new exponential SlabAllocator

src/browser/Factory.zig

@@ -24,7 +24,7 @@ const IS_DEBUG = builtin.mode == .Debug;
 const log = @import("../log.zig");
 const String = @import("../string.zig").String;
 
-const SlabAllocator = @import("../slab.zig").SlabAllocator(16);
+const SlabAllocator = @import("../slab.zig").SlabAllocator;
 
 const Page = @import("Page.zig");
 const Node = @import("webapi/Node.zig");
@@ -53,7 +53,7 @@ _slab: SlabAllocator,
 pub fn init(page: *Page) Factory {
     return .{
         ._page = page,
-        ._slab = SlabAllocator.init(page.arena),
+        ._slab = SlabAllocator.init(page.arena, 128),
     };
 }
 

src/slab.zig

@@ -4,21 +4,10 @@ const assert = std.debug.assert;
 const Allocator = std.mem.Allocator;
 const Alignment = std.mem.Alignment;
 
-pub fn SlabAllocator(comptime slot_count: usize) type {
-    comptime assert(std.math.isPowerOfTwo(slot_count));
-
-    const SlabKey = struct {
-        size: usize,
-        alignment: Alignment,
-    };
-
 const Slab = struct {
-        const Slab = @This();
-        const chunk_shift = std.math.log2_int(usize, slot_count);
-        const chunk_mask = slot_count - 1;
-
     alignment: Alignment,
     item_size: usize,
+    max_slot_count: usize,
 
     bitset: std.bit_set.DynamicBitSetUnmanaged,
     chunks: std.ArrayListUnmanaged([]u8),
@@ -27,12 +16,14 @@ pub fn SlabAllocator(comptime slot_count: usize) type {
         allocator: Allocator,
         alignment: Alignment,
         item_size: usize,
+        max_slot_count: usize,
     ) !Slab {
         return .{
             .alignment = alignment,
             .item_size = item_size,
             .bitset = try .initFull(allocator, 0),
             .chunks = .empty,
+            .max_slot_count = max_slot_count,
         };
     }
 
@@ -46,23 +37,42 @@ pub fn SlabAllocator(comptime slot_count: usize) type {
         self.chunks.deinit(allocator);
     }
 
-        inline fn toBitsetIndex(chunk_index: usize, slot_index: usize) usize {
+    inline fn calculateChunkSize(self: *Slab, chunk_index: usize) usize {
-            return chunk_index * slot_count + slot_index;
+        const safe_index: u6 = @intCast(@min(std.math.maxInt(u6), chunk_index));
+        const exponential = @as(usize, 1) << safe_index;
+        return @min(exponential, self.max_slot_count);
     }
 
-        inline fn chunkIndex(bitset_index: usize) usize {
+    inline fn toBitsetIndex(self: *Slab, chunk_index: usize, slot_index: usize) usize {
-            return bitset_index >> chunk_shift;
+        var offset: usize = 0;
+        for (0..chunk_index) |i| {
+            const chunk_size = self.calculateChunkSize(i);
+            offset += chunk_size;
+        }
+        return offset + slot_index;
     }
 
-        inline fn slotIndex(bitset_index: usize) usize {
+    inline fn toChunkAndSlotIndices(self: *Slab, bitset_index: usize) struct { usize, usize } {
-            return bitset_index & chunk_mask;
+        var offset: usize = 0;
+        var chunk_index: usize = 0;
+
+        while (chunk_index < self.chunks.items.len) : (chunk_index += 1) {
+            const chunk_size = self.calculateChunkSize(chunk_index);
+            if (bitset_index < offset + chunk_size) {
+                return .{ chunk_index, bitset_index - offset };
+            }
+
+            offset += chunk_size;
+        }
+
+        unreachable;
     }
 
     fn alloc(self: *Slab, allocator: Allocator) ![]u8 {
         if (self.bitset.findFirstSet()) |index| {
+            const chunk_index, const slot_index = self.toChunkAndSlotIndices(index);
+
             // if we have a free slot
-                const chunk_index = chunkIndex(index);
-                const slot_index = slotIndex(index);
             self.bitset.unset(index);
 
             const chunk = self.chunks.items[chunk_index];
@@ -87,13 +97,13 @@ pub fn SlabAllocator(comptime slot_count: usize) type {
 
         for (self.chunks.items, 0..) |chunk, i| {
             const chunk_start = @intFromPtr(chunk.ptr);
-                const chunk_end = chunk_start + (slot_count * self.item_size);
+            const chunk_end = chunk_start + chunk.len;
 
             if (addr >= chunk_start and addr < chunk_end) {
                 const offset = addr - chunk_start;
                 const slot_index = offset / self.item_size;
 
