Refactor PVF preparation memory stats (#6693)

* Refactor PVF preparation memory stats

The original purpose of this change was to gate metrics that are unsupported by
some systems behind conditional compilation directives (#[cfg]); see
https://github.com/paritytech/polkadot/pull/6675#discussion_r1099996209.

Then I started doing some random cleanups and simplifications and got a bit
carried away. 🙈 The code should be overall tidier than before.

Changes:
- Don't register unsupported metrics (e.g. `max_rss` on non-Linux systems)
- Introduce `PrepareStats` struct as an abstraction over the `Ok` values of
  `PrepareResult`. It is cleaner, and can be easily modified in the future.
- Other small changes

* Minor fixes to comments

* Fix compile errors

* Try to fix some Linux errors

* Mep

* Fix candidate-validation tests

* Update docstring

polkadot/node/core/candidate-validation/src/lib.rs

@@ -24,7 +24,8 @@
 #![warn(missing_docs)]
 
 use polkadot_node_core_pvf::{
-	InvalidCandidate as WasmInvalidCandidate, PrepareError, Pvf, ValidationError, ValidationHost,
+	InvalidCandidate as WasmInvalidCandidate, PrepareError, PrepareStats, Pvf, ValidationError,
+	ValidationHost,
 };
 use polkadot_node_primitives::{
 	BlockData, InvalidCandidate, PoV, ValidationResult, POV_BOMB_LIMIT, VALIDATION_CODE_BOMB_LIMIT,
@@ -654,7 +655,7 @@ trait ValidationBackend {
 		validation_result
 	}
 
-	async fn precheck_pvf(&mut self, pvf: Pvf) -> Result<Duration, PrepareError>;
+	async fn precheck_pvf(&mut self, pvf: Pvf) -> Result<PrepareStats, PrepareError>;
 }
 
 #[async_trait]
@@ -680,7 +681,7 @@ impl ValidationBackend for ValidationHost {
 			.map_err(|_| ValidationError::InternalError("validation was cancelled".into()))?
 	}
 
-	async fn precheck_pvf(&mut self, pvf: Pvf) -> Result<Duration, PrepareError> {
+	async fn precheck_pvf(&mut self, pvf: Pvf) -> Result<PrepareStats, PrepareError> {
 		let (tx, rx) = oneshot::channel();
 		if let Err(err) = self.precheck_pvf(pvf, tx).await {
 			// Return an IO error if there was an error communicating with the host.

polkadot/node/core/candidate-validation/src/tests.rs

@@ -377,7 +377,7 @@ impl ValidationBackend for MockValidateCandidateBackend {
 		result
 	}
 
-	async fn precheck_pvf(&mut self, _pvf: Pvf) -> Result<Duration, PrepareError> {
+	async fn precheck_pvf(&mut self, _pvf: Pvf) -> Result<PrepareStats, PrepareError> {
 		unreachable!()
 	}
 }
@@ -894,11 +894,11 @@ fn pov_decompression_failure_is_invalid() {
 }
 
 struct MockPreCheckBackend {
-	result: Result<Duration, PrepareError>,
+	result: Result<PrepareStats, PrepareError>,
 }
 
 impl MockPreCheckBackend {
-	fn with_hardcoded_result(result: Result<Duration, PrepareError>) -> Self {
+	fn with_hardcoded_result(result: Result<PrepareStats, PrepareError>) -> Self {
 		Self { result }
 	}
 }
@@ -914,7 +914,7 @@ impl ValidationBackend for MockPreCheckBackend {
 		unreachable!()
 	}
 
-	async fn precheck_pvf(&mut self, _pvf: Pvf) -> Result<Duration, PrepareError> {
+	async fn precheck_pvf(&mut self, _pvf: Pvf) -> Result<PrepareStats, PrepareError> {
 		self.result.clone()
 	}
 }
@@ -931,7 +931,7 @@ fn precheck_works() {
 
 	let (check_fut, check_result) = precheck_pvf(
 		ctx.sender(),
-		MockPreCheckBackend::with_hardcoded_result(Ok(Duration::default())),
+		MockPreCheckBackend::with_hardcoded_result(Ok(PrepareStats::default())),
 		relay_parent,
 		validation_code_hash,
 	)
@@ -977,7 +977,7 @@ fn precheck_invalid_pvf_blob_compression() {
 
 	let (check_fut, check_result) = precheck_pvf(
 		ctx.sender(),
-		MockPreCheckBackend::with_hardcoded_result(Ok(Duration::default())),
+		MockPreCheckBackend::with_hardcoded_result(Ok(PrepareStats::default())),
 		relay_parent,
 		validation_code_hash,
 	)

polkadot/node/core/pvf/src/artifacts.rs

@@ -14,7 +14,7 @@
 // You should have received a copy of the GNU General Public License
 // along with Polkadot.  If not, see <http://www.gnu.org/licenses/>.
 
