Estimation of capacity for users of an XMPP server based on memory usage and bandwidth using ChatGPT

1. Question

Assume a server with 2GB of memory and a bandwidth of 150Mbit/second, which runs a minimal Linux operating System, such as Alpine Linux, an XMPP server, eg PROSODY, and an XMPP web client, eg MOVIM. All connections use TLS encryption. The traffic only includes text messages, but no multimedia content, such as images, audio or video.
Questions:
1) How much memory is available for handling XMPP traffic, if the web client is not used by any users? How many XMPP users can simultaneously be active on such a server?
2) How much memory is available for handling XMPP traffic, if the web client is used by all users? How many XMPP users can simultaneously be active on such a server?
Please start by first estimating a plausible memory usage per XMPP user connecting with an external XMPP client to the server. Consider bandwidth limitations, TLS overhead, and spikes in usage.

2. Answer

Step 1: Estimating Memory Usage per XMPP User

An XMPP user’s memory consumption depends on:

  • Base memory overhead of the XMPP server (Prosody).
  • Memory used per XMPP connection (message handling, presence updates, and session state).
  • TLS overhead per connection.

Memory Estimates

  • Prosody Base Memory: ~50MB (idle)
  • Memory per Active XMPP Client:
    • Base connection overhead: 3MB
    • Active message processing, presence updates, TLS buffers: 2MB
    • Total per user: ~5MB per active connection

Step 2: Available Memory and Maximum Users (No Web Client Used)

System Memory Usage

  • Total RAM: 2GB = 2048MB
  • OS (Alpine Linux) + System Services: ~100MB
  • Prosody (base usage): ~50MB
  • Other services (logs, networking, security, etc.): ~100MB
  • Available for XMPP traffic:
    2048(100+50+100)=1798MB2048 – (100 + 50 + 100) = 1798MB
     

Max Users (No Web Client)

1798MB5MB359 simultaneous users\frac{1798MB}{5MB} \approx 359 \text{ simultaneous users}

 


Step 3: Available Memory and Maximum Users (With Web Client Used)

The Movim web client is a PHP-based XMPP client running on a web server (e.g., Nginx + PHP-FPM).

Memory Overhead for Movim

  • Nginx + PHP backend: 200MB
  • Per-user PHP-FPM session: 10MB per user
  • Prosody (base usage): 50MB

Available Memory for XMPP with Web Client

2048(100+50+100+200+10N)=159810N2048 – (100 + 50 + 100 + 200 + 10N) = 1598 – 10N

 

Each web client user also consumes 5MB for XMPP, so total per-user memory usage is:

10MB(Movim)+5MB(XMPP)=15MB10MB (\text{Movim}) + 5MB (\text{XMPP}) = 15MB

 

Max Users (With Web Client)

1598MB15MB106 simultaneous users\frac{1598MB}{15MB} \approx 106 \text{ simultaneous users}

 


Step 4: Bandwidth Considerations (Including TLS Overhead)

Your server has 150 Mbit/sec (18.75 MB/sec) available.

Estimating Per-User Bandwidth Consumption

  • Baseline XMPP text traffic: ~25 Kbit/sec per user (~3.125 KB/sec).
  • TLS overhead: ~20% extra for encryption.
  • Adjusted per-user bandwidth:
    25Kbit/sec×1.2=30Kbit/sec25 Kbit/sec \times 1.2 = 30 Kbit/sec
     

Max Users Based on Bandwidth

150,000Kbit/sec30Kbit/sec=5000 users\frac{150,000 Kbit/sec}{30 Kbit/sec} = 5000 \text{ users}

 

Bandwidth is not a bottleneck, as RAM limits the number of users long before reaching the bandwidth cap.


Step 5: Considering Spikes in Usage

  • Peak Traffic Scenarios:
    • Spikes in presence updates or burst message sending can temporarily increase bandwidth per user to ~50 Kbit/sec.
    • In this case, max bandwidth capacity drops to 3000 users, which is still well above the RAM-imposed limits.
  • TLS Session Resumption & Reconnects:
    • When many users reconnect simultaneously, TLS handshakes may spike CPU usage rather than memory or bandwidth.
    • This might require limiting reconnect rates in the server config.

Final Results Summary

Scenario Available RAM for XMPP Max Users (RAM) Max Users (Bandwidth) Actual Limit
No Web Client 1798MB ~359 ~5000 ~359 (RAM-limited)
With Web Client 1598MB – 10N ~106 ~5000 ~106 (RAM-limited)

Conclusion

  • RAM is the limiting factor, not bandwidth.
  • The server can support ~359 XMPP users when using external XMPP clients.
  • With Movim web clients, this drops to ~106 users due to PHP-FPM overhead.
  • TLS overhead slightly increases bandwidth but does not significantly impact user capacity.

Would you like me to consider any additional factors, such as CPU constraints or optimizations?

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