Swap

From VoIPmonitor.org
Revision as of 00:50, 6 January 2026 by Admin (talk | contribs) (Add guidance on kernel cache/buffer normal behavior, memory vs CPU diagnostic steps, and conditional buffer tuning (increase/decrease max_buffer_mem based on RAM availability))

Category:Installation

Swap Configuration

Swap usage leads to performance degradation on VoIPmonitor servers. For realtime packet processing, we highly recommend configuring swap space properly or disabling it entirely.

Understanding Swappiness

Most Linux distributions default to using swap when RAM usage exceeds 40%. The swappiness value controls this behavior: 

# Check current swappiness value
cat /proc/sys/vm/swappiness

A value of 60 means the system starts using swap when less than 60% of RAM is free. This is not optimal for realtime services like VoIPmonitor.

Swappiness Value Behavior
0 Use swap only when RAM is completely exhausted
5 Use swap only when critically low on RAM (recommended)
60 Default - start swapping at 40% free RAM (not recommended)
100 Aggressively use swap (not recommended for VoIPmonitor)

Configuring Swappiness

Temporary Change (Until Reboot)

# Set swappiness to 5 (use swap only when critically low on RAM)
echo '5' > /proc/sys/vm/swappiness

# Apply immediately by clearing swap
swapoff -a && swapon -a

Permanent Change

Add the following line to /etc/sysctl.conf: 

vm.swappiness=5

Then apply without reboot: 

sysctl -p

Clearing Swap

To move all data from swap back to RAM: 

swapoff -a
swapon -a

Disabling Swap Completely

Step 1
Disable swap immediately:
swapoff -a
Step 2
Comment out swap entries in /etc/fstab:
proc            /proc           proc    defaults        0       0
UUID=17e56a9a-0f42-44ca-90e8-570315708def /               xfs     relatime        0       1

# Swap entries commented out:
#UUID=0a403f5e-0505-4055-a219-70217b6b74d1 none            swap    sw              0       0
#UUID=4cf42564-4339-42ed-b7ec-29644a3085f1 none            swap    sw              0       0
Step 3
Monitor for OOM issues:
# Check for out-of-memory events
dmesg -T | grep -i "out of memory\|killed process"

Understanding Linux Cache and Buffer Memory

It is important to understand that high system cache and buffer usage is normal kernel behavior and should not be confused with actual memory consumption. The Linux kernel uses available RAM to cache disk I/O and buffer network packets, but this memory is considered free and can be immediately reclaimed when needed by applications. 

# Check memory with cache/buffers separated
free -h

# Example output interpretation:
# total        used        free      shared  buff/cache   available
# Mem:           31Gi       10Gi       2.0Gi       1.0Gi        19Gi        18Gi
#
# The "available" column (18Gi) is the usable memory for applications.
# The 19Gi in "buff/cache" is kernel-managed but reclaimable.

When diagnosing memory issues on VoIPmonitor servers, focus on the available column rather than total memory usage. High cache/buffer values indicate the kernel is efficiently using spare RAM, not that the system is running out of memory.

However, if the sniffer's internal 'working memory buffer/cache is filling up and causing crashes, you may need to adjust max_buffer_mem depending on your server's RAM capacity:

  • Insufficient RAM: Decrease max_buffer_mem to prevent OOM crashes
  • Sufficient RAM Available: Increase max_buffer_mem (e.g., to 10000) to handle high traffic bursts

Diagnostic Step: Identify Memory vs CPU Bottleneck

Before adjusting buffer settings, isolate whether the bottleneck is memory-related or CPU-related. Follow this diagnostic process:

Step 1
Temporarily disable all packet saving:
# In /etc/voipmonitor.conf, temporarily set all saves to no:
savesip = no
savertp = no
savertcp = no
savegraph = no
Step 2
Restart the sniffer:
systemctl restart voipmonitor
Step 3
Monitor the system:
# Watch memory usage
watch -n 1 free -h

# Check if the "buffer filling up" condition persists
journalctl -u voipmonitor -f
Step 4
Analyze the result:
  • If high memory usage/load persists' after disabling saves: The bottleneck is CPU-related. Focus on reducing processing load (disable audio transcoding, RTP analysis, etc.)
  • If memory usage drops significantly and stabilizes: The bottleneck was caused by packet storage operations. Restore saves after tuning buffer settings.

Checking VoIPmonitor Memory Configuration

After completing the diagnostic above, review and adjust VoIPmonitor's internal memory configuration based on your findings. 

# Check current memory-related settings
grep -E '^(max_buffer_mem|ringbuffer|packetbuffer)' /etc/voipmonitor.conf
Parameter Default Description When to Increase When to Decrease
ringbuffer 50 MB Ringbuffer size per interface. Recommended >= 500 for >100 Mbit traffic. Max 2000. High traffic bursts, packet loss observed RAM limited, packet loss not observed
max_buffer_mem 2000 MB Maximum buffer memory for packet buffers. Sufficient RAM available (>16GB), buffer filling up on high traffic RAM limited (<8GB), OOM crashes occurring
packetbuffer_enable yes Enable packet buffer cache. Generally keep enabled for performance Disable if causing excessive memory pressure
packetbuffer_compress no Compress packet buffer (saves RAM, uses CPU). RAM constrained, CPU available CPU constrained, RAM available

Buffer Tuning Guidelines:

  • Increasing Buffers (when RAM is available): Set max_buffer_mem = 10000 or higher on servers with 32GB+ RAM to handle traffic spikes without packet loss.
  • Decreasing Buffers (when RAM is limited): Reduce ringbuffer to 20-50 and max_buffer_mem to 500-1000 on servers with 8GB or less RAM to prevent OOM crashes.

See Sniffer Configuration for complete memory and buffer documentation.

Checking Current Memory and Swap Usage

# Check memory and swap status
free -m

# Monitor memory usage over time
watch -n 1 free -m

# Check which processes use most memory
ps aux --sort=-%mem | head -10

See Also

AI Summary for RAG

Summary: Linux swap configuration for VoIPmonitor servers. Covers swappiness tuning (recommended value 5), disabling swap completely, understanding Linux cache/buffer memory as normal kernel behavior (reclaimable, not consumed), diagnostic steps to identify memory vs CPU bottleneck (temporarily disable savesip, savertp, savertcp, savegraph and monitor if high load persists), and conditional guidance for VoIPmonitor buffer memory settings (ringbuffer, max_buffer_mem): INCREASE max_buffer_mem (e.g., to 10000) when sufficient RAM available (>16GB) to handle high traffic bursts, DECREASE max_buffer_mem (to 500-1000) when RAM limited (<8GB) to prevent OOM crashes.

Keywords: swap, swappiness, memory, RAM, performance, ringbuffer, max_buffer_mem, OOM, out of memory, packetbuffer, cache, buffer, kernel memory, diagnostic, memory bottleneck, CPU bottleneck, savesip, savertp, savertcp, savegraph, disable packet saving

Key Questions:

  • How to configure swap for VoIPmonitor?
  • How to disable swap on Linux?
  • What is swappiness and how to configure it?
  • How to reduce VoIPmonitor memory usage?
  • What are ringbuffer and max_buffer_mem settings?
  • Should I increase or decrease max_buffer_mem for my server?
  • How do I know if memory issues are caused by VoIPmonitor buffers or CPU bottlenecks?
  • How do I diagnose memory vs CPU bottleneck in VoIPmonitor?
  • What does high cache/buffer usage mean in Linux?
  • Why does free command show high memory usage but system is fine?
Retrieved from "https://www.voipmonitor.org/doc/index.php?title=Swap&oldid=7438"