-use crate::{error::PrepareError, host::PrepareResultSender};
+use crate::{error::PrepareError, host::PrepareResultSender, prepare::PrepareStats};
 use always_assert::always;
 use polkadot_parachain::primitives::ValidationCodeHash;
 use std::{
@@ -101,8 +101,8 @@ pub enum ArtifactState {
 		/// This is updated when we get the heads up for this artifact or when we just discover
 		/// this file.
 		last_time_needed: SystemTime,
-		/// The CPU time that was taken preparing this artifact.
+		/// Stats produced by successful preparation.
-		cpu_time_elapsed: Duration,
+		prepare_stats: PrepareStats,
 	},
 	/// A task to prepare this artifact is scheduled.
 	Preparing {
@@ -177,12 +177,12 @@ impl Artifacts {
 		&mut self,
 		artifact_id: ArtifactId,
 		last_time_needed: SystemTime,
-		cpu_time_elapsed: Duration,
+		prepare_stats: PrepareStats,
 	) {
 		// See the precondition.
 		always!(self
 			.artifacts
-			.insert(artifact_id, ArtifactState::Prepared { last_time_needed, cpu_time_elapsed })
+			.insert(artifact_id, ArtifactState::Prepared { last_time_needed, prepare_stats })
 			.is_none());
 	}
 

polkadot/node/core/pvf/src/error.rs

@@ -14,12 +14,13 @@
 // You should have received a copy of the GNU General Public License
 // along with Polkadot.  If not, see <http://www.gnu.org/licenses/>.
 
+use crate::prepare::PrepareStats;
 use parity_scale_codec::{Decode, Encode};
-use std::{any::Any, fmt, time::Duration};
+use std::{any::Any, fmt};
 
-/// Result of PVF preparation performed by the validation host. Contains the elapsed CPU time if
+/// Result of PVF preparation performed by the validation host. Contains stats about the preparation if
 /// successful
-pub type PrepareResult = Result<Duration, PrepareError>;
+pub type PrepareResult = Result<PrepareStats, PrepareError>;
 
 /// An error that occurred during the prepare part of the PVF pipeline.
 #[derive(Debug, Clone, Encode, Decode)]

polkadot/node/core/pvf/src/host.rs

@@ -456,9 +456,9 @@ async fn handle_precheck_pvf(
 
 	if let Some(state) = artifacts.artifact_state_mut(&artifact_id) {
 		match state {
-			ArtifactState::Prepared { last_time_needed, cpu_time_elapsed } => {
+			ArtifactState::Prepared { last_time_needed, prepare_stats } => {
 				*last_time_needed = SystemTime::now();
-				let _ = result_sender.send(Ok(*cpu_time_elapsed));
+				let _ = result_sender.send(Ok(prepare_stats.clone()));
 			},
 			ArtifactState::Preparing { waiting_for_response, num_failures: _ } =>
 				waiting_for_response.push(result_sender),
@@ -725,8 +725,8 @@ async fn handle_prepare_done(
 	}
 
 	*state = match result {
-		Ok(cpu_time_elapsed) =>
+		Ok(prepare_stats) =>
-			ArtifactState::Prepared { last_time_needed: SystemTime::now(), cpu_time_elapsed },
+			ArtifactState::Prepared { last_time_needed: SystemTime::now(), prepare_stats },
 		Err(error) => {
 			let last_time_failed = SystemTime::now();
 			let num_failures = *num_failures + 1;
@@ -834,7 +834,7 @@ fn pulse_every(interval: std::time::Duration) -> impl futures::Stream<Item = ()>
 #[cfg(test)]
 mod tests {
 	use super::*;
-	use crate::{InvalidCandidate, PrepareError};
+	use crate::{prepare::PrepareStats, InvalidCandidate, PrepareError};
 	use assert_matches::assert_matches;
 	use futures::future::BoxFuture;
 
