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Merge pull request #22823 from ziglang/SmpAllocator-freelist-accounting

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std.heap.SmpAllocator: back to simple free implementation
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Andrew Kelley 2025-02-08 21:27:22 -08:00 committed by GitHub
commit 604ed5281c
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1 changed files with 4 additions and 42 deletions
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lib/std/heap/SmpAllocator.zig
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@ -51,10 +51,6 @@ const slab_len: usize = @max(std.heap.page_size_max, 64 * 1024);
/// Because of storing free list pointers, the minimum size class is 3. /// Because of storing free list pointers, the minimum size class is 3.
const min_class = math.log2(@sizeOf(usize)); const min_class = math.log2(@sizeOf(usize));
const size_class_count = math.log2(slab_len) - min_class; const size_class_count = math.log2(slab_len) - min_class;
/// When a freelist length exceeds this number, a `free` will rotate up to
/// `max_free_search` times before pushing.
const max_freelist_len: u8 = 16;
const max_free_search = 1;
/// Before mapping a fresh page, `alloc` will rotate this many times. /// Before mapping a fresh page, `alloc` will rotate this many times.
const max_alloc_search = 1; const max_alloc_search = 1;
@ -73,8 +69,6 @@ const Thread = struct {
next_addrs: [size_class_count]usize = @splat(0), next_addrs: [size_class_count]usize = @splat(0),
/// For each size class, points to the freed pointer. /// For each size class, points to the freed pointer.
frees: [size_class_count]usize = @splat(0), frees: [size_class_count]usize = @splat(0),
/// For each size class, tracks the number of items in the freelist.
freelist_lens: [size_class_count]u8 = @splat(0),
fn lock() *Thread { fn lock() *Thread {
var index = thread_index; var index = thread_index;
@ -142,7 +136,6 @@ fn alloc(context: *anyopaque, len: usize, alignment: mem.Alignment, ra: usize) ?
defer t.unlock(); defer t.unlock();
const node: *usize = @ptrFromInt(top_free_ptr); const node: *usize = @ptrFromInt(top_free_ptr);
t.frees[class] = node.*; t.frees[class] = node.*;
t.freelist_lens[class] -|= 1;
return @ptrFromInt(top_free_ptr); return @ptrFromInt(top_free_ptr);
} }
@ -160,7 +153,6 @@ fn alloc(context: *anyopaque, len: usize, alignment: mem.Alignment, ra: usize) ?
// slab alignment here ensures the % slab len earlier catches the end of slots. // slab alignment here ensures the % slab len earlier catches the end of slots.
const slab = PageAllocator.map(slab_len, .fromByteUnits(slab_len)) orelse return null; const slab = PageAllocator.map(slab_len, .fromByteUnits(slab_len)) orelse return null;
t.next_addrs[class] = @intFromPtr(slab) + slot_size; t.next_addrs[class] = @intFromPtr(slab) + slot_size;
t.freelist_lens[class] = 0;
return slab; return slab;
} }
@ -214,42 +206,12 @@ fn free(context: *anyopaque, memory: []u8, alignment: mem.Alignment, ra: usize)
} }
const node: *usize = @alignCast(@ptrCast(memory.ptr)); const node: *usize = @alignCast(@ptrCast(memory.ptr));
var search_count: u8 = 0;
var t = Thread.lock(); const t = Thread.lock();
outer: while (true) {
const freelist_len = t.freelist_lens[class];
if (freelist_len < max_freelist_len) {
@branchHint(.likely);
defer t.unlock(); defer t.unlock();
node.* = t.frees[class]; node.* = t.frees[class];
t.frees[class] = @intFromPtr(node); t.frees[class] = @intFromPtr(node);
return;
}
if (search_count >= max_free_search) {
defer t.unlock();
t.freelist_lens[class] = freelist_len +| 1;
node.* = t.frees[class];
t.frees[class] = @intFromPtr(node);
return;
}
t.unlock();
const cpu_count = getCpuCount();
assert(cpu_count != 0);
var index = thread_index;
while (true) {
index = (index + 1) % cpu_count;
t = &global.threads[index];
if (t.mutex.tryLock()) {
thread_index = index;
search_count += 1;
continue :outer;
}
}
}
} }
fn sizeClassIndex(len: usize, alignment: mem.Alignment) usize { fn sizeClassIndex(len: usize, alignment: mem.Alignment) usize {