Copyright © 2016 Intel Corporation. All Rights Reserved.
This section contains descriptions of flags that were included implicitly
by other flags, but which do not have a permanent home at SPEC.
- ulimit -s <n>
-
Sets the stack size to n kbytes, or unlimited to allow the stack size
to grow without limit.
- numactl --interleave=all "runspec command"
-
Launching a process with numactl --interleave=all sets the memory interleave policy so that memory will be allocated using
round robin on nodes. When memory cannot be allocated on the current interleave target fall back to other nodes.
- Free the file system page cache
-
The command "echo 1> /proc/sys/vm/drop_caches" is used to free up the filesystem page cache.
- Using numactl to bind processes and memory to cores
-
For multi-copy runs or single copy runs on systems with multiple sockets, it is advantageous to bind a process to a particular core. Otherwise, the OS may arbitrarily move your process from one core to another. This can affect performance. To help, SPEC allows the use of a "submit" command where users can specify a utility to use to bind processes. We have found the utility 'numactl' to be the best choice.
numactl runs processes with a specific NUMA scheduling or memory placement policy. The policy is set for a command and inherited by all of its children. The numactl flag "--physcpubind" specifies which core(s) to bind the process. "-l" instructs numactl to keep a process memory on the local node while "-m" specifies which node(s) to place a process memory. For full details on using numactl, please refer to your Linux documentation, 'man numactl'
- Linux Huge Page settings
-
In order to take advantage of large pages, your system must be configured to use large pages.
To configure your system for huge pages perform the following steps:
Create a mount point for the huge pages: "mkdir /mnt/hugepages"
The huge page file system needs to be mounted when the systems reboots. Add the following to a system boot configuration file before any services are started: "mount -t hugetlbfs nodev /mnt/hugepages"
Set vm/nr_hugepages=N in your /etc/sysctl.conf file where N is the maximum number of pages the system may allocate.
Reboot to have the changes take effect. (Not necessary on some operating systems like RedHat Enterprise Linux 5.5).
Note that further information about huge pages may be found in your Linux documentation file: /usr/src/linux/Documentation/vm/hugetlbpage.txt
Transparent Huge Pages
On RedHat EL 6 and later, Transparent Hugepages increases the memory page size from 4 kilobytes to 2 megabytes. Transparent Hugepages provides significant performance advantages on systems with highly contended resources
and large memory workloads. If memory utilization is too high or memory is badly fragmented which prevents hugepages being allocated, the kernel will assign smaller 4k pages instead. Hugepages are used by default
if /sys/kernel/mm/redhat_transparent_hugepage/enabled is set to always.
- HUGETLB_MORECORE
-
Set this environment variable to "yes" to enable applications to use large pages.
- KMP_STACKSIZE
-
Specify stack size to be allocated for each thread.
- KMP_AFFINITY
-
KMP_AFFINITY = < physical | logical >, starting-core-id
specifies the static mapping of user threads to physical cores. For example,
if you have a system configured with 8 cores, OMP_NUM_THREADS=8 and
KMP_AFFINITY=physical,0 then thread 0 will mapped to core 0, thread 1 will be mapped to core 1, and
so on in a round-robin fashion.
KMP_AFFINITY = granularity=fine,scatter
The value for the environment variable KMP_AFFINITY affects how the threads from an auto-parallelized program are scheduled across processors.
Specifying granularity=fine selects the finest granularity level, causes each OpenMP thread to be bound to a single thread context.
This ensures that there is only one thread per core on cores supporting HyperThreading Technology
Specifying scatter distributes the threads as evenly as possible across the entire system.
Hence a combination of these two options, will spread the threads evenly across sockets, with one thread per physical core.
- OMP_NUM_THREADS
-
Sets the maximum number of threads to use for OpenMP* parallel regions if no
other value is specified in the application. This environment variable
applies to both -openmp and -parallel (Linux and Mac OS X) or /Qopenmp and /Qparallel (Windows).
Example syntax on a Linux system with 8 cores:
export OMP_NUM_THREADS=8
- Intel Turbo Boost Technology:
-
Enabling this option allows the processor cores to automatically increase their frequency if they are running below power and temperature, thereby increasing performance. By default, this option is enabled.