@@ -1056,8 +1056,12 @@ mod tests {
 		let mut builder = Builder::default();
 		builder.cleanup_pulse_interval = Duration::from_millis(100);
 		builder.artifact_ttl = Duration::from_millis(500);
-		builder.artifacts.insert_prepared(artifact_id(1), mock_now, Duration::default());
+		builder
-		builder.artifacts.insert_prepared(artifact_id(2), mock_now, Duration::default());
+			.artifacts
+			.insert_prepared(artifact_id(1), mock_now, PrepareStats::default());
+		builder
+			.artifacts
+			.insert_prepared(artifact_id(2), mock_now, PrepareStats::default());
 		let mut test = builder.build();
 		let mut host = test.host_handle();
 
@@ -1129,7 +1133,7 @@ mod tests {
 		test.from_prepare_queue_tx
 			.send(prepare::FromQueue {
 				artifact_id: artifact_id(1),
-				result: Ok(Duration::default()),
+				result: Ok(PrepareStats::default()),
 			})
 			.await
 			.unwrap();
@@ -1145,7 +1149,7 @@ mod tests {
 		test.from_prepare_queue_tx
 			.send(prepare::FromQueue {
 				artifact_id: artifact_id(2),
-				result: Ok(Duration::default()),
+				result: Ok(PrepareStats::default()),
 			})
 			.await
 			.unwrap();
@@ -1197,7 +1201,7 @@ mod tests {
 		test.from_prepare_queue_tx
 			.send(prepare::FromQueue {
 				artifact_id: artifact_id(1),
-				result: Ok(Duration::default()),
+				result: Ok(PrepareStats::default()),
 			})
 			.await
 			.unwrap();
@@ -1304,7 +1308,7 @@ mod tests {
 		test.from_prepare_queue_tx
 			.send(prepare::FromQueue {
 				artifact_id: artifact_id(2),
-				result: Ok(Duration::default()),
+				result: Ok(PrepareStats::default()),
 			})
 			.await
 			.unwrap();
@@ -1454,7 +1458,7 @@ mod tests {
 		test.from_prepare_queue_tx
 			.send(prepare::FromQueue {
 				artifact_id: artifact_id(1),
-				result: Ok(Duration::default()),
+				result: Ok(PrepareStats::default()),
 			})
 			.await
 			.unwrap();
@@ -1630,7 +1634,7 @@ mod tests {
 		test.from_prepare_queue_tx
 			.send(prepare::FromQueue {
 				artifact_id: artifact_id(1),
-				result: Ok(Duration::default()),
+				result: Ok(PrepareStats::default()),
 			})
 			.await
 			.unwrap();

polkadot/node/core/pvf/src/lib.rs

@@ -108,6 +108,7 @@ pub mod testing;
 pub use sp_tracing;
 
 pub use error::{InvalidCandidate, PrepareError, PrepareResult, ValidationError};
+pub use prepare::PrepareStats;
 pub use priority::Priority;
 pub use pvf::Pvf;
 

polkadot/node/core/pvf/src/metrics.rs

@@ -16,6 +16,7 @@
 
 //! Prometheus metrics related to the validation host.
 
+use crate::prepare::MemoryStats;
 use polkadot_node_metrics::metrics::{self, prometheus};
 
 /// Validation host metrics.
@@ -73,24 +74,24 @@ impl Metrics {
 		self.0.as_ref().map(|metrics| metrics.execution_time.start_timer())
 	}
 
-	/// Observe max_rss for preparation.
+	/// Observe memory stats for preparation.
-	pub(crate) fn observe_preparation_max_rss(&self, max_rss: f64) {
+	#[allow(unused_variables)]
+	pub(crate) fn observe_preparation_memory_metrics(&self, memory_stats: MemoryStats) {
 		if let Some(metrics) = &self.0 {
-			metrics.preparation_max_rss.observe(max_rss);
+			#[cfg(target_os = "linux")]
-		}
+			if let Some(max_rss) = memory_stats.max_rss {
-	}
+				metrics.preparation_max_rss.observe(max_rss as f64);
+			}
 
-	/// Observe max resident memory for preparation.
+			#[cfg(any(target_os = "linux", feature = "jemalloc-allocator"))]
-	pub(crate) fn observe_preparation_max_resident(&self, max_resident_kb: f64) {
+			if let Some(tracker_stats) = memory_stats.memory_tracker_stats {
-		if let Some(metrics) = &self.0 {
+				// We convert these stats from B to KB to match the unit of `ru_maxrss` from `getrusage`.
-			metrics.preparation_max_resident.observe(max_resident_kb);
+				let max_resident_kb = (tracker_stats.resident / 1024) as f64;
-		}
+				let max_allocated_kb = (tracker_stats.allocated / 1024) as f64;
-	}
 
