1. Question

How does memory usage per user compare between an IRC server and an XMPP server for text only communication when external clients are used to connect to the server?

2. Answer

Comparing CPU and Bandwidth Usage: IRC vs. XMPP (Text-Only Communication, External Clients)

Now, let’s compare CPU usage and bandwidth usage per user between IRC and XMPP servers.

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1. CPU Usage Comparison

CPU usage depends on how the protocol handles connections, messages, and encryption.

IRC Server CPU Usage

✅ Simple text-based protocol → Low processing overhead.
✅ No XML parsing or message validation → Minimal CPU load.
❌ No built-in encryption (unless TLS is enabled separately).

• Idle state: ~1-5% CPU
• Active chat (~1000 users): ~5-20% CPU
• High traffic (mass joins, large channels): Spikes to ~50%+ CPU
• TLS Overhead (if enabled): Moderate increase (~10-20% CPU usage boost).

Why is IRC low CPU?
IRC sends plain text messages with minimal processing.
No presence updates or contact lists to manage.

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XMPP Server CPU Usage

✅ XML-based structured communication → Requires parsing & validation.
✅ TLS encryption enforced → Cryptographic overhead on CPU.
✅ Presence tracking & roster management → Constant background processing.

• Idle state: ~5-10% CPU
• Active chat (~100 users): ~20-50% CPU
• High traffic (presence updates, group chats): ~60-90% CPU
• TLS Overhead: Encryption can add 30-50% CPU load, especially for frequent reconnections.

Why is XMPP higher CPU?
Parsing XML is CPU-intensive.
Encryption (TLS) adds cryptographic processing.
Roster & Presence Management means the server constantly updates user state.

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2. Bandwidth Usage Comparison

Both protocols are lightweight, but XMPP generates more background traffic due to presence updates.

IRC Bandwidth Per User

• Idle (connected, no messages): ~0.1 Kbit/sec (~0.012 KB/sec)
• Active chat: ~10-25 Kbit/sec (~1.25-3.125 KB/sec) per user
• High activity (large channels, rapid messages): ~50 Kbit/sec (~6.25 KB/sec)

Why is IRC lower bandwidth?
No background presence updates—only messages are transmitted.
Simple text format (1:1 mapping of input/output).

Max Users Based on 150 Mbit/sec Bandwidth

$$\frac{150,000 Kbit/sec}{25 Kbit/sec} \approx 6,000 \text{ users}$$

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XMPP Bandwidth Per User

• Idle (connected, no messages): ~2-5 Kbit/sec (~0.25-0.625 KB/sec)
• Active chat: ~25-50 Kbit/sec (~3.125-6.25 KB/sec) per user
• High activity (group chats, presence updates): ~100 Kbit/sec (~12.5 KB/sec)

Why is XMPP higher bandwidth?
Frequent presence updates generate extra traffic.
Messages are wrapped in XML, increasing size.

Max Users Based on 150 Mbit/sec Bandwidth

$$\frac{150,000 Kbit/sec}{50 Kbit/sec} \approx 3,000 \text{ users}$$

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3. Summary: Performance Comparison Table

Feature	IRC Server	XMPP Server
Memory Per User	~0.5MB	~5MB
CPU Usage (Idle)	1-5%	5-10%
CPU Usage (Active 100 Users)	5-20%	20-50%
CPU Usage (High Traffic, 1000 Users)	50%+	60-90%
Bandwidth (Idle Per User)	~0.1 Kbit/sec	~2-5 Kbit/sec
Bandwidth (Active Per User)	~10-25 Kbit/sec	~25-50 Kbit/sec
Bandwidth (High Traffic Per User)	~50 Kbit/sec	~100 Kbit/sec
Max Users on 2GB RAM	~3,596	~359
Max Users on 150 Mbit/sec	~6,000	~3,000

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4. Key Takeaways

1. IRC is significantly more efficient in memory, CPU, and bandwidth.
2. XMPP has higher CPU and bandwidth costs due to encryption, XML parsing, and presence tracking.
3. If scaling to thousands of users on limited hardware, IRC is better.
4. XMPP offers modern features like presence and encryption but needs more resources.

Would you like a comparison including storage requirements or performance tuning strategies?

Reference

Igl, W. (2025). Prompt “How does memory usage per user compare between an IRC server and an XMPP server for text only communication when external clients are used to connect to the server?” Retrieved 2025-02-06