- Intel Hyper Threading Technology:
-
Enabling this option allows processor resources to be used more efficiently, enabling multiple threads to run
on each core and increasing processor throughput, improving overall performance on threaded software. By default, this option is enabled.
- Enhanced Intel SpeedStep:
-
Enabling this option allows the system to dynamically adjust processor voltage and core frequency. This technology can result in decreased average power consumption and decreased average heat production. By default, this option is enabled.
- Core Multi Processing:
-
This option specifies the number of logical processor cores that can run on the server. This option sets he state of logical processor cores in a package. If you disable this setting, Hyper Threading is also disabled.
- Virtualization Technology:
-
This option allows the user whether the processor uses Intel Virtualization Technology, which allows a platform to run multiple operating systems and applications in independent partitions.
This can be one of the following:
Disabled - The processor does not permit virtualization.
enabled — The processor allows multiple operating systems in independent partitions.
Platform Default — The BIOS option uses the value for this attribute contained in the BIOS defaults for the server type and vendor.
By default this BIOS option is enabled.
- Direct Cache Access:
-
Enabling this option allows processors to increase I/O performance by placing data from I/O devices directly into the processor cache. This setting helps to reduce cache misses. By default, this option is enabled.
- Power Technology:
-
This BIOS option enables the user to configure the CPU power management settings such as Enhanced Intel SpeedStep, Intel Turbo Boost Technology and Processor Power State C6.
Settings in Custom will allow the user to change individual settings for the BIOS parameters in the preceding list. You must select this option if you want to change any of these BIOS parameters.
Settings in Energy Efficient will determines the best settings for the BIOS parameters in the preceding list and ignores the individual settings for these parameters.
Settings in Disabled state do not perform any CPU power management and any settings for the BIOS parameters in the preceding list are ignored
- Processor C1 Enhanced:
-
Enabling this option allows the processor to transition to its minimum frequency upon entering C1. This setting does not take effect until after you have rebooted the server.
In disabled state, the CPU continues to run at its maximum frequency in C1 state. Users should disable this option for performing application benchmarking.
In enabled state, the CPU transitions to its minimum frequency. This option saves the maximum amount of power in the C1 state. By default, C1 Enhanced is disabled.
- Processor State C6:
-
Enabling this option allows the processor to send the C6 report to the operating system.
Users should disable this option for performing application benchmarking. By default, C6 is disabled.
- Energy Performance:
-
This BIOS option allows you to determine whether system Performance or energy efficiency is more important on server. This can be one of the following: Balanced Energy, Balanced Performance, Energy Efficient and Performance.
Balanced Performance optimized to maximum power savings with minimal impact on performance and it is enabled by default.
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 option is only selectable if “Power Technology" is set to "Custom".
- CPU Performance:
-
This BIOS option allows the enabling/disabling of a processor mechanism in 3 modes: Enterprise, High-Throughput and HPC.
- Setting this BIOS option in Enterprise and High-Throughput modes, will enable all the prefetchers (Hardware prefetcher, Adjacent-cache-line prefetcher, DCU streamer prefetcher and DCU-IP prefetcher), disable Data Reuse Technology and LLC Prefetch. By default, CPU Performance is set to Enterprise.
- Setting this BIOS option in HPC mode will enable all the prefetchers and enable Data Reuse Technology. Intel Xeon processors have several layers of cache. Each core has a tiny Layer 1 cache,sometimes referred to as the data cache unit (DCU),that has 32 KB for instructions and 32 KB for data. Slightly bigger is the Layer 2 cache, with 256 KB shared between data and instructions for each core.
Hardware prefetcher (Layer 2): The hardware prefetcher prefetches additional streams of instructions and data into the Layer 2 cache upon detection of an access stride. This behavior is more likely to occur during operations that sort through sequential data, such as database table scans and clustered index scans, or that run a tight loop in code.You can specify whether the processor allows the Intel hardware prefetcher to fetch streams of data and instructions from memory into the unified second-level cache when necessary.
The setting can be one of the following:
- Disabled:The hardware prefetcher is not used.
- Enabled:The processor uses the hardware prefetcher when cache problems are detected.
