pool size guess

Cargo.lock

@@ -21,6 +21,8 @@ version = "0.1.0"
 dependencies = [
  "memmap",
  "num_cpus",
+ "rustc_version",
+ "smallvec",
 ]
 
 [[package]]
@@ -43,6 +45,36 @@ dependencies = [
  "libc",
 ]
 
+[[package]]
+name = "rustc_version"
+version = "0.2.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "138e3e0acb6c9fb258b19b67cb8abd63c00679d2851805ea151465464fe9030a"
+dependencies = [
+ "semver",
+]
+
+[[package]]
+name = "semver"
+version = "0.9.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "1d7eb9ef2c18661902cc47e535f9bc51b78acd254da71d375c2f6720d9a40403"
+dependencies = [
+ "semver-parser",
+]
+
+[[package]]
+name = "semver-parser"
+version = "0.7.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "388a1df253eca08550bef6c72392cfe7c30914bf41df5269b68cbd6ff8f570a3"
+
+[[package]]
+name = "smallvec"
+version = "1.5.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "ae524f056d7d770e174287294f562e95044c68e88dec909a00d2094805db9d75"
+
 [[package]]
 name = "winapi"
 version = "0.3.9"

Cargo.toml

@@ -10,3 +10,8 @@ edition = "2018"
 [dependencies]
 memmap = "0.7.0"
 num_cpus = "1.13.0"
+smallvec = "1.5.1"
+
+[build-dependencies]
+rustc_version = "0.2"
+num_cpus = "1.13.0"

build.rs

@@ -0,0 +1,46 @@
+
+extern crate rustc_version;
+extern crate num_cpus;
+
+use rustc_version::{version, version_meta, Channel};
+use std::io::{self, Write};
+use std::fs::OpenOptions;
+
+pub const DEFAULT_CPU_FILE: &str = "./src/consts.rs";
+
+fn write_numcpus() -> io::Result<()>
+{
+    let mut file = OpenOptions::new()
+	.write(true)
+	.create(true)
+	.truncate(true)
+	.open(DEFAULT_CPU_FILE)?;
+
+    writeln!(&mut file, "pub const CPUS: usize = {};", num_cpus::get())?;
+    writeln!(&mut file, "pub type PoolContainer<T> = [T; CPUS+1];")?;
+    file.flush()?;
+    Ok(())
+}
+
+fn main() {
+    // Assert we haven't travelled back in time
+    assert!(version().unwrap().major >= 1);
+
+    // Set cfg flags depending on release channel
+    match version_meta().unwrap().channel {
+        Channel::Stable => {
+            println!("cargo:rustc-cfg=stable");
+        }
+        Channel::Beta => {
+            println!("cargo:rustc-cfg=beta");
+        }
+        Channel::Nightly => {
+            println!("cargo:rustc-cfg=nightly");
+        }
+        Channel::Dev => {
+            println!("cargo:rustc-cfg=dev");
+        }
+    }
+
+    write_numcpus().expect("Failed to write comptime cpu info to source")
+}

src/consts.rs

@@ -0,0 +1,2 @@
+pub const CPUS: usize = 8;
+pub type PoolContainer<T> = [T; CPUS+1];

src/main.rs

@@ -6,6 +6,8 @@ mod map;
 mod job;
 mod work;
 
+mod consts;
+
 fn main() {
     println!("Hello, world!");
 }

src/pool.rs

@@ -66,9 +66,15 @@ pub fn join_all<T>(i: impl IntoIterator<Item = JoinHandle<T>>)
     }
 }
 
+/// Vector type used for thread pool waiting.
+///
+/// This container is compiled will an estimate of enough stack storage for a reasonably sized thread pool.
+/// The number of processors of the CPU this binary was built on is used as this estimate, and will only heap allocate when spawning more threads that number.
+pub type PoolContainer = smallvec::SmallVec<[JoinHandle<()>; crate::consts::CPUS]>;
+
 /// A pool container that ensures worker threads are waited on before termination.
 #[derive(Debug)]
-pub struct Pool(Vec<JoinHandle<()>>);
+pub struct Pool(PoolContainer);
 
 impl Pool
 {
@@ -81,7 +87,7 @@ impl Pool
     /// Detach the pool but do not join its worker threads.
     #[inline] pub fn detach(mut self)
     {
-	self.0 = Vec::default();
+	self.0 = Default::default();
     }
 
     /// The number of threads in the pool
@@ -102,11 +108,11 @@ impl FromIterator<JoinHandle<()>> for Pool
 impl IntoIterator for Pool
 {
     type Item= JoinHandle<()>;
-    type IntoIter = std::vec::IntoIter<JoinHandle<()>>;
+    type IntoIter = smallvec::IntoIter<[JoinHandle<()>; crate::consts::CPUS]>;
 
     fn into_iter(mut self) -> Self::IntoIter
     {
-	std::mem::replace(&mut self.0, Vec::default()).into_iter()
+	std::mem::replace(&mut self.0, Default::default()).into_iter()
     }
 }
 
@@ -115,7 +121,7 @@ impl Drop for Pool
     fn drop(&mut self)
     {
 	if self.0.len() > 0 {
-	    join_all(std::mem::replace(&mut self.0, Vec::default()));
+	    join_all(std::mem::replace(&mut self.0, Default::default()));
 	}
     }
 }