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diff --git a/docs/release/userguide/tuning.userguide.rst b/docs/release/userguide/tuning.userguide.rst new file mode 100644 index 000000000..3673ae2d4 --- /dev/null +++ b/docs/release/userguide/tuning.userguide.rst @@ -0,0 +1,102 @@ +.. This work is licensed under a Creative Commons Attribution 4.0 International License. + +.. http://creativecommons.org/licenses/by/4.0 + +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. |