Low Latency Tunning Suggestion

The correct configuration is critical for improving the NFV performance/latency.Even working on the same codebase, configurations can cause wildly different performance/latency results.

There are many combinations of configurations, from hardware configuration to Operating System configuration and application level configuration. And there is no one simple configuration that works for every case. To tune a specific scenario, it’s important to know the behaviors of different configurations and their impact.

Platform Configuration

Some hardware features can be configured through firmware interface(like BIOS) but others may not be configurable (e.g. SMI on most platforms).

• Power management: Most power management related features save power at the expensive of latency. These features include: Intel®Turbo Boost Technology, Enhanced Intel®SpeedStep, Processor C state and P state. Normally they should be disabled but, depending on the real-time application design and latency requirements, there might be some features that can be enabled if the impact on deterministic execution of the workload is small.
• Hyper-Threading: The logic cores that share resource with other logic cores can introduce latency so the recommendation is to disable this feature for realtime use cases.
• Legacy USB Support/Port 60/64 Emulation: These features involve some emulation in firmware and can introduce random latency. It is recommended that they are disabled.
• SMI (System Management Interrupt): SMI runs outside of the kernel code and can potentially cause latency. It is a pity there is no simple way to disable it. Some vendors may provide related switches in BIOS but most machines do not have this capability.

Operating System Configuration

• CPU isolation: To achieve deterministic latency, dedicated CPUs should be allocated for realtime application. This can be achieved by isolating cpus from kernel scheduler. Please refer to http://lxr.free-electrons.com/source/Documentation/kernel-parameters.txt#L1608 for more information.
• Memory allocation: Memory shoud be reserved for realtime applications and usually hugepage should be used to reduce page fauts/TLB misses.
• IRQ affinity: All the non-realtime IRQs should be affinitized to non realtime CPUs to reduce the impact on realtime CPUs. Some OS distributions contain an irqbalance daemon which balances the IRQs among all the cores dynamically. It should be disabled as well.
• Device assignment for VM: If a device is used in a VM, then device passthrough is desirable. In this case,the IOMMU should be enabled.
• Tickless: Frequent clock ticks cause latency. CONFIG_NOHZ_FULL should be enabled in the linux kernel. With CONFIG_NOHZ_FULL, the physical CPU will trigger many fewer clock tick interrupts(currently, 1 tick per second). This can reduce latency because each host timer interrupt triggers a VM exit from guest to host which causes performance/latency impacts.
• TSC: Mark TSC clock source as reliable. A TSC clock source that seems to be unreliable causes the kernel to continuously enable the clock source watchdog to check if TSC frequency is still correct. On recent Intel platforms with Constant TSC/Invariant TSC/Synchronized TSC, the TSC is reliable so the watchdog is useless but cause latency.
• Idle: The poll option forces a polling idle loop that can slightly improve the performance of waking up an idle CPU.
• RCU_NOCB: RCU is a kernel synchronization mechanism. Refer to http://lxr.free-electrons.com/source/Documentation/RCU/whatisRCU.txt for more information. With RCU_NOCB, the impact from RCU to the VNF will be reduced.
• Disable the RT throttling: RT Throttling is a Linux kernel mechanism that occurs when a process or thread uses 100% of the core, leaving no resources for the Linux scheduler to execute the kernel/housekeeping tasks. RT Throttling increases the latency so should be disabled.
• NUMA configuration: To achieve the best latency. CPU/Memory and device allocated for realtime application/VM should be in the same NUMA node.