Swap space is essential for system stability, allowing the kernel to move less-frequently-used memory pages to disk when physical RAM is exhausted. Here’s how to set up a dedicated swap partition. Step 1: Create the Partition Use fdisk, gdisk (for GPT disks), or parted to create a new partition. For modern systems with GPT partition…
Linux can manage multiple swap partitions or files simultaneously, each with an assigned priority. The kernel allocates swap pages from highest to lowest priority, with round-robin allocation among equally-prioritized areas. Setting Swap Priority with swapon Use the swapon command to activate a swap partition and set its priority: swapon -p 10 /dev/sdc1 The -p flag…
To disable all active swap spaces on Linux, run: swapoff -a The -a flag targets all currently enabled swap devices and files. This forces the kernel and applications to rely entirely on physical RAM. When You’d Do This Disabling swap is useful for: Performance testing: Eliminating swap I/O variance to get consistent benchmarks Latency-sensitive workloads:…
Kernel parameters often need to be passed to Xen DomUs during boot or for troubleshooting. Common scenarios include booting into single-user mode for recovery, enabling kernel debugging, or adjusting memory/CPU settings at runtime. Using the extra parameter The simplest method is passing kernel parameters via the extra= option when creating or starting a DomU. For…
The conventional wisdom that smaller code is always better has been thoroughly invalidated on modern processors. A statement from kernel development circles captured this well: for performance-critical paths like interrupt handlers, large-but-correctly-aligned-and-optimized code consistently outperforms byte-packed alternatives. Consider this comparison: mov eax, [foo] add eax, 1 mov [foo], eax versus: inc [foo] The three-instruction sequence…
Archiving or copying a 1TB sparse file containing only 32MB of actual data is impractical with naive approaches—you’ll waste time and I/O bandwidth copying zeros. The solution is using SEEK_HOLE and SEEK_DATA flags with lseek(), which allow tools to skip over hole regions entirely. How SEEK_HOLE and SEEK_DATA work The Linux kernel provides two additional…
When you mmap() a range larger than your system’s total physical memory plus swap, the kernel normally reserves swap space to back the mapping. If that reservation would exceed available swap, mmap() fails with ENOMEM. The MAP_NORESERVE flag lets you bypass this reservation requirement. Understanding the Default Behavior By default, when you call mmap() with…
When you need to debug kernel behavior without full instrumentation frameworks, module parameters offer a pragmatic shortcut. Rather than immediately reaching for perf, kprobes, or eBPF, you can add simple toggles and state tracking directly in the code you’re investigating, then control them at runtime through sysfs. Frameworks like perf, kprobes, and eBPF are powerful…
QEMU/KVM networking operates across multiple layers — TAP devices, QEMU’s packet queuing, virtio descriptors, and kernel bypass mechanisms. Understanding this stack is essential for debugging connectivity issues and tuning performance in production environments. TAP Device Fundamentals TAP (TUN/TAP) devices create a bridge between the guest and host network stack. A typical QEMU/KVM configuration looks like:…
Most popular task monitoring tools (top, iotop, procfs) sample disk I/O information at granularity limited by kernel timer frequency—typically once per second. This sampling rate misses short-lived I/O bursts and makes it difficult to accurately measure which tasks are truly I/O bound. eBPF and kernel tracepoints let you capture microsecond-level accuracy without heavy overhead. Why…
Xen remains a solid hypervisor choice for specific workloads, though KVM dominates most Linux distributions today. This guide covers installing a paravirtualized DomU on Xen using modern tooling and Rocky Linux 9, which requires specific kernel parameters during installation. Why Paravirtualization Matters Paravirtualized guests communicate directly with the hypervisor rather than emulating hardware, resulting in…
USB technology continues evolving with each generation delivering significant speed improvements. Linux provides comprehensive kernel support across all modern standards through dedicated drivers. USB 2.0 Speed: 480 Mbps (theoretical ~60 MB/s) Status: Legacy but still common. Nearly all systems retain USB 2.0 ports for backward compatibility. Linux support: Built-in via the ehci-hcd driver in all…
Grab a release from kernel.org. Recent stable versions (6.x and later) are recommended for production systems: wget https://www.kernel.org/pub/linux/kernel/v6.x/linux-6.12.tar.gz tar xf linux-6.12.tar.gz cd linux-6.12/ If wget isn’t available, use curl: curl -O https://www.kernel.org/pub/linux/kernel/v6.x/linux-6.12.tar.gz Verify the tarball Always verify the kernel’s integrity using checksums and signatures published on kernel.org. This is essential for production systems: wget https://www.kernel.org/pub/linux/kernel/v6.x/linux-6.12.tar.gz.asc…
The Linux kernel provides the taskstats API to monitor detailed per-task statistics, including I/O wait time, CPU scheduling delays, and memory reclamation waits. Understanding how these metrics are collected at the kernel level is essential for diagnosing performance issues and understanding process behavior. How the Kernel Records I/O Delays The kernel records I/O wait delays…
QEMU/KVM uses global locks to synchronize threads, which is necessary for correctness but causes performance problems at scale. When multiple I/O requests and vCPU threads contend for the same lock, you get lock contention that manifests as scheduling jitter, reduced system scalability, and degraded I/O performance. IOThreads (the modern implementation of dataplane) address this by…
CFS (Completely Fair Scheduling) is the default process scheduler in modern Linux kernels. Understanding actual timeslice allocation—how long a process runs before being context-switched out—is essential for performance debugging, verifying fairness assumptions, and identifying scheduling anomalies. The challenge is that /proc interfaces and ps commands show scheduling policy but not the actual on-CPU duration per…
The Completely Fair Scheduler (CFS) is the default process scheduler in Linux kernels since 2.6.23. It allocates CPU time proportionally based on process priority, but the actual timeslices and preemption behavior depend on three key tunable parameters: sched_latency_ns, sched_min_granularity_ns, and sched_wakeup_granularity_ns. Understanding how these work together is essential for tuning scheduler behavior on production systems….
In computer systems, resources must be balanced across available hardware to maximize performance. The Linux kernel runs migration threads as per-CPU daemons to handle this work. These threads manage task migration between CPUs, CPU hotplug operations, and stop-machine synchronization. You can observe migration threads on any modern system: $ ps aux | grep migration root…
CPU affinity binds a process or thread to specific CPU cores, reducing cache misses, improving performance isolation, and managing NUMA locality. The sched_setaffinity() system call is the primary mechanism in Linux. Understanding how it works at the kernel level clarifies scheduler internals and helps you use it effectively. System Call Entry Point The sched_setaffinity() syscall…
Systemd replaced traditional init on most modern Linux distributions, shifting terminology from “runlevels” to “targets.” If you’re migrating from older systems, here’s the mapping: Runlevel Systemd Target Purpose 0 poweroff.target Shutdown 1 rescue.target Single-user mode 2, 3, 4 multi-user.target Multi-user, no GUI 5 graphical.target Multi-user with GUI 6 reboot.target Reboot The two targets you’ll use…