SPEC CPU2006/CPU2017 Flag Description - Platform settings

Operating System Tuning Parameters

cpupower frequency-set

cpupower utility is a collection of tools for power efficiency of processor. frequency-set sub-command controls settings for processor frequency. "-g [governor]" specifies a policy to select processor frequency. The performance governor statically sets frequency of the processor cores specified by "-c" option to the highest possible for maximum performance.

cpupower idle-set

idle-set sub-command of cpupower utility controls a processor idle state (C-state) of the kernel. "-d [state_no]>" option disables a specific processor idle state. Disabling idle state can reduce the idle-wakeup delay, but it results in substantially higher power consumption. By default, processor idle states of all CPU cores are set.

isolcpus

This kernel option excludes a specified processor from load balancing by the kernel scheduler. This prevents the scheduler from scheduling any user-space threads on this processor.

nohz_full

This kernel option sets adaptive tick mode (NOHZ_FULL) to specified processors. Since the number of interrupts is reduced to ones per second, latency-sensitive applications can take advantage of it.

numa_balancing

This OS setting controls automatic NUMA balancing on memory mapping and process placement. Setting 0 disables this feature. It is enabled by default (1).

sched_min_granularity_ns

This OS setting controls the minimal preemption granularity for CPU bound tasks. As the number of runnable tasks increases, CFS(Complete Fair Scheduler), the scheduler of the Linux kernel, decreases the timeslices of tasks. If the number of runnable tasks exceeds sched_latency_ns/sched_min_granularity_ns, the timeslice becomes number_of_running_tasks * sched_min_granularity_ns. The default value is 4000000 (ns).

sched_wakeup_granularity_ns

This OS setting controls the wake-up preemption granularity. Increasing this variable reduces wake-up preemption, reducing disturbance of compute bound tasks. Lowering it improves wake-up latency and throughput for latency critical tasks, particularly when a short duty cycle load component must compete with CPU bound components. The default value is 2500000 (ns).

Firmware / BIOS / Microcode Settings

CPU C1E Support

Enabling this option which is the default allows the processor to transmit to its minimum frequency when entering the power state C1. If the switch is disabled the CPU stays at its maximum frequency in C1. Because of the increase of power consumption users should only select this option after performing application benchmarking to verify improved performance in their environment.

Energy Performance:

This BIOS switch allows 4 options: "Balanced performance", "Performance", "Balanced Energy" and "Energy Efficient". The default is "Balanced Performance" optimized to maximum power savings with minimal impact on performance. "Performance" disables all power management options with any impact on performance. "Balanced Energy" is optimized for power efficiency and "Energy Efficient" for power savings. The BIOS switch is only selectable if the BIOS switch "Power Technology" is set to "Custom".

The two options "Balanced Performance" and "Balanced Energy" should always be the first choice as both options optimize the efficiency of the system. In cases where the performance is not sufficient or the power consumption is too high the two options "Performance" or "Energy Efficient" could be an alternative.

Intel Hyper-Threading Technology

This BIOS option enables or disables additional hardware thread which shares same physical core. Generally "Enabled" is recommended but disabling it makes sense for the application which requires the shortest possible response times. Default setting is "Enabled".

Intel Virtualization Technology

This BIOS option enables or disables additional virtualization functions of the CPU. If the server is not used for virtualization, this option should be set to "Disabled". This can result in energy savings. Default setting is "Enabled".

Patrol Scrub

This BIOS option enables or disables the so-called memory scrubbing, which cyclically accesses the main memory of the system in the background regardless of the operating system in order to detect and correct memory errors in a preventive way. The time of this memory test cannot be influenced and can under certain circumstances result in losses in performance. The disabling of the Patrol Scrub option increases the probability of discovering memory errors in case of active accesses by the operating system. Until these errors are correctable, the ECC technology of the memory modules ensures that the system continues to run in a stable way. However, too many correctable memory errors increase the risk of discovering non-correctable errors, which then result in a system standstill.

QPI Link Frequency Select

This switch allows the configuration of the QPI link speed. Default is auto, which configures the optimal link speed automatically.

QPI snoop mode:

There are 4 snoop mode options for how to maintain cache coherency across the Intel QPI fabric, each with varying memory latency & bandwidth characteristics depending on how the snoop traffic is generated.

There are 3 BIOS switches to select one of these 4 modes. The corresponding settings are given below together with the description of the 4 snoop modes.

Cluster on Die

(BIOS switch settings to enable this mode: "COD Enable" = Enabled and "Early Snoop" = Disabled and "Home Snoop Dir OSB" = Disabled)

This mode logically splits a socket into 2 NUMA domains that are exposed to the OS with half the amount of cores & LLC assigned to each NUMA domain in a socket. This mode utilizes an on-die directory cache & in memory directory bits to determine whether a snoop needs to be sent. Use this mode for highly NUMA optimized workloads to get the lowest local memory latency & highest local memory bandwidth for NUMA workloads.

"Home Directory Snoop with OSB" is the Opportunistic Snoop Broadcast (OSB) directory mode. The HA can choose to do a speculative home snoop broadcast even before the directory information has been collected and checked.

In "Home Snoop" and "Early Snoop" modes, snoops are always sent -- they just originate from different places: the caching agent (earlier) in "Early Snoop" mode and the home agent (later) in "Home Snoop" mode.

Home Snoop

(BIOS switch settings to enable this mode: "COD Enable" = Disabled and "Early Snoop" = Disabled and "Home Snoop Dir OSB" = Disabled)

Use Home Snoop mode for NUMA workloads that are memory bandwidth sensitive and need both local & remote memory bandwidth.

Early Snoop

(BIOS switch settings to enable this mode: "COD Enable" = Disabled and "Early Snoop" = Enabled and "Home Snoop Dir OSB" = Disabled)

Use Early Snoop mode for workloads that are memory latency sensitive or for workloads that benefit from fast cache-to-cache transfer latencies from the remote socket. Snoops are sent out earlier, which is why memory latency is lower in this mode.

Home Directory Snoop with OSB

(BIOS switch settings to enable this mode: "COD Enable" = Disabled and "Early Snoop" = Disabled and "Home Snoop Dir OSB" = Enabled)

Use Home Directory Snoop with OSB under very lightly loaded conditions.

Uncore Frequency Override:

This BIOS switch allows 3 options: "Disabled", "Maximum" and "Nominal". The default is "Disabled" optimized for energy efficiency. "Maximum" sets the uncore frequency to the fixed maximum uncore frequency available. "Nominal" reduces the uncore frequency to the nominal value.

Setting this option to "Maximum" may improve performance but also increase the power consumption of the system. Users should only select this option after performing application benchmarking to verify improved performance in their environment.

Utilization Profile:

This BIOS switch allows 2 options: "Even" and "Unbalanced". The default is "Even" and the best choice for all workloads utilizing the whole system. In cases where the utilization is highly concentrated on few resources of the system the performance of the application could be improved by setting to "Unbalanced".

Setting this option to "Unbalanced" may improve performance but also increase the power consumption of the system. Users should only select this option after performing application benchmarking to verify improved performance in their environment.

VT-d

This BIOS option enables or disables I/O virtualization functions of the CPU. If the server is not used for virtualization, this option should be set to "Disabled". Default setting is "Enabled".