-	/// Observe max allocated memory for preparation.
+				metrics.preparation_max_resident.observe(max_resident_kb);
-	pub(crate) fn observe_preparation_max_allocated(&self, max_allocated_kb: f64) {
+				metrics.preparation_max_allocated.observe(max_allocated_kb);
-		if let Some(metrics) = &self.0 {
+			}
-			metrics.preparation_max_allocated.observe(max_allocated_kb);
 		}
 	}
 }
@@ -106,8 +107,11 @@ struct MetricsInner {
 	execute_finished: prometheus::Counter<prometheus::U64>,
 	preparation_time: prometheus::Histogram,
 	execution_time: prometheus::Histogram,
+	#[cfg(target_os = "linux")]
 	preparation_max_rss: prometheus::Histogram,
+	#[cfg(any(target_os = "linux", feature = "jemalloc-allocator"))]
 	preparation_max_allocated: prometheus::Histogram,
+	#[cfg(any(target_os = "linux", feature = "jemalloc-allocator"))]
 	preparation_max_resident: prometheus::Histogram,
 }
 
@@ -226,6 +230,7 @@ impl metrics::Metrics for Metrics {
 				)?,
 				registry,
 			)?,
+			#[cfg(target_os = "linux")]
 			preparation_max_rss: prometheus::register(
 				prometheus::Histogram::with_opts(
 					prometheus::HistogramOpts::new(
@@ -238,6 +243,7 @@ impl metrics::Metrics for Metrics {
 				)?,
 				registry,
 			)?,
+			#[cfg(any(target_os = "linux", feature = "jemalloc-allocator"))]
 			preparation_max_resident: prometheus::register(
 				prometheus::Histogram::with_opts(
 					prometheus::HistogramOpts::new(
@@ -250,6 +256,7 @@ impl metrics::Metrics for Metrics {
 				)?,
 				registry,
 			)?,
+			#[cfg(any(target_os = "linux", feature = "jemalloc-allocator"))]
 			preparation_max_allocated: prometheus::register(
 				prometheus::Histogram::with_opts(
 					prometheus::HistogramOpts::new(

polkadot/node/core/pvf/src/prepare/memory_stats.rs

@@ -27,18 +27,18 @@
 //! <https://github.com/paritytech/polkadot/issues/6472#issuecomment-1381941762> for more
 //! background.
 
-use crate::{metrics::Metrics, LOG_TARGET};
 use parity_scale_codec::{Decode, Encode};
-use std::io;
 
 /// Helper struct to contain all the memory stats, including [`MemoryAllocationStats`] and, if
 /// supported by the OS, `ru_maxrss`.
-#[derive(Encode, Decode)]
+#[derive(Clone, Debug, Default, Encode, Decode)]
 pub struct MemoryStats {
 	/// Memory stats from `tikv_jemalloc_ctl`.
+	#[cfg(any(target_os = "linux", feature = "jemalloc-allocator"))]
 	pub memory_tracker_stats: Option<MemoryAllocationStats>,
 	/// `ru_maxrss` from `getrusage`. A string error since `io::Error` is not `Encode`able.
-	pub max_rss: Option<Result<i64, String>>,
+	#[cfg(target_os = "linux")]
+	pub max_rss: Option<i64>,
 }
 
 /// Statistics of collected memory metrics.
@@ -51,44 +51,14 @@ pub struct MemoryAllocationStats {
 	pub allocated: u64,
 }
 