Adjacent-cache-line prefetcher (Layer 2): The adjacent-cache-line prefetcher always prefetches the next cache line. Although this approach works well when data is accessed sequentially in memory, it can quickly litter the small Layer 2 cache with unneeded instructions and data if the system is not accessing data sequentially, causing frequently accessed instructions and code to leave the cache to make room for the adjacent-line data or instructions.You can specify whether the processor fetches cache lines in even or odd pairs instead of fetching just the required line.
The setting can be one of the following:
- Disabled:The processor fetches only the required line.
- Enabled:The processor fetches both the required line and its paired line.
DCU streamer prefetcher (Layer 1): Like the hardware prefetcher, the DCU streamer prefetcher prefetches additional streams of instructions or data upon detection of an access stride; however, it stores the streams in the tiny Layer 1 cache instead of the Layer 2 cache. This prefetcher is a Layer 1 data cache prefetcher. It detects multiple loads from the same cache line that occur within a time limit. Making the assumption that the next cache line is also required, the prefetcher loads the next line in advance to the Layer 1 cache from the Layer 2 cache or the main memory.
The setting can be one of the following:
- Disabled:The processor does not try to anticipate cache read requirements and fetches only explicitly requested lines.
- Enabled:The DCU prefetcher analyzes the cache read pattern and prefetches the next line in the cache if it determines that it may be needed.
DCU-IP prefetcher (Layer 1): The DCU-IP prefetcher predictably prefetches data into the Layer 1 cache on the basis of the recent instruction pointer load instruction history. You can specify whether the processor uses the DCU-IP prefetch mechanism to analyze historical cache access patterns and preload the most relevant lines in the Layer 1 cache.
The setting can be one of the following:
- Disabled:The processor does not preload any cache data.
- Enabled:The DCU-IP prefetcher preloads the Layer 1 cache with the data it determines to be the most relevant.
- LLC Prefetch:
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This BIOS option configures the processor last level cache (LLC) prefetch feature as a result of the non-inclusive cache architecture. The LLC prefetcher exists on top of other prefetchers that can prefetch data into the core data cache unit (DCU) and mid-level cache (MLC). In some cases, setting this option to disabled can improve performance.
Values for this BIOS option can be:
Disabled: Disables the LLC prefetcher and forces data to fill in the MLC. The other core prefetchers are unaffected.
Enabled: Gives the core prefetcher the ability to prefetch data directly to the LLC.
By default, LLC prefetch option is disabled.
- Power Performance Tuning:
-
This BIOS option determines how aggressively the CPU will be power managed and placed into turbo. With “BIOS Controls”, the system controls the setting. Selecting "OS Controls” allows the operating system to control it. By default, OS Control is enabled.
- Memory Power Saving Mode:
-
This BIOS option controls the DIMM power savings mode policy.
Setting this BIOS option in Disabled, DIMMs do not enter power saving mode.
Setting this BIOS option in Slow, DIMMs can enter power saving mode, but the requirements are higher. Therefore, DIMMs enter power saving mode less frequently.
Setting this BIOS option in Fast, DIMMs enter power saving mode as often as possible.
Setting this BIOS option in Auto, BIOS controls when a DIMM enters power saving mode based on the DIMM configuration.
By default, Memory Power Saving Mode is set too Disabled.
- LV DDR Mode or Low Voltage DDR Mode and DRAM Clock Throttling:
-
This BIOS option controls the prioritization of memory operations.
Setting this BIOS option in Power-saving-mode will prioritize low voltage memory operations over high frequency memory operations. This mode may lower memory frequency in order to keep the voltage low. By default, Power-saving-mode is enabled.
Setting this BIOS option in Performance-mode will prioritize high frequency operations over low voltage operations.
- Closed Loop Thermal Throttling:
-
This BIOS option allows the user to enable/disable temperature-based memory throttling. By default this BIOS option is enabled. By default this BIOS option is enabled.
By enabling this BIOS option, the system BIOS will initiate memory throttling to manage memory performance by limiting bandwidth to the DIMMs, therefore capping the power consumption and preventing the DIMMs from overheating.
- Memory RAS Configuration:
-
This BIOS option allows the user to configure memory reliability, availability and serviceability (RAS).
