SPEC CPU2006 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.

DCU Streamer Prefetcher

This BIOS switch allows 2 options: "Enabled" and "Disabled". The default is "Enabled".

This prefetcher is a L1 data cache prefetcher, which detects multiple loads from the same cache line done within a time limit, in order to then prefetch the next line from the L2 cache or the main memory into the L1 cache based on the assumption that the next cache line will also be needed.

Enable CPU HWPM(HWPM Support):

This BIOS switch allows 4 options: "Native Mode", "Disabled", "Out of Band Mode" and "Native Mode with No legacy Support". The default is "Native Mode".

With Hardware Power Management(HWPM) the processors provides a flexible interface between Hardware and Platform for performance management and improving energy efficiency.

In Native Mode the HWPM operates cooperatively with the OS via a software interface to provide constraints and hints.

When disabled, system does not use HWPM.

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.

Fan Control

This BIOS switch allows 2 options: "Auto" and "Full". The default setting is "Auto", which allows the system to control the fan speed according to the system temperature. If "Full" is selected, the system runs fans at 100% speed and it may improve the system performance. But it increases the power consumption of the system.

IMC Interleaving

This BIOS option controls the interleaving between the Integrated Memory Controllers (IMCs). There are two IMCs per socket in Skylake Server. If IMC Interleaving is set to 2-way, addresses will be interleaved between the two IMCs. If IMC Interleaving is set to 1-way, there will be no interleaving. If SNC is disabled, IMC Interleaving should be set to 2-way. If SNC is enabled, IMC Interleaving should be set to 1-way. Default setting is "Auto".

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".

Link Frequency Select

This switch allows the configuration of the Intel Ultra Path Interconnect (UPI) link speed. Default is auto, which configures the optimal link speed automatically. It can be set "9.6 GT/s", "10.4 GT/s" or "Auto".

LLC Dead Line Alloc

This BIOS switch allows 2 options: "Enabled" and "Disabled". The default is "Enabled". In the Skylake non-inclusive cache scheme, the mid-level cache (MLC) evictions are filled into the last-level cache (LLC). When lines are evicted from the MLC, the core can flag them as "dead" (i.e., not likely to be read again). The LLC has the option to drop dead lines and not fill them in the LLC. If the Dead Line LLC Alloc feature is disabled, dead lines will always be dropped and will never fill into the LLC. This can help save space in the LLC and prevent the LLC from evicting useful data. However, if the Dead Line LLC Alloc feature is enabled, the LLC can opportunistically fill dead lines into the LLC if there is free space available.

Override OS Energy Performance

This BIOS switch allows 2 options: "Enabled" and "Disabled". The default is "Disabled". The power control unit (PCU) in the processors takes on the central role of controlling the energy-saving options. The PCU can be parameterized in order to consequently control the settings more in the direction of energy efficiency or in the direction of maximum performance. The default setting allows you to control energy-saving options through the operating system by its power plan. If enabled, PCU overrides the setting of the operating system and controls the energy-saving options based on the settings in the BIOS.

Package C State limit

This BIOS option allows 6 options: "C0", "C2", "C6", "C6(Retention)", "No Limit" and "Auto". The default setting is "Auto". Package C-states is one of energy-saving options of the processor, which not only allow the individual cores of a processor, but the entire processor chip to be put into a type of sleep state. As a result, power consumption is even further reduced. But the "waking-up time" that is required to change from the lower package C-states to the active (C0) state is even longer in comparison with the CPU or core C-states. If the "C0" setting is made in the BIOS, the processor chip always remains active. It can improve the performance of latency sensitive workloads.

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.

Stale AtoS (Directory AtoS)

This BIOS switch allows 2 options: "Enabled" and "Disabled". The default is "Disabled".

The in-memory directory has three states: I, A, and S. I (invalid) state means the data is clean and does not exist in any other socket's cache. A (snoopAll) state means the data may exist in another socket in exclusive or modified state. S (Shared) state means the data is clean and may be shared across one or more socket's caches.

When doing a read to memory, if the directory line is in the A state we must snoop all the other sockets because another socket may have the line in modified state. If this is the case, the snoop will return the modified data. However, it may be the case that a line is read in A state and all the snoops come back a miss. This can happen if another socket read the line earlier and then silently dropped it from its cache without modifying it.

If Stale AtoS feature is enabled, in the situation where a line in A state returns only snoop misses, the line will transition to S state. That way, subsequent reads to the line will encounter it in S state and not have to snoop, saving latency and snoop bandwidth. Stale AtoS may be beneficial in a workload where there are many cross-socket reads.

Sub NUMA Cluster

Sub NUMA Cluster (SNC) breaks up the last-level cache (LLC) into two disjoint clusters based on address range, with each cluster bound to one memory controller. SNC improves average latency to the LLC/memory and is a replacement for the "Cluster On Die" (COD) feature found in previous processor families. For a multi-socketed system, all SNC clusters are mapped to unique NUMA domains. IMC Interleaving must be set to the correct value to correspond with SNC enable/disable. If SNC and IMC Interleave are both set to Auto, the result will be SNC disabled (only one cluster per socket) with 2-way IMC interleave. If SNC is set to Enable, IMC Interleave should be set to 1-way, which will result in two clusters per socket. The BIOS switch "Sub NUMA Clustering" allows 3 options: "auto", "enabled" and "disabled". The default setting is "enabled" (PRIMERGY servers), "auto" (PRIMEQUEST servers).

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".