Chapter 1. Real-time kernel tuning in RHEL 8

Latency, or response time, refers to the time from an event and to the system response. It is generally measured in microseconds (μs).

For most applications running under a Linux environment, basic performance tuning can improve latency sufficiently. For those industries where latency must be low, accountable, and predictable, Red Hat has a replacement kernel that can be set for latency to meet those requirements. RHEL for Real Time 8 provides seamless integration with RHEL 8 and offers clients the opportunity to measure, configure, and record latency times within their organization.

RHEL for Real Time 8 is designed for well-tuned systems and for applications with extremely high determinism requirements. Kernel system tuning offers the vast majority of the improvement in determinism. Before you begin, perform general system tuning of the standard RHEL 8 system before using RHEL for Real Time 8.

Warning

Failure to perform these tasks might prevent getting consistent performance from a RHEL Real Time deployment.

1.1. Tuning guidelines

  • Real-time tuning is an iterative process; you will almost never be able to tweak a few variables and know that the change is the best that can be achieved. Be prepared to spend days or weeks narrowing down the set of tuning configurations that work best for your system.

    Additionally, always make long test runs. Changing some tuning parameters then doing a five minute test run is not a good validation of a set of tunes. Make the length of your test runs adjustable and run them for longer than a few minutes. Try to narrow down to a few different tuning configuration sets with test runs of a few hours, then run those sets for many hours or days at a time to try and catch corner-cases of highest latency or resource exhaustion.

  • Build a measurement mechanism into your application, so that you can accurately gauge how a particular set of tuning changes affect the application’s performance. Anecdotal evidence (for example, "The mouse moves more smoothly.") is usually wrong and varies from person to person. Do hard measurements and record them for later analysis.
  • It is very tempting to make multiple changes to tuning variables between test runs, but doing so means that you do not have a way to narrow down which tune affected your test results. Keep the tuning changes between test runs as small as you can.
  • It is also tempting to make large changes when tuning, but it is almost always better to make incremental changes. You will find that working your way up from the lowest to highest priority values will yield better results in the long run.
  • Use the available tools. The tuna tuning tool makes it easy to change processor affinities for threads and interrupts, thread priorities and to isolate processors for application use. The taskset and chrt command line utilities allow you to do most of what Tuna does. If you run into performance problems, the ftrace and perf utilities can help locate latency issues.
  • Rather than hard-coding values into your application, use external tools to change policy, priority and affinity. Using external tools allows you to try many different combinations and simplifies your logic. Once you have found some settings that give good results, you can either add them to your application, or set up startup logic to implement the settings when the application starts.

1.2. Thread scheduling policies

Linux uses three main thread scheduling policies.

  • SCHED_OTHER (sometimes called SCHED_NORMAL)

    This is the default thread policy and has dynamic priority controlled by the kernel. The priority is changed based on thread activity. Threads with this policy are considered to have a real-time priority of 0 (zero).

  • SCHED_FIFO (First in, first out)

    A real-time policy with a priority range of from 1 - 99, with 1 being the lowest and 99 the highest. SCHED_FIFO threads always have a higher priority than SCHED_OTHER threads (for example, a SCHED_FIFO thread with a priority of 1 will have a higher priority than any SCHED_OTHER thread). Any thread created as a SCHED_FIFO thread has a fixed priority and will run until it is blocked or preempted by a higher priority thread.

  • SCHED_RR (Round-Robin)

    SCHED_RR is a modification of SCHED_FIFO. Threads with the same priority have a quantum and are round-robin scheduled among all equal priority SCHED_RR threads. This policy is rarely used.

1.3. Balancing logging parameters

The syslog server forwards log messages from programs over a network. The less often this occurs, the larger the pending transaction is likely to be. If the transaction is very large, it can cause an I/O spike. To prevent this, keep the interval reasonably small.

The system logging daemon, syslogd, is used to collect messages from different programs. It also collects information reported by the kernel from the kernel logging daemon, klogd. Typically, syslogd logs to a local file, but it can also be configured to log over a network to a remote logging server.

Procedure

To enable remote logging:

  1. Configure the machine to which the logs will be sent. For more information, see Remote Syslogging with rsyslog on Red Hat Enterprise Linux.

  2. Configure each system that will send logs to the remote log server, so that its syslog output is written to the server, rather than to the local file system. To do so, edit the /etc/rsyslog.conf file on each client system. For each of the logging rules defined in that file, replace the local log file with the address of the remote logging server. 

    # Log all kernel messages to remote logging host.
kern.*     @my.remote.logging.server

    The example above configures the client system to log all kernel messages to the remote machine at @my.remote.logging.server.

    Alternatively, you can configure syslogd to log all locally generated system messages, by adding the following line to the /etc/rsyslog.conf file: 

    # Log all messages to a remote logging server:
.     @my.remote.logging.server
Important

The syslogd daemon does not include built-in rate limiting on its generated network traffic. Therefore, Red Hat recommends that when using RHEL for Real Time systems, only log messages that are required to be remotely logged by your organization. For example, kernel warnings, authentication requests, and the like. Other messages should be logged locally.