-/// Gets the `ru_maxrss` for the current thread if the OS supports `getrusage`. Otherwise, just
+/// Module for the memory tracker. The memory tracker runs in its own thread, where it polls memory
-/// returns `None`.
+/// usage at an interval.
-pub fn get_max_rss_thread() -> Option<io::Result<i64>> {
+///
-	// `c_long` is either `i32` or `i64` depending on architecture. `i64::from` always works.
+/// NOTE: Requires jemalloc enabled.
-	#[cfg(target_os = "linux")]
-	let max_rss = Some(getrusage::getrusage_thread().map(|rusage| i64::from(rusage.ru_maxrss)));
-	#[cfg(not(target_os = "linux"))]
-	let max_rss = None;
-	max_rss
-}
-
-/// Helper function to send the memory metrics, if available, to prometheus.
-pub fn observe_memory_metrics(metrics: &Metrics, memory_stats: MemoryStats, pid: u32) {
-	if let Some(max_rss) = memory_stats.max_rss {
-		match max_rss {
-			Ok(max_rss) => metrics.observe_preparation_max_rss(max_rss as f64),
-			Err(err) => gum::warn!(
-				target: LOG_TARGET,
-				worker_pid = %pid,
-				"error getting `ru_maxrss` in preparation thread: {}",
-				err
-			),
-		}
-	}
-
-	if let Some(tracker_stats) = memory_stats.memory_tracker_stats {
-		// We convert these stats from B to KB to match the unit of `ru_maxrss` from `getrusage`.
-		let resident_kb = (tracker_stats.resident / 1024) as f64;
-		let allocated_kb = (tracker_stats.allocated / 1024) as f64;
-
-		metrics.observe_preparation_max_resident(resident_kb);
-		metrics.observe_preparation_max_allocated(allocated_kb);
-	}
-}
-
 #[cfg(any(target_os = "linux", feature = "jemalloc-allocator"))]
 pub mod memory_tracker {
 	use super::*;
+	use crate::LOG_TARGET;
 	use std::{
 		sync::mpsc::{Receiver, RecvTimeoutError, Sender},
 		time::Duration,
@@ -183,13 +153,15 @@ pub mod memory_tracker {
 	pub async fn get_memory_tracker_loop_stats(
 		fut: JoinHandle<Result<MemoryAllocationStats, String>>,
 		tx: Sender<()>,
+		worker_pid: u32,
 	) -> Option<MemoryAllocationStats> {
 		// Signal to the memory tracker thread to terminate.
 		if let Err(err) = tx.send(()) {
 			gum::warn!(
 				target: LOG_TARGET,
-				worker_pid = %std::process::id(),
+				%worker_pid,
-				"worker: error sending signal to memory tracker_thread: {}", err
+				"worker: error sending signal to memory tracker_thread: {}",
+				err
 			);
 			None
 		} else {
@@ -199,7 +171,7 @@ pub mod memory_tracker {
 				Ok(Err(err)) => {
 					gum::warn!(
 						target: LOG_TARGET,
-						worker_pid = %std::process::id(),
+						%worker_pid,
 						"worker: error occurred in the memory tracker thread: {}", err
 					);
 					None
@@ -207,7 +179,7 @@ pub mod memory_tracker {
 				Err(err) => {
 					gum::warn!(
 						target: LOG_TARGET,
-						worker_pid = %std::process::id(),
+						%worker_pid,
 						"worker: error joining on memory tracker thread: {}", err
 					);
 					None
@@ -217,13 +189,19 @@ pub mod memory_tracker {
 	}
 }
 
+/// Module for dealing with the `ru_maxrss` (peak resident memory) stat from `getrusage`.
+///
+/// NOTE: `getrusage` with the `RUSAGE_THREAD` parameter is only supported on Linux. `RUSAGE_SELF`
+/// works on MacOS, but we need to get the max rss only for the preparation thread. Gettng it for
+/// the current process would conflate the stats of previous jobs run by the process.
 #[cfg(target_os = "linux")]
-mod getrusage {
+pub mod max_rss_stat {
+	use crate::LOG_TARGET;
 	use libc::{getrusage, rusage, timeval, RUSAGE_THREAD};
 	use std::io;
 
 	/// Get the rusage stats for the current thread.
-	pub fn getrusage_thread() -> io::Result<rusage> {
+	fn getrusage_thread() -> io::Result<rusage> {
 		let mut result = rusage {
 			ru_utime: timeval { tv_sec: 0, tv_usec: 0 },
 			ru_stime: timeval { tv_sec: 0, tv_usec: 0 },
@@ -247,4 +225,25 @@ mod getrusage {
 		}
 		Ok(result)
 	}
+
+	/// Gets the `ru_maxrss` for the current thread.
+	pub fn get_max_rss_thread() -> io::Result<i64> {
+		// `c_long` is either `i32` or `i64` depending on architecture. `i64::from` always works.
+		getrusage_thread().map(|rusage| i64::from(rusage.ru_maxrss))
+	}
+
+	/// Extracts the max_rss stat and logs any error.
+	pub fn extract_max_rss_stat(max_rss: io::Result<i64>, worker_pid: u32) -> Option<i64> {
+		max_rss
+			.map_err(|err| {
+				gum::warn!(
+					target: LOG_TARGET,
+					%worker_pid,
+					"error getting `ru_maxrss` in preparation thread: {}",
+					err
+				);
+				err
+			})
+			.ok()
+	}
 }

polkadot/node/core/pvf/src/prepare/mod.rs

@@ -27,6 +27,16 @@ mod pool;
 mod queue;
 mod worker;
 
