick a node with a single cpu, and the maximum clock available
2. install a low latency kernel
   • apt install linux-lowlatency-hwe-18.04 linux-tools-lowlatency-hwe-18.04
3. usage of tuned-adm
   • tuned is a perfomance optimization project that wraps many configuration methods
   • apt install tuned

Monitoring tools

• htop
• i7z
• hwloc (to show pci and numa layout)

Results

Dell R740 results

transition latency	cstate
1	0
2	1
10	10

tuned latency	cstate	workload	mhz
1	0	idle	3300
1	0	stress -c 48	3300
1	0	stress —matrix 48	3000
1	0	mprime -t	2300

• USRP 2974:
  • C0 2500mhz all core
  • C0 2300mhz stress —matrix 0
  • C0 2000mhz prime95 avx
• dell r740xd xeon gold
  • 3300mhz c0 all core
  • 3000mhz c0 stress-ng —matrix 0
  • 2300mhz c0 p95 avx512