Additional resources

  • syslog(3) man page
  • rsyslog.conf(5) man page
  • rsyslogd(8) man page

1.4. Improving performance by avoiding running unnecessary applications

Every running application uses system resources. Ensuring that there are no unnecessary applications running on your system can significantly improve performance.

Prerequisites

  • You have root permissions on the system.

Procedure

  1. Do not run the graphical interface where it is not absolutely required, especially on servers.

    Check if the system is configured to boot into the GUI by default: 

    # systemctl get-default

  2. If the output of the command is graphical.target, configure the system to boot to text mode: 

    # systemctl set-default multi-user.target

  3. Unless you are actively using a Mail Transfer Agent (MTA) on the system you are tuning, disable it. If the MTA is required, ensure it is well-tuned or consider moving it to a dedicated machine.

    For more information, refer to the MTA’s documentation.

    Important

    MTAs are used to send system-generated messages, which are executed by programs such as cron. This includes reports generated by logging functions like logwatch(). You will not be able to receive these messages if the MTAs on your machine are disabled.

  4. Peripheral devices, such as mice, keyboards, webcams send interrupts that may negatively affect latency. If you are not using a graphical interface, remove all unused peripheral devices and disable them.

    For more information, refer to the devices' documentation.

  5. Check for automated cron jobs that might impact performance. 

    # crontab -l

    Disable the crond service or any unneeded cron jobs.

  6. Check your system for third-party applications and any components added by external hardware vendors, and remove any that are unnecessary.

Additional resources

  • cron(8) man page

1.5. Non-Uniform Memory Access

The taskset utility only works on CPU affinity and has no knowledge of other NUMA resources such as memory nodes. If you want to perform process binding in conjunction with NUMA, use the numactl command instead of taskset.

For more information about the NUMA API, see Andi Kleen’s whitepaper An NUMA API for Linux.

Additional resources

  • numactl(8) man page

1.6. Ensuring that debugfs is mounted

The debugfs file system is specially designed for debugging and making information available to users. It is mounted automatically in RHEL 8 in the /sys/kernel/debug/ directory.

Note

The debugfs file system is mounted using the ftrace and trace-cmd commands.

Procedure

To verify that debugfs is mounted:

  • Run the following command: 

    # mount | grep ^debugfs
debugfs on /sys/kernel/debug type debugfs (rw,nosuid,nodev,noexec,relatime,seclabel)

    If debugfs is mounted, the command displays the mount point and properties for debugfs.

    If debugfs is not mounted, the command returns nothing.

1.7. InfiniBand in RHEL for Real Time

InfiniBand is a type of communications architecture often used to increase bandwidth, improve quality of service (QOS), and provide for failover. It can also be used to improve latency by using the Remote Direct Memory Access (RDMA) mechanism.

The support for InfiniBand on RHEL for Real Time is the same as the support available on Red Hat Enterprise Linux 8. For more information, see Configuring InfiniBand and RDMA networks.

1.8. Using RoCEE and High-Performance Networking

RoCEE (RDMA over Converged Enhanced Ethernet) is a protocol that implements Remote Direct Memory Access (RDMA) over Ethernet networks. It allows you to maintain a consistent, high-speed environment in your data centers, while providing deterministic, low latency data transport for critical transactions.

High Performance Networking (HPN) is a set of shared libraries that provides RoCEE interfaces into the kernel. Instead of going through an independent network infrastructure, HPN places data directly into remote system memory using standard Ethernet infrastructure, resulting in less CPU overhead and reduced infrastructure costs.

Support for RoCEE and HPN under RHEL for Real Time does not differ from the support offered under RHEL 8.

Additional resources

1.9. Reducing CPU performance spikes

The kernel command line skew_tick parameter smooths jitter on moderate to large systems with latency-sensitive applications running. A common source of latency spikes on a real time Linux system is when multiple CPUs contend on common locks in the Linux kernel timer tick handler.

Prerequisites

  • You have administrator permissions.

Procedure

  • Set the skew_tick boot parameter to 1.

1.10. Real time scheduling issues and solutions

This section provides information about real time scheduling issues and the available solutions.

Real time scheduling policies

The two real time scheduling policies in RHEL for Real Time share one main characteristic: they run until they are preempted by a higher priority thread or until they "wait", either by sleeping or performing I/O. In the case of SCHED_RR, a thread may be preempted by the operating system so that another thread of equal SCHED_RR priority may run. In either of these cases, no provision is made by the POSIX specifications that define the policies for allowing lower priority threads to get any CPU time.

