linux tune

cpu

governor strategy

• Check Available CPU Governors

  # Run the following command to check available CPU governors:
  cat /sys/devices/system/cpu/cpu0/cpufreq/scaling_available_governors
  
  # You should see something like below info:
  # conservative ondemand userspace powersave performance schedutil

• Check Current CPU Governor

  cat /sys/devices/system/cpu/cpu*/cpufreq/scaling_governor
  or
  cpupower frequency-info --policy # should install cpupower command by 'sudo dnf install kernel-tools -y'

• Set CPU Governor to Performance temporarily

  # To temporarily set the CPU governor to performance (until reboot):
  for cpu in /sys/devices/system/cpu/cpu*/cpufreq/scaling_governor; do
    echo performance | sudo tee $cpu
  done

• Set CPU Governor Persistent

  # First, install cpupower:
  sudo dnf install kernel-tools -y
  
  # Then, set the governor:
  sudo cpupower frequency-set -g performance
  
  # To apply this setting at boot, enable the service:
  sudo systemctl enable --now cpupower.service

nvme

I/O scheduler

• check current scheduler pattern

  cat /sys/block/nvme[01]\*/queue/scheduler
  
  # console print below info
  none [mq-deadline] kyber bfq

  The scheduler currently in use is indicated by the square brackets [ ].

  Common NVMe I/O Schedulers

  1. none – Default for NVMe, provides minimal latency.

  2. mq-deadline – Multi-queue deadline scheduler, balances fairness and performance.

  3. kyber – Optimized for high-performance SSDs.

  4. bfq – Budget Fair Queueing, useful for low-latency workloads.

• Change the I/O Scheduler

  # To set the none scheduler for nvme0n1, use:
  echo none | sudo tee /sys/block/nvme0n1/queue/scheduler

  When to Change the Scheduler?

  • Use none (default) if latency is the priority (most NVMe drives handle queuing internally).

  • Use mq-deadline if fairness in I/O operations is needed.

  • Use kyber for workloads requiring fast response time.

  • Use bfq for interactive desktop workloads.

ulimit

open file size

To configure the maximum open file size (ulimit) for a specific user

• Check Current Limits

  # To see the current limits for a user:
  ulimit -n # Check open file limit
  ulimit -a # Check all limits
  
  # To check system-wide limits:
  cat /proc/sys/fs/file-max

• Set Open File Limits for a Specific User

  The changes in limits.conf are only applied to new login sessions

  sudo vim /etc/security/limits.conf
  
  # Add the following lines (replace username with the actual user):
  username soft nofile 65535
  username hard nofile 65535

​ soft: The default limit the user gets.

​ hard: The maximum limit a user can set.

Feature	/etc/security/limits.conf	/etc/security/limits.d/*.conf
Default File	Yes	No (Custom files in limits.d directory)
Priority	Read first	Read after limits.conf
Organization	All rules in one file	Modular approach, one file per service or user
Example	* soft nofile 65536	dongw soft nofile 65536 (in /etc/security/limits.d/dongw.conf)
Syntax	Same syntax for both files	Same syntax as limits.conf

HP and THP

Huge Pages (HP) and Transparent Huge Pages (THP)

Understanding Huge Pages (HP) and Transparent Huge Pages (THP)

Huge Pages (HP): A manually configured fixed-size memory allocation system, beneficial for workloads that require large contiguous memory allocations.

Transparent Huge Pages (THP): An automated memory management feature that dynamically allocates large memory pages based on usage patterns.

for performance-sensitive applications, THP can sometimes cause performance issues due to fragmentation and unexpected latency spikes. Thus, it’s often recommended to disable THP and manually configure HP.

• Checking Current Huge Pages Configuration

  cat /proc/meminfo | grep HugePages
  
  # Example output:
  AnonHugePages:  16850944 kB # Memory allocated via THP.
  ShmemHugePages:        0 kB
  FileHugePages:         0 kB
  HugePages_Total:       0 # Number of configured huge pages.
  HugePages_Free:        0 # Available huge pages.
  HugePages_Rsvd:        0
  HugePages_Surp:        0

• Configuring Static Huge Pages

  Step 1: Set the Number of Huge Pages

  # To allocate a specific number of 2MB huge pages, calculate based on your memory requirements. Example:
  echo 1024 | sudo tee /proc/sys/vm/nr_hugepages
  
  # Make it persistent:
  echo "vm.nr_hugepages=1024" | sudo tee -a /etc/sysctl.conf
  sysctl -p

  Step 2: Allocate Huge Pages at Boot (Recommended)

  sudo vim /etc/default/grub
  
  # Add hugepages kernel parameter:
  GRUB_CMDLINE_LINUX_DEFAULT="default_hugepagesz=2M hugepagesz=2M hugepages=1024"
  
  # Update GRUB:
  sudo grub2-mkconfig -o /boot/grub2/grub.cfg
  
  # Reboot the system:
  sudo reboot

  Step 3: Mount Huge Pages File System (Optional)

  # To enable shared access to huge pages:
  mkdir -p /mnt/huge
  mount -t hugetlbfs nodev /mnt/huge
  # To make this persistent, add to /etc/fstab:
  nodev /mnt/huge hugetlbfs defaults 0 0

• Disabling Transparent Huge Pages (THP)

  THP should be disabled for performance-sensitive applications

  Step 1: Disable THP at Runtime

  echo never | sudo tee /sys/kernel/mm/transparent_hugepage/enabled
  echo never | sudo tee /sys/kernel/mm/transparent_hugepage/defrag

  Step 2: Make THP Disabled at Boot

  sudo vim /etc/default/grub
  
  # Add:
  GRUB_CMDLINE_LINUX_DEFAULT="transparent_hugepage=never"
  
  # Update GRUB:
  sudo grub2-mkconfig -o /boot/grub2/grub.cfg
  
  # Reboot:
  sudo reboot

• Verification After Reboot

  # Check Huge Pages Allocation
  cat /proc/meminfo | grep HugePages
  
  # Check THP Status
  cat /sys/kernel/mm/transparent_hugepage/enabled
  # Expected output:
  [always] madvise never

Summary: Should You Configure HP & THP Together?

For performance-sensitive applications, it’s recommended to disable THP and manually configure Huge Pages (HP) for better performance.

THP can cause latency spikes and memory fragmentation, making it less ideal for high-performance file systems.