Setting this BIOS option in Maximum Performance, system performance is optimized and enabled by default.
Setting this BIOS option in Mirroring, system reliability is optimized by using half the system memory as backup.
Setting this BIOS option in Lockstep, if the DIMM pairs in the server have an identical type, size, and organization and are populated across the SMI channels, you can enable lockstep mode to minimize memory access latency and provide better performance.
Setting this BIOS option in Sparing, system reliability is enhanced with a degree of memory redundancy while making more memory available to the operating system than mirroring.
- DRAM Refresh Rate:
-
This option controls the refresh interval rate for internal memory. By default, the refresh interval rate set as Auto, which is 2X DRAM refresh for every 32ns.
Setting this BIOS option in 1X, DRAM cells are refreshed every 64ns.
- Patrol Scrub:
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This BIOS option is memory RAS feature which runs a background memory scrub against all DIMMs and it can negatively impact performance.
By default, this option is enabled. Disabling this option, improves performance.
- QPI Snoop Configuration:
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There are 4 snoop mode options for how to maintain cache coherency across the Intel QPI fabric, each with varying memory latency and bandwidth characteristics depending on how the snoop traffic is generated.
Cluster on Die (COD) mode logically splits a socket into 2 NUMA domains that are exposed to the OS with half the amount of cores and LLC assigned to each NUMA domain in a socket. This mode utilizes an on-die directory cache and 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 and highest local memory bandwidth for NUMA workloads.
Home Directory Snoop with OSB is the Opportunistic Snoop Broadcast (OSB) directory mode, the HA could choose to do speculative home snoop broadcast under very lightly loaded conditions 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.
- Use Home Snoop mode for NUMA workloads that are memory bandwidth sensitive and need both local and remote memory bandwidth.
- 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.
- Sub Numa Clustering:
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This BIOS option provides similar localization benefits as cluster-on-die (COD), without some of COD’s downsides. SNC breaks up the LLC into two disjoint clusters based on address range, with each cluster bound to a subset of the memory controllers in the system. SNC improves average latency to the LLC (last level cache) and memory. SNC is a replacement for the 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.
Values for this BIOS option can be:
Disabled: The LLC is treated as one cluster when this option is disabled
Enabled: Utilizes LLC capacity more efficiently and reduces latency due to core/IMC proximity. This may provide performance improvement on NUMA-aware operating systems
By default this BIOS option set to Disabled.
- IMC Interleaving:
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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. By default, IMC Interleaving is set to Auto, which is 2-way Interleaving.
- High Bandwidth:
-
Enabling this option allows the chipset to defer memory transactions and
process them out of order for optimal performance.
- submit= MYMASK=`printf '0x%x' \$((1<< \$SPECCOPYNUM))`; /usr/bin/taskset \$MYMASK $command
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When running multiple copies of benchmarks, the SPEC config file feature submit is sometimes used to cause individual jobs to be bound to specific processors. This specific submit command is used for Linux. The description of the elements of the command are:
/usr/bin/taskset [options] [mask] [pid | command [arg] ... ]
:
taskset is used to set or retreive the CPU affinity of a running process given its PID or to launch a new COMMAND with a given CPU affinity. The CPU affinity is represented as a bitmask, with the lowest order bit corresponding to the first logical CPU and highest order bit corresponding to the last logical CPU. When the taskset returns, it is guaranteed that the given program has been scheduled to a legal CPU. :
The default behaviour of taskset is to run a new command with a given affinity mask: :
taskset [mask] [command] [arguments]
- $MYMASK: The bitmask (in hexadecimal) corresponding to a specific SPECCOPYNUM. For example, $MYMASK value for the first copy of a rate run will be 0x00000001, for the second copy of the rate will be 0x00000002 etc. Thus, the first copy of the rate run will have a CPU affinity of CPU0, the second copy will have the affinity CPU1 etc.
- $command: Program to be started, in this case, the benchmark instance to be started. :
![[user]](https://www.spec.org/auto/cpu2017/images/user.png)
![[suite]](https://www.spec.org/auto/cpu2017/images/suite.png)
![[benchmark]](https://www.spec.org/auto/cpu2017/images/benchmark.png)