Fix remaining flaky CodSpeed microbenchmarks

encodings/runend/Cargo.toml

@@ -30,6 +30,7 @@ workspace = true
 [dev-dependencies]
 divan = { workspace = true }
 itertools = { workspace = true }
+mimalloc = { workspace = true }
 rand = { workspace = true }
 rstest = { workspace = true }
 vortex-array = { workspace = true, features = ["_test-harness"] }

encodings/runend/benches/run_end_compress.rs

@@ -7,6 +7,7 @@ use std::sync::LazyLock;
 
 use divan::Bencher;
 use itertools::repeat_n;
+use mimalloc::MiMalloc;
 use vortex_array::IntoArray;
 use vortex_array::RecursiveCanonical;
 use vortex_array::VortexSessionExecute;
@@ -19,6 +20,12 @@ use vortex_runend::RunEnd;
 use vortex_runend::compress::runend_encode;
 use vortex_session::VortexSession;
 
+// `runend_encode` allocates its output buffers inside the timed region, so route allocation
+// through vendored mimalloc to keep glibc malloc (which varies across runner images) out of
+// the measured trace.
+#[global_allocator]
+static GLOBAL: MiMalloc = MiMalloc;
+
 fn main() {
     divan::main();
 }

vortex-array/benches/cast_decimal.rs

@@ -14,6 +14,7 @@
 use std::sync::LazyLock;
 
 use divan::Bencher;
+use mimalloc::MiMalloc;
 use rand::prelude::*;
 use vortex_array::ArrayRef;
 use vortex_array::Canonical;
@@ -28,6 +29,12 @@ use vortex_array::validity::Validity;
 use vortex_buffer::BufferMut;
 use vortex_session::VortexSession;
 
+// The copy casts allocate their output buffer inside the timed region, so route allocation
+// through vendored mimalloc to keep glibc malloc (which varies across runner images) out of
+// the measured trace.
+#[global_allocator]
+static GLOBAL: MiMalloc = MiMalloc;
+
 fn main() {
     divan::main();
 }

vortex-buffer/benches/vortex_bitbuffer.rs

@@ -176,7 +176,12 @@ fn slice_arrow_buffer(bencher: Bencher, length: usize) {
         });
 }
 
-#[divan::bench(args = INPUT_SIZE)]
+/// A 128-bit `true_count` is a popcount over two `u64` words, so the measurement is fixed
+/// dispatch and harness overhead plus binary code layout rather than the count itself. Only
+/// sizes where the popcount loop dominates are worth measuring.
+const TRUE_COUNT_INPUT_SIZE: &[usize] = &[1024, 2048, 16_384, 65_536];
+
+#[divan::bench(args = TRUE_COUNT_INPUT_SIZE)]
 fn true_count_vortex_buffer(bencher: Bencher, length: usize) {
     let buffer = BitBuffer::from_iter((0..length).map(true_count_pattern));
 
@@ -185,7 +190,7 @@ fn true_count_vortex_buffer(bencher: Bencher, length: usize) {
         .bench_refs(|buffer| buffer.true_count())
 }
 
-#[divan::bench(args = INPUT_SIZE)]
+#[divan::bench(args = TRUE_COUNT_INPUT_SIZE)]
 fn true_count_arrow_buffer(bencher: Bencher, length: usize) {
     let buffer = Arrow(BooleanBuffer::from_iter(
         (0..length).map(true_count_pattern),