--- /dev/null
+.. 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.
Code Review / kvmfornfv.git / blobdiff