+pub use memory_stats::MemoryStats;
 pub use pool::start as start_pool;
 pub use queue::{start as start_queue, FromQueue, ToQueue};
 pub use worker::worker_entrypoint;
+
+use parity_scale_codec::{Decode, Encode};
+
+/// Preparation statistics, including the CPU time and memory taken.
+#[derive(Debug, Clone, Default, Encode, Decode)]
+pub struct PrepareStats {
+	cpu_time_elapsed: std::time::Duration,
+	memory_stats: MemoryStats,
+}

polkadot/node/core/pvf/src/prepare/queue.rs

@@ -495,6 +495,7 @@ mod tests {
 	use crate::{
 		error::PrepareError,
 		host::{LENIENT_PREPARATION_TIMEOUT, PRECHECK_PREPARATION_TIMEOUT},
+		prepare::PrepareStats,
 	};
 	use assert_matches::assert_matches;
 	use futures::{future::BoxFuture, FutureExt};
@@ -622,7 +623,7 @@ mod tests {
 		test.send_from_pool(pool::FromPool::Concluded {
 			worker: w,
 			rip: false,
-			result: Ok(Duration::default()),
+			result: Ok(PrepareStats::default()),
 		});
 
 		assert_eq!(test.poll_and_recv_from_queue().await.artifact_id, pvf(1).as_artifact_id());
@@ -660,7 +661,7 @@ mod tests {
 		test.send_from_pool(pool::FromPool::Concluded {
 			worker: w1,
 			rip: false,
-			result: Ok(Duration::default()),
+			result: Ok(PrepareStats::default()),
 		});
 
 		assert_matches!(test.poll_and_recv_to_pool().await, pool::ToPool::StartWork { .. });
@@ -710,7 +711,7 @@ mod tests {
 		test.send_from_pool(pool::FromPool::Concluded {
 			worker: w1,
 			rip: false,
-			result: Ok(Duration::default()),
+			result: Ok(PrepareStats::default()),
 		});
 		assert_eq!(test.poll_and_recv_to_pool().await, pool::ToPool::Kill(w1));
 	}
@@ -746,7 +747,7 @@ mod tests {
 		test.send_from_pool(pool::FromPool::Concluded {
 			worker: w1,
 			rip: true,
-			result: Ok(Duration::default()),
+			result: Ok(PrepareStats::default()),
 		});
 
 		// Since there is still work, the queue requested one extra worker to spawn to handle the

polkadot/node/core/pvf/src/prepare/worker.rs

@@ -14,13 +14,16 @@
 // You should have received a copy of the GNU General Public License
 // along with Polkadot.  If not, see <http://www.gnu.org/licenses/>.
 
+#[cfg(target_os = "linux")]
+use super::memory_stats::max_rss_stat::{extract_max_rss_stat, get_max_rss_thread};
 #[cfg(any(target_os = "linux", feature = "jemalloc-allocator"))]
 use super::memory_stats::memory_tracker::{get_memory_tracker_loop_stats, memory_tracker_loop};
-use super::memory_stats::{get_max_rss_thread, observe_memory_metrics, MemoryStats};
+use super::memory_stats::MemoryStats;
 use crate::{
 	artifacts::CompiledArtifact,
 	error::{PrepareError, PrepareResult},
 	metrics::Metrics,
+	prepare::PrepareStats,
 	worker_common::{
 		bytes_to_path, cpu_time_monitor_loop, framed_recv, framed_send, path_to_bytes,
 		spawn_with_program_path, tmpfile_in, worker_event_loop, IdleWorker, SpawnErr, WorkerHandle,
@@ -118,12 +121,11 @@ pub async fn start_work(
 
