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Start byte-based backpressure earlier #1179

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Feb 26, 2024
Merged

Start byte-based backpressure earlier #1179

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21 changes: 15 additions & 6 deletions upstairs/src/guest.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -343,6 +343,8 @@ struct BackpressureConfig {
bytes_start: u64,
/// Scale for byte-based quadratic backpressure
bytes_scale: f64,
/// Maximum bytes-based backpressure
bytes_max_delay: Duration,

/// When should queue-based backpressure start?
queue_start: f64,
Expand Down Expand Up @@ -393,8 +395,12 @@ impl Guest {
bw_tokens: 0,
backpressure_us: backpressure_us.clone(),
backpressure_config: BackpressureConfig {
bytes_start: 1024u64.pow(3), // Start at 1 GiB
bytes_scale: 9.3e-8, // Delay of 10ms at 2 GiB in-flight
// Byte-based backpressure
bytes_start: 100 * 1024u64.pow(2), // Start at 100 MiB
bytes_scale: 6e-8, // Delay of 15ms at 2 GiB in-flight
bytes_max_delay: Duration::from_millis(15),

// Queue-based backpressure
queue_start: 0.05,
queue_max_delay: Duration::from_millis(5),
},
Expand Down Expand Up @@ -918,10 +924,13 @@ impl GuestIoHandle {
// the upstairs and downstairs) is particularly high. If so,
// apply some backpressure by delaying host operations, with a
// quadratically-increasing delay.
let d1 = (bytes.saturating_sub(self.backpressure_config.bytes_start)
as f64
* self.backpressure_config.bytes_scale)
.powf(2.0) as u64;
let d1 = (self.backpressure_config.bytes_max_delay.as_micros() as u64)
.min(
(bytes.saturating_sub(self.backpressure_config.bytes_start)
as f64
* self.backpressure_config.bytes_scale)
.powf(2.0) as u64,
);

// Compute an alternate delay based on queue length
let d2 = self
Expand Down