How to Master Linux Resource Limits: Boost Your Server Stability

Introduction

When writing programs we often overlook system resource limits, yet these limits can cause serious problems such as forgotten open file descriptors, malloc failures, or stack overflows. This article shows how to identify and prevent these issues.

User‑Level Resource Limits

The ulimit command displays user‑level limits, which are defined in /etc/security/limits.conf (and files under /etc/security/limits.d). These limits affect users logging in via PAM but do not affect system services.

Typical limits of interest include:

Stack size: 8 MB

Maximum processes: over 30 000

File descriptor limit: 1024

Soft limits can be lowered by the user, while hard limits are set by root and can only be increased by root. Use ulimit -a to view all limits, or ulimit -Ha / ulimit -Sa for hard and soft limits respectively.

Experiment 1 – Increase File Descriptor Limit

Add the following snippet to /etc/security/limits.conf (or a file in /etc/security/limits.d) and reboot:

*               soft    nofile          10240
*               hard    nofile          10240

After reboot, verify the new limit with ulimit -n. A test program that attempts to open 10 240 temporary files succeeds, opening 10 217 files (the remainder are used by the program itself and shared libraries).

Experiment 2 – Increase Stack Size

Add the following lines to /etc/security/limits.conf:

*               soft    stack           8192000
*               hard    stack           8192000

Compile and run a C program that recursively allocates stack space. The program crashes with a segmentation fault when the stack reaches roughly 8 192 000 B, confirming the new limit.

System‑Level Resource Limits

Key system‑wide limits include:

Maximum open files per process: 1 billion

PID limit: ~4 million file‑max (kernel‑enforced max file descriptors): 6 million

Total threads limit: 60 000

Maximum memory‑mapped areas per process: 60 000

Redis File Descriptor Adjustment

When starting redis‑server, you may see a log line such as:

Increased maximum number of open files to 10032 (it was originally set to 1024).

Redis raises the limit by calling setrlimit(RLIMIT_NOFILE, &limit).

prlimit Command

The prlimit command provides functionality similar to ulimit but can operate on any running process.

Conclusion

Linux resource limits are crucial for server‑side developers. Adjusting them via /etc/security/limits.conf (and rebooting) helps fully utilize system capabilities and avoid unexpected failures.