 		match result {
 			// Received bytes from worker within the time limit.
-			Ok(Ok((prepare_result, memory_stats))) =>
+			Ok(Ok(prepare_result)) =>
 				handle_response(
 					metrics,
 					IdleWorker { stream, pid },
 					prepare_result,
-					memory_stats,
 					pid,
 					tmp_file,
 					artifact_path,
@@ -162,13 +164,12 @@ async fn handle_response(
 	metrics: &Metrics,
 	worker: IdleWorker,
 	result: PrepareResult,
-	memory_stats: Option<MemoryStats>,
+	worker_pid: u32,
-	pid: u32,
 	tmp_file: PathBuf,
 	artifact_path: PathBuf,
 	preparation_timeout: Duration,
 ) -> Outcome {
-	let cpu_time_elapsed = match result {
+	let PrepareStats { cpu_time_elapsed, memory_stats } = match result.clone() {
 		Ok(result) => result,
 		// Timed out on the child. This should already be logged by the child.
 		Err(PrepareError::TimedOut) => return Outcome::TimedOut,
@@ -179,7 +180,7 @@ async fn handle_response(
 		// The job didn't complete within the timeout.
 		gum::warn!(
 			target: LOG_TARGET,
-			worker_pid = %pid,
+			%worker_pid,
 			"prepare job took {}ms cpu time, exceeded preparation timeout {}ms. Clearing WIP artifact {}",
 			cpu_time_elapsed.as_millis(),
 			preparation_timeout.as_millis(),
@@ -190,7 +191,7 @@ async fn handle_response(
 
 	gum::debug!(
 		target: LOG_TARGET,
-		worker_pid = %pid,
+		%worker_pid,
 		"promoting WIP artifact {} to {}",
 		tmp_file.display(),
 		artifact_path.display(),
@@ -201,7 +202,7 @@ async fn handle_response(
 		Err(err) => {
 			gum::warn!(
 				target: LOG_TARGET,
-				worker_pid = %pid,
+				%worker_pid,
 				"failed to rename the artifact from {} to {}: {:?}",
 				tmp_file.display(),
 				artifact_path.display(),
@@ -213,9 +214,7 @@ async fn handle_response(
 
 	// If there were no errors up until now, log the memory stats for a successful preparation, if
 	// available.
-	if let Some(memory_stats) = memory_stats {
+	metrics.observe_preparation_memory_metrics(memory_stats);
-		observe_memory_metrics(metrics, memory_stats, pid);
-	}
 
 	outcome
 }
@@ -299,19 +298,11 @@ async fn recv_request(stream: &mut UnixStream) -> io::Result<(Vec<u8>, PathBuf,
 	Ok((code, tmp_file, preparation_timeout))
 }
 
-async fn send_response(
+async fn send_response(stream: &mut UnixStream, result: PrepareResult) -> io::Result<()> {
-	stream: &mut UnixStream,
+	framed_send(stream, &result.encode()).await
-	result: PrepareResult,
-	memory_stats: Option<MemoryStats>,
-) -> io::Result<()> {
-	framed_send(stream, &result.encode()).await?;
-	framed_send(stream, &memory_stats.encode()).await
 }
 
-async fn recv_response(
+async fn recv_response(stream: &mut UnixStream, pid: u32) -> io::Result<PrepareResult> {
-	stream: &mut UnixStream,
-	pid: u32,
-) -> io::Result<(PrepareResult, Option<MemoryStats>)> {
 	let result = framed_recv(stream).await?;
 	let result = PrepareResult::decode(&mut &result[..]).map_err(|e| {
 		// We received invalid bytes from the worker.
@@ -327,14 +318,7 @@ async fn recv_response(
 			format!("prepare pvf recv_response: failed to decode result: {:?}", e),
 		)
 	})?;
-	let memory_stats = framed_recv(stream).await?;
+	Ok(result)
-	let memory_stats = Option::<MemoryStats>::decode(&mut &memory_stats[..]).map_err(|e| {
-		io::Error::new(
-			io::ErrorKind::Other,
-			format!("prepare pvf recv_response: failed to decode memory stats: {:?}", e),
-		)
-	})?;
-	Ok((result, memory_stats))
 }
 
 /// The entrypoint that the spawned prepare worker should start with. The `socket_path` specifies
@@ -362,10 +346,11 @@ async fn recv_response(
 pub fn worker_entrypoint(socket_path: &str) {
 	worker_event_loop("prepare", socket_path, |rt_handle, mut stream| async move {
 		loop {
+			let worker_pid = std::process::id();
 			let (code, dest, preparation_timeout) = recv_request(&mut stream).await?;
 			gum::debug!(
 				target: LOG_TARGET,
-				worker_pid = %std::process::id(),
+				%worker_pid,
 				"worker: preparing artifact",
 			);
 
