Add the documentation for kvm4nfv project

[kvmfornfv.git] / docs / all / tunning.rst

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+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 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
+  deamon 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.