This characteristic of real-time threads means that it is easy to write an application which monopolizes 100% of a given CPU. However, this causes problems for the operating system. For example, the operating system is responsible for managing both system-wide and per-CPU resources and must periodically examine data structures describing these resources and perform housekeeping activities with them. But if a core is monopolized by a SCHED_FIFO thread, it cannot perform its housekeeping tasks. Eventually the entire system becomes unstable, potentially crashing.

On the RHEL for Real Time kernel, interrupt handlers run as threads with a SCHED_FIFO priority. The default priority is 50. A cpu-hog thread with a SCHED_FIFO or SCHED_RR policy higher than the interrupt handler threads can prevent interrupt handlers from running. This causes programs waiting for data signaled by those interrupts to be starved and fail.

Real time scheduler throttling

RHEL for Real Time includes with a safeguard mechanism that allows the system administrator to allocate bandwith for use by real time tasks. This safeguard mechanism is known as real time scheduler throttling. Real time scheduler throttling is controlled by two parameters in the /proc file system:

  • /proc/sys/kernel/sched_rt_period_us

    Defines the period in μs (microseconds) to be considered 100% of CPU bandwidth. The default value is 1,000,000 μs (1 second). Changes to the value of the period must be very well thought out, as a period too long or too small are equally dangerous.

  • /proc/sys/kernel/sched_rt_runtime_us

    The total bandwidth available for all real-time tasks. The default value is 950,000 μs (0.95 s), which is 95% of the CPU bandwidth. Setting the value to -1 means that real time tasks may use up to 100% of CPU time. This is only adequate when the real time tasks are well engineered and have no obvious caveats, such as unbounded polling loops.

    The default values for the real time throttling mechanism define that the real time tasks can use 95% of the CPU time. The remaining 5% will be devoted to non-real time tasks, such as tasks running under SCHED_OTHER and similar scheduling policies. It is important to note that if a single real time task occupies that 95% CPU time slot, the remaining real time tasks on that CPU will not run. Only non-real time tasks use the remaining 5% of CPU time.

    The impact of the default values include the following:

    • Rogue real time tasks do not lock up the system by not allowing non-real time tasks to run.
    • Real time tasks have at most 95% of CPU time available for them, which can affect their performance.

Real time thread starvation

Thread starvation occurs when a thread is on a CPU run queue for longer than the starvation threshold and does not make progress. A common cause of thread starvation is to run a fixed-priority polling application, such as SCHED_FIFO or SCHED_RR bound to a CPU. Since the polling application does not block for I/O, this can prevent other threads, such as kworkers, from running on that CPU.

An early attempt to reduce thread starvation is called as real-time throttling. In real-time throttling, each CPU has a portion of the execution time dedicated to non real-time tasks. The default setting for throttling is on with 95 percent of the CPU for real-time tasks and 5 percent reserved for non real-time tasks. This works if you have a single real-time task causing starvation but does not work if there are multiple real-time tasks assigned to a CPU.

  • The stalld mechanism

    The stalld mechanism is an alternative for real-time throttling and avoids some of the throttling drawbacks. stalld is a daemon to periodically monitor the state of each thread in the system and looks for threads that are on the run queue for a specified length of time without being run. stalld temporarily changes that thread to use the SCHED_DEADLINE policy and gives the thread a small slice of time on the specified CPU. The thread then runs, and when the time slice is used, the thread returns to its original scheduling policy and stalld continues to monitor thread states.

    Housekeeping CPUs are CPUs that run all daemons, shell processes, kernel threads, interrupt handlers, and all work that can be dispatched from isolated CPUs. For housekeeping CPUs with real-time throttling disabled, stalld monitors the CPU running the main workload. It allows the CPU to run as a SCHED_FIFO busy loop, detecting stalled threads and improving their priority when required with a previously defined acceptable added noise. stalld can be a preference if the real-time throttling mechanism causes an unreasonable noise in the main workload.

    With stalld, you can more precisely control the noise introduced by boosting starved threads. stalld includes the shell script /usr/bin/throttlectl, which automatically disables RT throttling when stalld is run. You can list the current throttling values by using the /usr/bin/throttlectl show script.

  • Disabling real-time throttling

    The following two files in the /proc filesystem can control real-time throttling:

    • /proc/sys/kernel/sched_rt_period_us, specifies the number of microseconds in a period and defaults to 1 million, which is one second.

    • /proc/sys/kernel/sched_rt_runtime_us, specifies the number of microseconds that can be used by a real-time task before throttling occurs and it defaults to 950,000 or 95% of the available CPU cycles. You can disable throttling by passing a value of -1 into the sched_rt_runtime_us file as echo -1 > /proc/sys/kernel/sched_rt_runtime_us.

      Note

      The stalld mechanism causes conflicts with the real-time throttling mechanism. At machine startup, stalld automatically invokes the throttlectl script with a value of off. It saves the current throttling values and writes a -1 to the runtime file. When the stalld service is shut down, the throttlectl script runs with a value of on. It restores the previous saved values for real-time throttling.

Additional resources