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|
use crate::page::{
slot::{Generation, RefCount},
Addr,
};
use crate::Pack;
use std::{fmt, marker::PhantomData};
/// Configuration parameters which can be overridden to tune the behavior of a slab.
pub trait Config: Sized {
/// The maximum number of threads which can access the slab.
///
/// This value (rounded to a power of two) determines the number of shards
/// in the slab. If a thread is created, accesses the slab, and then terminates,
/// its shard may be reused and thus does not count against the maximum
/// number of threads once the thread has terminated.
const MAX_THREADS: usize = DefaultConfig::MAX_THREADS;
/// The maximum number of pages in each shard in the slab.
///
/// This value, in combination with `INITIAL_PAGE_SIZE`, determines how many
/// bits of each index are used to represent page addresses.
const MAX_PAGES: usize = DefaultConfig::MAX_PAGES;
/// The size of the first page in each shard.
///
/// When a page in a shard has been filled with values, a new page
/// will be allocated that is twice as large as the previous page. Thus, the
/// second page will be twice this size, and the third will be four times
/// this size, and so on.
///
/// Note that page sizes must be powers of two. If this value is not a power
/// of two, it will be rounded to the next power of two.
const INITIAL_PAGE_SIZE: usize = DefaultConfig::INITIAL_PAGE_SIZE;
/// Sets a number of high-order bits in each index which are reserved from
/// user code.
///
/// Note that these bits are taken from the generation counter; if the page
/// address and thread IDs are configured to use a large number of bits,
/// reserving additional bits will decrease the period of the generation
/// counter. These should thus be used relatively sparingly, to ensure that
/// generation counters are able to effectively prevent the ABA problem.
const RESERVED_BITS: usize = 0;
}
pub(crate) trait CfgPrivate: Config {
const USED_BITS: usize = Generation::<Self>::LEN + Generation::<Self>::SHIFT;
const INITIAL_SZ: usize = next_pow2(Self::INITIAL_PAGE_SIZE);
const MAX_SHARDS: usize = next_pow2(Self::MAX_THREADS - 1);
const ADDR_INDEX_SHIFT: usize = Self::INITIAL_SZ.trailing_zeros() as usize + 1;
fn page_size(n: usize) -> usize {
Self::INITIAL_SZ * 2usize.pow(n as _)
}
fn debug() -> DebugConfig<Self> {
DebugConfig { _cfg: PhantomData }
}
fn validate() {
assert!(
Self::INITIAL_SZ.is_power_of_two(),
"invalid Config: {:#?}",
Self::debug(),
);
assert!(
Self::INITIAL_SZ <= Addr::<Self>::BITS,
"invalid Config: {:#?}",
Self::debug()
);
assert!(
Generation::<Self>::BITS >= 3,
"invalid Config: {:#?}\ngeneration counter should be at least 3 bits!",
Self::debug()
);
assert!(
Self::USED_BITS <= WIDTH,
"invalid Config: {:#?}\ntotal number of bits per index is too large to fit in a word!",
Self::debug()
);
assert!(
WIDTH - Self::USED_BITS >= Self::RESERVED_BITS,
"invalid Config: {:#?}\nindices are too large to fit reserved bits!",
Self::debug()
);
assert!(
RefCount::<Self>::MAX > 1,
"invalid config: {:#?}\n maximum concurrent references would be {}",
Self::debug(),
RefCount::<Self>::MAX,
);
}
#[inline(always)]
fn unpack<A: Pack<Self>>(packed: usize) -> A {
A::from_packed(packed)
}
#[inline(always)]
fn unpack_addr(packed: usize) -> Addr<Self> {
Self::unpack(packed)
}
#[inline(always)]
fn unpack_tid(packed: usize) -> crate::Tid<Self> {
Self::unpack(packed)
}
#[inline(always)]
fn unpack_gen(packed: usize) -> Generation<Self> {
Self::unpack(packed)
}
}
impl<C: Config> CfgPrivate for C {}
/// Default slab configuration values.
#[derive(Copy, Clone)]
pub struct DefaultConfig {
_p: (),
}
pub(crate) struct DebugConfig<C: Config> {
_cfg: PhantomData<fn(C)>,
}
pub(crate) const WIDTH: usize = std::mem::size_of::<usize>() * 8;
pub(crate) const fn next_pow2(n: usize) -> usize {
let pow2 = n.count_ones() == 1;
let zeros = n.leading_zeros();
1 << (WIDTH - zeros as usize - pow2 as usize)
}
// === impl DefaultConfig ===
impl Config for DefaultConfig {
const INITIAL_PAGE_SIZE: usize = 32;
#[cfg(target_pointer_width = "64")]
const MAX_THREADS: usize = 4096;
#[cfg(target_pointer_width = "32")]
// TODO(eliza): can we find enough bits to give 32-bit platforms more threads?
const MAX_THREADS: usize = 128;
const MAX_PAGES: usize = WIDTH / 2;
}
impl fmt::Debug for DefaultConfig {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
Self::debug().fmt(f)
}
}
impl<C: Config> fmt::Debug for DebugConfig<C> {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
f.debug_struct(std::any::type_name::<C>())
.field("initial_page_size", &C::INITIAL_SZ)
.field("max_shards", &C::MAX_SHARDS)
.field("max_pages", &C::MAX_PAGES)
.field("used_bits", &C::USED_BITS)
.field("reserved_bits", &C::RESERVED_BITS)
.field("pointer_width", &WIDTH)
.field("max_concurrent_references", &RefCount::<C>::MAX)
.finish()
}
}
#[cfg(test)]
mod tests {
use super::*;
use crate::test_util;
use crate::Slab;
#[test]
#[cfg_attr(loom, ignore)]
#[should_panic]
fn validates_max_refs() {
struct GiantGenConfig;
// Configure the slab with a very large number of bits for the generation
// counter. This will only leave 1 bit to use for the slot reference
// counter, which will fail to validate.
impl Config for GiantGenConfig {
const INITIAL_PAGE_SIZE: usize = 1;
const MAX_THREADS: usize = 1;
const MAX_PAGES: usize = 1;
}
let _slab = Slab::<usize>::new_with_config::<GiantGenConfig>();
}
#[test]
#[cfg_attr(loom, ignore)]
fn big() {
let slab = Slab::new();
for i in 0..10000 {
println!("{:?}", i);
let k = slab.insert(i).expect("insert");
assert_eq!(slab.get(k).expect("get"), i);
}
}
#[test]
#[cfg_attr(loom, ignore)]
fn custom_page_sz() {
let slab = Slab::new_with_config::<test_util::TinyConfig>();
for i in 0..4096 {
println!("{}", i);
let k = slab.insert(i).expect("insert");
assert_eq!(slab.get(k).expect("get"), i);
}
}
}
|
