Change MaximumClientWritesPending to 64

[lneoe]

Adjust MaximumClientWritesPending default value: from 1024*8 to 64

• Current default value is too high, causing significant memory overhead when maintaining a large number of connections (tens of thousands)
• Testing shows that reducing this value significantly decreases memory usage, optimizing resource utilization

Technical Impact

• Minimal impact on small-scale deployments
• Significantly improves memory usage in high-concurrency scenarios
• No effect on core functionality and performance

[thedevop]

@lneoe, for my use case (also large number of connections), I also set this to a much smaller value (128) than the default. However, many do not necessary use in environments with large number of connections. For those use cases, it is preferable to have a higher value to prevent possible loss of delivery.

[lneoe]

@lneoe, for my use case (also large number of connections), I also set this to a much smaller value (128) than the default. However, many do not necessary use in environments with large number of connections. For those use cases, it is preferable to have a higher value to prevent possible loss of delivery.

Yes, larger values can prevent message loss, but this consideration is primarily necessary in scenarios with very frequent message transmission and relatively slow consumption rates.
In my actual application scenario, the server maintains approximately 33,000 simultaneous connections, consuming about 5GB of memory that never gets garbage collected. Memory consumption became a bottleneck, preventing the server from creating additional connections. After reducing this value, we now support 50,000 simultaneous connections while consuming only about 2.8GB of memory. Of course, 64 isn't appropriate for every scenario, but the original default value was indeed too large and easily led to excessive memory growth.

[thedevop]

Are you using the broker in embedded mode? if so, you can consult #453 for additional server settings that can reduce memory usage.

[lneoe]

Are you using the broker in embedded mode? if so, you can consult #453 for additional server settings that can reduce memory usage.

Yes, I'm using embedded mode, and I've also configured it through setting my own mqtt.Options. The main reason for submitting this PR is considering that such a large default value isn't necessary for most scenarios, and we often use default settings because they typically represent a more universal solution.