pageserver,safekeeper: increase heap profiling frequency to 2 MB (#10362)

## Problem

Currently, the heap profiling frequency is every 1 MB allocated. Taking
a profile stack trace takes about 1 µs, and allocating 1 MB takes about
15 µs, so the overhead is about 6.7% which is a bit high. This is a
fixed cost regardless of whether heap profiles are actually accessed.

## Summary of changes

Increase the heap profiling sample frequency from 1 MB to 2 MB, which
reduces the overhead to about 3.3%. This seems acceptable, considering
performance-sensitive code will avoid allocations as far as possible
anyway.

pageserver/src/bin/pageserver.rs

@@ -53,10 +53,12 @@ project_build_tag!(BUILD_TAG);
 #[global_allocator]
 static GLOBAL: tikv_jemallocator::Jemalloc = tikv_jemallocator::Jemalloc;
 
-/// Configure jemalloc to sample allocations for profiles every 1 MB (1 << 20).
+/// Configure jemalloc to profile heap allocations by sampling stack traces every 2 MB (1 << 21).
+/// This adds roughly 3% overhead for allocations on average, which is acceptable considering
+/// performance-sensitive code will avoid allocations as far as possible anyway.
 #[allow(non_upper_case_globals)]
 #[export_name = "malloc_conf"]
-pub static malloc_conf: &[u8] = b"prof:true,prof_active:true,lg_prof_sample:20\0";
+pub static malloc_conf: &[u8] = b"prof:true,prof_active:true,lg_prof_sample:21\0";
 
 const PID_FILE_NAME: &str = "pageserver.pid";
 

safekeeper/benches/receive_wal.rs

@@ -21,14 +21,13 @@ const KB: usize = 1024;
 const MB: usize = 1024 * KB;
 const GB: usize = 1024 * MB;
 
-/// Use jemalloc, and configure it to sample allocations for profiles every 1 MB.
+/// Use jemalloc and enable profiling, to mirror bin/safekeeper.rs.
-/// This mirrors the configuration in bin/safekeeper.rs.
 #[global_allocator]
 static GLOBAL: tikv_jemallocator::Jemalloc = tikv_jemallocator::Jemalloc;
 
 #[allow(non_upper_case_globals)]
 #[export_name = "malloc_conf"]
-pub static malloc_conf: &[u8] = b"prof:true,prof_active:true,lg_prof_sample:20\0";
+pub static malloc_conf: &[u8] = b"prof:true,prof_active:true,lg_prof_sample:21\0";
 
 // Register benchmarks with Criterion.
 criterion_group!(

safekeeper/src/bin/safekeeper.rs

@@ -51,10 +51,12 @@ use utils::{
 #[global_allocator]
 static GLOBAL: tikv_jemallocator::Jemalloc = tikv_jemallocator::Jemalloc;
 
-/// Configure jemalloc to sample allocations for profiles every 1 MB (1 << 20).
+/// Configure jemalloc to profile heap allocations by sampling stack traces every 2 MB (1 << 21).
+/// This adds roughly 3% overhead for allocations on average, which is acceptable considering
+/// performance-sensitive code will avoid allocations as far as possible anyway.
 #[allow(non_upper_case_globals)]
 #[export_name = "malloc_conf"]
-pub static malloc_conf: &[u8] = b"prof:true,prof_active:true,lg_prof_sample:20\0";
+pub static malloc_conf: &[u8] = b"prof:true,prof_active:true,lg_prof_sample:21\0";
 
 const PID_FILE_NAME: &str = "safekeeper.pid";
 const ID_FILE_NAME: &str = "safekeeper.id";