@@ -387,28 +372,29 @@ pub fn worker_entrypoint(socket_path: &str) {
 			// Spawn another thread for preparation.
 			let prepare_fut = rt_handle
 				.spawn_blocking(move || {
-					let prepare_result = prepare_artifact(&code);
+					let result = prepare_artifact(&code);
 
 					// Get the `ru_maxrss` stat. If supported, call getrusage for the thread.
-					let max_rss = get_max_rss_thread();
+					#[cfg(target_os = "linux")]
+					let result = result.map(|artifact| (artifact, get_max_rss_thread()));
 
-					(prepare_result, max_rss)
+					result
 				})
 				.fuse();
 
 			pin_mut!(cpu_time_monitor_fut);
 			pin_mut!(prepare_fut);
 
-			let (result, memory_stats) = select_biased! {
+			let result = select_biased! {
 				// If this future is not selected, the join handle is dropped and the thread will
 				// finish in the background.
 				join_res = cpu_time_monitor_fut => {
-					let result = match join_res {
+					match join_res {
 						Ok(Some(cpu_time_elapsed)) => {
 							// Log if we exceed the timeout and the other thread hasn't finished.
 							gum::warn!(
 								target: LOG_TARGET,
-								worker_pid = %std::process::id(),
+								%worker_pid,
 								"prepare job took {}ms cpu time, exceeded prepare timeout {}ms",
 								cpu_time_elapsed.as_millis(),
 								preparation_timeout.as_millis(),
@@ -417,28 +403,29 @@ pub fn worker_entrypoint(socket_path: &str) {
 						},
 						Ok(None) => Err(PrepareError::IoErr("error communicating over finished channel".into())),
 						Err(err) => Err(PrepareError::IoErr(err.to_string())),
-					};
+					}
-					(result, None)
 				},
-				compilation_res = prepare_fut => {
+				prepare_res = prepare_fut => {
 					let cpu_time_elapsed = cpu_time_start.elapsed();
 					let _ = cpu_time_monitor_tx.send(());
 
-					match compilation_res.unwrap_or_else(|err| (Err(PrepareError::IoErr(err.to_string())), None)) {
+					match prepare_res.unwrap_or_else(|err| Err(PrepareError::IoErr(err.to_string()))) {
-						(Err(err), _) => {
+						Err(err) => {
 							// Serialized error will be written into the socket.
-							(Err(err), None)
+							Err(err)
 						},
-						(Ok(compiled_artifact), max_rss) => {
+						Ok(ok) => {
 							// Stop the memory stats worker and get its observed memory stats.
 							#[cfg(any(target_os = "linux", feature = "jemalloc-allocator"))]
 							let memory_tracker_stats =
-								get_memory_tracker_loop_stats(memory_tracker_fut, memory_tracker_tx).await;
+								get_memory_tracker_loop_stats(memory_tracker_fut, memory_tracker_tx, worker_pid).await;
-							#[cfg(not(any(target_os = "linux", feature = "jemalloc-allocator")))]
+							#[cfg(target_os = "linux")]
-							let memory_tracker_stats = None;
+							let (ok, max_rss) = ok;
 							let memory_stats = MemoryStats {
+								#[cfg(any(target_os = "linux", feature = "jemalloc-allocator"))]
 								memory_tracker_stats,
-								max_rss: max_rss.map(|inner| inner.map_err(|e| e.to_string())),
+								#[cfg(target_os = "linux")]
+								max_rss: extract_max_rss_stat(max_rss, worker_pid),
 							};
 
 							// Write the serialized artifact into a temp file.
@@ -450,19 +437,19 @@ pub fn worker_entrypoint(socket_path: &str) {
 
 							gum::debug!(
 								target: LOG_TARGET,
-								worker_pid = %std::process::id(),
+								%worker_pid,
 								"worker: writing artifact to {}",
 								dest.display(),
 							);
-							tokio::fs::write(&dest, &compiled_artifact).await?;
+							tokio::fs::write(&dest, &ok).await?;
 
-							(Ok(cpu_time_elapsed), Some(memory_stats))
+							Ok(PrepareStats{cpu_time_elapsed, memory_stats})
 						},
 					}
 				},
 			};
 
-			send_response(&mut stream, result, memory_stats).await?;
+			send_response(&mut stream, result).await?;
 		}
 	});
 }