pagebench: add CPU profiling support (#12478)

## Problem

The new communicator gRPC client has significantly worse Pagebench
performance than a basic gRPC client. We need to find out why.

## Summary of changes

Add a `pagebench --profile` flag which takes a client CPU profile of the
benchmark and writes a flamegraph to `profile.svg`.

Cargo.lock

@@ -4296,6 +4296,7 @@ dependencies = [
  "pageserver_client",
  "pageserver_client_grpc",
  "pageserver_page_api",
+ "pprof",
  "rand 0.8.5",
  "reqwest",
  "serde",

pageserver/pagebench/Cargo.toml

@@ -16,6 +16,7 @@ futures.workspace = true
 hdrhistogram.workspace = true
 humantime.workspace = true
 humantime-serde.workspace = true
+pprof.workspace = true
 rand.workspace = true
 reqwest.workspace = true
 serde.workspace = true

pageserver/pagebench/src/main.rs

@@ -1,4 +1,7 @@
+use std::fs::File;
+
 use clap::Parser;
+use tracing::info;
 use utils::logging;
 
 /// Re-usable pieces of code that aren't CLI-specific.
@@ -24,7 +27,18 @@ mod cmd {
 
 /// Component-level performance test for pageserver.
 #[derive(clap::Parser)]
-enum Args {
+struct Args {
+    /// Takes a client CPU profile into profile.svg. The benchmark must exit cleanly before it's
+    /// written, e.g. via --runtime.
+    #[arg(long)]
+    profile: bool,
+
+    #[command(subcommand)]
+    subcommand: Subcommand,
+}
+
+#[derive(clap::Subcommand)]
+enum Subcommand {
     Basebackup(cmd::basebackup::Args),
     GetPageLatestLsn(cmd::getpage_latest_lsn::Args),
     TriggerInitialSizeCalculation(cmd::trigger_initial_size_calculation::Args),
@@ -33,25 +47,46 @@ enum Args {
     IdleStreams(cmd::idle_streams::Args),
 }
 
-fn main() {
+fn main() -> anyhow::Result<()> {
     logging::init(
         logging::LogFormat::Plain,
         logging::TracingErrorLayerEnablement::Disabled,
         logging::Output::Stderr,
-    )
-    .unwrap();
+    )?;
     logging::replace_panic_hook_with_tracing_panic_hook().forget();
 
     let args = Args::parse();
-    match args {
-        Args::Basebackup(args) => cmd::basebackup::main(args),
-        Args::GetPageLatestLsn(args) => cmd::getpage_latest_lsn::main(args),
-        Args::TriggerInitialSizeCalculation(args) => {
+
+    // Start a CPU profile if requested.
+    let mut profiler = None;
+    if args.profile {
+        profiler = Some(
+            pprof::ProfilerGuardBuilder::default()
+                .frequency(1000)
+                .blocklist(&["libc", "libgcc", "pthread", "vdso"])
+                .build()?,
+        );
+    }
+
+    match args.subcommand {
+        Subcommand::Basebackup(args) => cmd::basebackup::main(args),
+        Subcommand::GetPageLatestLsn(args) => cmd::getpage_latest_lsn::main(args),
+        Subcommand::TriggerInitialSizeCalculation(args) => {
             cmd::trigger_initial_size_calculation::main(args)
         }
-        Args::OndemandDownloadChurn(args) => cmd::ondemand_download_churn::main(args),
-        Args::AuxFiles(args) => cmd::aux_files::main(args),
-        Args::IdleStreams(args) => cmd::idle_streams::main(args),
+        Subcommand::OndemandDownloadChurn(args) => cmd::ondemand_download_churn::main(args),
+        Subcommand::AuxFiles(args) => cmd::aux_files::main(args),
+        Subcommand::IdleStreams(args) => cmd::idle_streams::main(args),
+    }?;
+
+    // Generate a CPU flamegraph if requested.
+    if let Some(profiler) = profiler {
+        let report = profiler.report().build()?;
+        drop(profiler); // stop profiling
+        let file = File::create("profile.svg")?;
+        report.flamegraph(file)?;
+        info!("wrote CPU profile flamegraph to profile.svg")
     }
-    .unwrap()
+
+    Ok(())
 }