configurable dynamic stream concurrency scalar

Signed-off-by: Jason Volk <jason@zemos.net>

src/core/config/mod.rs

@@ -1626,6 +1626,33 @@ pub struct Config {
 	#[serde(default = "default_db_pool_queue_mult")]
 	pub db_pool_queue_mult: usize,
 
+	/// Sets the initial value for the concurrency of streams. This value simply
+	/// allows overriding the default in the code. The default is 32, which is
+	/// the same as the default in the code. Note this value is itself
+	/// overridden by the computed stream_width_scale, unless that is disabled;
+	/// this value can serve as a fixed-width instead.
+	///
+	/// default: 32
+	#[serde(default = "default_stream_width_default")]
+	pub stream_width_default: usize,
+
+	/// Scales the stream width starting from a base value detected for the
+	/// specific system. The base value is the database pool worker count
+	/// determined from the hardware queue size (e.g. 32 for SSD or 64 or 128+
+	/// for NVMe). This float allows scaling the width up or down by multiplying
+	/// it (e.g. 1.5, 2.0, etc). The maximum result can be the size of the pool
+	/// queue (see: db_pool_queue_mult) as any larger value will stall the tokio
+	/// task. The value can also be scaled down (e.g. 0.5)  to improve
+	/// responsiveness for many users at the cost of throughput for each.
+	///
+	/// Setting this value to 0.0 causes the stream width to be fixed at the
+	/// value of stream_width_default. The default is 1.0 to match the
+	/// capabilities detected for the system.
+	///
+	/// default: 1.0
+	#[serde(default = "default_stream_width_scale")]
+	pub stream_width_scale: f32,
+
 	/// Number of sender task workers; determines sender parallelism. Default is
 	/// '0' which means the value is determined internally, likely matching the
 	/// number of tokio worker-threads or number of cores, etc. Override by
@@ -2436,3 +2463,7 @@ fn default_db_pool_workers() -> usize {
 fn default_db_pool_workers_limit() -> usize { 64 }
 
 fn default_db_pool_queue_mult() -> usize { 4 }
+
+fn default_stream_width_default() -> usize { 32 }
+
+fn default_stream_width_scale() -> f32 { 1.0 }