Introduction to Preheat

Preheat is an adaptive readahead daemon that accelerates application startup times on Linux systems by intelligently preloading frequently used programs into memory.

What Preheat Does

When you launch an application, the operating system must read its executable file and shared libraries from disk into memory. On mechanical hard drives, this disk I/O is the primary bottleneck causing slow startup times. Even on SSDs, cold starts can be noticeably slower than warm starts.

Preheat solves this by:

Monitoring which applications you run
Learning your usage patterns using statistical models
Predicting which applications you’re likely to run next
Preloading those files into the disk cache proactively

The result: applications start faster because their files are already in memory when you launch them.

The Problem It Solves

Consider a typical workflow:

Without Preheat:
┌──────────┐     ┌──────────┐     ┌──────────┐
│ You      │ --> │ Launch   │ --> │ Wait for │ --> App Ready
│ Click    │     │ Firefox  │     │ Disk I/O │     (3-5 sec)
└──────────┘     └──────────┘     └──────────┘

With Preheat:
┌──────────┐     ┌──────────┐     ┌──────────┐
│ Preheat  │     │ Firefox  │     │ You      │
│ Preloads │     │ Files in │ --> │ Click    │ --> App Ready
│ Firefox  │     │ Cache    │     │ Firefox  │     (< 1 sec)
└──────────┘     └──────────┘     └──────────┘

Typical Improvements

Scenario	Without Preheat	With Preheat	Improvement
Browser (cold start)	4-8 seconds	1-2 seconds	60-75%
IDE (cold start)	10-20 seconds	3-6 seconds	50-70%
Office suite	5-10 seconds	1-3 seconds	60-70%

Results vary based on hardware and usage patterns.

Design Goals

Primary Goals

Automatic operation: Once installed, preheat runs without intervention
Low overhead: Minimal CPU usage (<1%) and controlled memory consumption
Adaptive learning: Improves predictions as it learns your habits
Safe defaults: Conservative settings that work well out of the box

Secondary Goals

systemd integration for proper service management
Flexible configuration for power users
Graceful handling of low-memory conditions
Persistent state across reboots

What Preheat Does NOT Do

Understanding limitations is as important as understanding capabilities:

Preheat Does NOT…	Reason
Speed up running applications	It only accelerates startup, not runtime
Help applications not in the prediction model	New or rarely-used apps aren’t predicted
Work well with insufficient RAM	Preloading requires free memory headroom
Improve SSD performance significantly	SSDs are already fast; gains are marginal
Replace proper system optimization	It complements, not replaces, other tuning

How It Differs from Similar Tools

vs. systemd-readahead (deprecated)

systemd-readahead recorded boot-time file access and replayed it
Preheat learns ongoing usage patterns, not just boot sequences
Preheat adapts to changing workflows

vs. preload (original)

Preheat is based on preload’s concepts
Updated for modern systems and systemd
Enhanced configuration options
Maintained and actively developed

vs. Early-OOM / zram

These tools manage memory pressure
Preheat proactively fills available memory with useful data
They can work together complementarily

Supported Environments

Operating Systems

Preheat works on any Debian-based Linux distribution:

Debian 11+
Ubuntu 20.04+
Linux Mint
Other apt-based distributions

Hardware Requirements

Component	Minimum	Recommended
RAM	2 GB	4+ GB (more headroom for caching)
Storage	Any	Slower storage benefits most
Kernel	2.6+	4.0+

When Preheat Helps Most

Slower storage (HDD, SATA SSD) with unused RAM sitting idle
Repetitive workflows — launching the same apps daily
Older hardware where cold starts feel noticeably slow
Desktop/laptop use with predictable launch patterns

When Preheat Helps Less

Fast NVMe storage — already quick startup times
Very limited RAM (<2 GB) — no headroom for caching
Server workloads — persistent services, not interactive apps
Random usage patterns — unpredictable usage defeats prediction

Architecture Overview

┌─────────────────────────────────────────────────────────┐
│                      PREHEAT DAEMON                     │
├────────────────┬────────────────┬───────────────────────┤
│   MONITORING   │   PREDICTION   │      PRELOADING       │
│                │                │                       │
│  /proc scanner │  Markov chain  │  readahead(2) calls   │
│  Process maps  │  Correlation   │  Parallel I/O         │
│  Library deps  │  Scoring       │  Memory management    │
└───────┬────────┴───────┬────────┴───────────┬───────────┘
        │                │                    │
        v                v                    v
┌───────────────┐ ┌──────────────┐ ┌─────────────────────┐
│ State File    │ │ Config File  │ │ Disk Cache (kernel) │
│ (persistent)  │ │ (user)       │ │ (memory)            │
└───────────────┘ └──────────────┘ └─────────────────────┘

For detailed architecture documentation, see Architecture.

Getting Started

Ready to try preheat? Follow these steps:

Install preheat - Get it running on your system
Quick start guide - Verify it’s working
Configuration - Customize for your needs

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