Round minimum queue capacity up to a power of two (#56)

Author: apfitzge

src/lib.rs

@@ -12,6 +12,14 @@ pub(crate) const VERSION_MAJOR: u16 = 2;
 pub(crate) const VERSION_PATCH: u16 = 0;
 pub(crate) const VERSION: u32 = (VERSION_MAJOR as u32) << 16 | VERSION_PATCH as u32;
 
+pub(crate) const fn normalized_capacity(capacity: usize) -> usize {
+    if capacity == 0 {
+        0
+    } else {
+        capacity.next_power_of_two()
+    }
+}
+
 /// `AtomicUsize` with 64-byte alignment for better performance.
 #[derive(Default)]
 #[repr(C, align(64))]

src/mpmc.rs

@@ -1,6 +1,8 @@
 //! DPDK-style bounded MPMC ring queue
 
-use crate::{error::Error, shmem::MappedRegion, CacheAlignedAtomicSize, VERSION};
+use crate::{
+    error::Error, normalized_capacity, shmem::MappedRegion, CacheAlignedAtomicSize, VERSION,
+};
 use core::{marker::PhantomData, ptr::NonNull, sync::atomic::Ordering};
 use std::{
     fs::File,
@@ -313,7 +315,7 @@ unsafe impl<T> Sync for Consumer<T> {}
 /// page-size requirements.
 pub const fn minimum_file_size<T: Sized>(capacity: usize) -> usize {
     let buffer_offset = SharedQueueHeader::buffer_offset::<T>();
-    buffer_offset + capacity * core::mem::size_of::<T>()
+    buffer_offset + normalized_capacity(capacity) * core::mem::size_of::<T>()
 }
 
 struct SharedQueue<T> {
@@ -965,6 +967,17 @@ mod tests {
         assert_eq!(values, vec![100, 101]);
     }
 
+    #[test]
+    fn test_minimum_file_size_rounds_up_capacity() {
+        let file = create_temp_shmem_file().unwrap();
+        let producer = unsafe { Producer::<u64>::create(&file, minimum_file_size::<u64>(3)) }
+            .expect("create failed");
+        let consumer = unsafe { Consumer::<u64>::join(&file) }.expect("join failed");
+
+        assert_eq!(producer.queue.capacity(), 4);
+        assert_eq!(consumer.queue.capacity(), 4);
+    }
+
     #[test]
     fn test_multiple_producers_consumers() {
         let (file, producer, consumer) = create_test_queue::<Item>(BUFFER_SIZE);

src/spsc.rs

@@ -1,4 +1,6 @@
-use crate::{error::Error, shmem::MappedRegion, CacheAlignedAtomicSize, VERSION};
+use crate::{
+    error::Error, normalized_capacity, shmem::MappedRegion, CacheAlignedAtomicSize, VERSION,
+};
 use core::ptr::NonNull;
 use std::{
     fs::File,
@@ -15,7 +17,7 @@ const MAGIC: u64 = 0x7368_6171_7370_7363; // b"shaqspsc"
 /// page-size requirements.
 pub const fn minimum_file_size<T: Sized>(capacity: usize) -> usize {
     let buffer_offset = SharedQueueHeader::buffer_offset::<T>();
-    buffer_offset + capacity * core::mem::size_of::<T>()
+    buffer_offset + normalized_capacity(capacity) * core::mem::size_of::<T>()
 }
 
 /// Producer side of the SPSC shared queue.
@@ -612,4 +614,13 @@ mod tests {
         assert_eq!(*val, 7);
         consumer.finalize();
     }
+
+    #[test]
+    fn test_minimum_file_size_rounds_up_capacity() {
+        let file = create_temp_shmem_file().unwrap();
+        let producer = unsafe { Producer::<u64>::create(&file, minimum_file_size::<u64>(3)) }
+            .expect("create failed");
+
+        assert_eq!(producer.capacity(), 4);
+    }
 }