CPU2017 Flag Description - Platform settings for New H3C systems

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.

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.

Firmware / BIOS / Microcode Settings

Enhanced Halt State (C1E)

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. The default is "Enabled".

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.

Hardware Prefetcher

The hardware prefetcher operates transparently, without programmer intervention, to fetch streams of data and instruction from memory into the unified second-level cache. The prefetcher is capable of handling multiple streams in either the forward or backward direction. It is triggered when successive cache misses occur in the last-level cache and a stride in the access pattern is detected, such as in the case of loop iterations that access array elements. The prefetching occurs up to a page boundary. This feature can be disabled through the BIOS. Default is Enable.

Enforce DDR Memory Frequency POR

This BIOS switch allows 2 options: "POR" and "Disabled". Enable to enforce POR restriction for DDR5 frequency and voltage programming. Memory speeds will be capped accordingly. Disabling allows user selection of additional supported memory speeds.

Isoc Mode

Enabling the isochronous (ISOC) mode option reduces the credits available for memory traffic. For memory requests, this option reduces latency at the expense of throughput under heavy loads. Select Enable to enable Isochronous support to meet QoS (Quality of Service) requirements. This feature is especially important for Virtualization Technology.

Adjacent Cache Prefetch

The Adjacent Cache-Line Prefetch mechanism, like automatic hardware prefetch, operates without programmer intervention. When enabled through the BIOS, two 64-byte cache lines are fetched into a 128-byte sector, regardless of whether the additional cache line has been requested or not. In applications with relatively poor spatial locality, the cache miss ratio is higher. A cache miss on an Intel Pentium 4 processor-based system (with adjacent sector prefetch enabled) brings in 128 byte, leading to higher bus utilization (assuming that the application did not need the other 64 bytes). When adjacent sector prefetch is disabled, an Intel Pentium4 processor-based system only fetches 64 bytes. The other 64 bytes of the sector in the last-level cache are not used unless the application explicitly issues a load to that address. Disabling adjacent sector prefetch on Intel Pentium4 processor-based systems can reduce bus traffic. Default is Enable.

Hardware P-States

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.

EIST PSD Function

Function of the Enhanced Intel SpeedStep Technology (EIST).

EIST reduces the latency inherent with changing the voltage-frequency pair (P-state), thus allowing those transitions to occur more frequently. This allows for more granular, demand-based switching and can optimize the power-to-performance balance, based on the demands of the applications.

ENERGY_PERF_BIAS_CFG mode

This BIOS switch allows 4 options: "Balanced Performance", "Performance", "Balanced Power" and "Power". 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 Power" is optimized for power efficiency and "Power" for power savings. The BIOS switch is only selectable if the BIOS switch "Power Performance Tuning" is set to "BIOS Controls EPB".

The two options "Balanced Performance" and "Balanced Power" 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 "Power" could be an alternative.

Enable LP [Global]

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

LLC dead line alloc

This BIOS switch allows 3 options: "Disabled","Enabled" and "Auto". The default is "Enabled". In the Sapphire 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". 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.

Package C State

This BIOS option allows 6 options: "C0/C1 state", "C2 state", "C6(non Retention) state", "C6(Retention) state", "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

This BIOS switch allows 3 options: "Disabled","Enabled" and "Auto". The default is "Auto".

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.

SNC

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. The BIOS switch "Sub NUMA Clustering" allows 4 options: "Auto", "Disabled" ,"Enable SNC2 (2-clusters)" and "Enable SNC4 (4-clusters)". The default setting is "Auto".

Intel VT for Directed I/O

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".

VMX

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".

XPT Prefetch

This option configures the processor Xtended Prediciton Table (XPT) prefetch feature. The XPT prefetch exists on top of other prefetchers that that can prefetch data in the core DCU, MLC, and LLC. The XPT prefetcher will issue a speculative DRAM read request in parallel to an LLC lookup. This prefetch bypasses the LLC, saving latency. In some cases, setting this option to disabled can improve performance. In some cases, setting this option to disabled can improve performance. Typically, setting this option to enable provides better performance. This option must be enabled when Sub-NUMA Clustering is enabled. Values for this BIOS option can be: Enabled: Allows a read request sent to the LLC to speculatively issue a copy of the read to the memory controller requesting the prefetch. Disabled: Does not allow the LLC to speculatively issue copies of reads. Disabling this will also disables Sub-NUMA Cluster (SNC).