-                    const bitset_index = toBitsetIndex(i, slot_index);
+                const bitset_index = self.toBitsetIndex(i, slot_index);
                 assert(!self.bitset.isSet(bitset_index));
 
                 self.bitset.set(bitset_index);
@@ -105,7 +115,8 @@ pub fn SlabAllocator(comptime slot_count: usize) type {
     }
 
     fn allocateChunk(self: *Slab, allocator: Allocator) !void {
-            const chunk_len = self.item_size * slot_count;
+        const next_chunk_size = self.calculateChunkSize(self.chunks.items.len);
+        const chunk_len = self.item_size * next_chunk_size;
 
         const chunk_ptr = allocator.rawAlloc(
             chunk_len,
@@ -116,7 +127,7 @@ pub fn SlabAllocator(comptime slot_count: usize) type {
         const chunk = chunk_ptr[0..chunk_len];
         try self.chunks.append(allocator, chunk);
 
-            const new_capacity = self.chunks.items.len * slot_count;
+        const new_capacity = self.bitset.bit_length + next_chunk_size;
         try self.bitset.resize(allocator, new_capacity, true);
     }
 
@@ -137,7 +148,7 @@ pub fn SlabAllocator(comptime slot_count: usize) type {
         const total_slots = self.bitset.bit_length;
         const free_slots = self.bitset.count();
         const used_slots = total_slots - free_slots;
-            const bytes_allocated = self.chunks.items.len * slot_count * self.item_size;
+        const bytes_allocated = total_slots * self.item_size;
         const bytes_in_use = used_slots * self.item_size;
 
         const utilization_ratio = if (bytes_allocated > 0)
@@ -160,10 +171,17 @@ pub fn SlabAllocator(comptime slot_count: usize) type {
     }
 };
 
-    return struct {
+const SlabKey = struct {
+    size: usize,
+    alignment: Alignment,
+};
+
+pub const SlabAllocator = struct {
     const Self = @This();
 
     child_allocator: Allocator,
+    max_slot_count: usize,
+
     slabs: std.ArrayHashMapUnmanaged(SlabKey, Slab, struct {
         const Context = @This();
 
@@ -179,10 +197,13 @@ pub fn SlabAllocator(comptime slot_count: usize) type {
         }
     }, false) = .empty,
 
-        pub fn init(child_allocator: Allocator) Self {
+    pub fn init(child_allocator: Allocator, max_slot_count: usize) Self {
+        assert(std.math.isPowerOfTwo(max_slot_count));
+
         return .{
             .child_allocator = child_allocator,
             .slabs = .empty,
+            .max_slot_count = max_slot_count,
         };
     }
 
@@ -365,6 +386,7 @@ pub fn SlabAllocator(comptime slot_count: usize) type {
                 self.child_allocator,
                 alignment,
                 len,
+                self.max_slot_count,
             ) catch return null;
         }
 
@@ -384,14 +406,13 @@ pub fn SlabAllocator(comptime slot_count: usize) type {
         list.free(ptr);
     }
 };
-}
 
 const testing = std.testing;
 
-const TestSlabAllocator = SlabAllocator(32);
+const TestSlabAllocator = SlabAllocator;
 
 test "slab allocator - basic allocation and free" {
-    var slab_alloc = TestSlabAllocator.init(testing.allocator);
+    var slab_alloc = TestSlabAllocator.init(testing.allocator, 16);
     defer slab_alloc.deinit();
 
     const allocator = slab_alloc.allocator();
@@ -409,7 +430,7 @@ test "slab allocator - basic allocation and free" {
 }
 
 test "slab allocator - multiple allocations" {
-    var slab_alloc = TestSlabAllocator.init(testing.allocator);
+    var slab_alloc = TestSlabAllocator.init(testing.allocator, 16);
     defer slab_alloc.deinit();
 
     const allocator = slab_alloc.allocator();
@@ -432,7 +453,7 @@ test "slab allocator - multiple allocations" {
 }
 
 test "slab allocator - no coalescing (different size classes)" {
-    var slab_alloc = TestSlabAllocator.init(testing.allocator);
+    var slab_alloc = TestSlabAllocator.init(testing.allocator, 16);
     defer slab_alloc.deinit();
 
     const allocator = slab_alloc.allocator();
@@ -459,7 +480,7 @@ test "slab allocator - no coalescing (different size classes)" {
 }
 
 test "slab allocator - reuse freed memory" {
-    var slab_alloc = TestSlabAllocator.init(testing.allocator);
+    var slab_alloc = TestSlabAllocator.init(testing.allocator, 16);
     defer slab_alloc.deinit();
 
     const allocator = slab_alloc.allocator();
@@ -477,7 +498,7 @@ test "slab allocator - reuse freed memory" {
 }
 
 test "slab allocator - multiple size classes" {
-    var slab_alloc = TestSlabAllocator.init(testing.allocator);
+    var slab_alloc = TestSlabAllocator.init(testing.allocator, 16);
     defer slab_alloc.deinit();
 
     const allocator = slab_alloc.allocator();
@@ -501,7 +522,7 @@ test "slab allocator - multiple size classes" {
 }
 
 test "slab allocator - various sizes" {
-    var slab_alloc = TestSlabAllocator.init(testing.allocator);
+    var slab_alloc = TestSlabAllocator.init(testing.allocator, 16);
     defer slab_alloc.deinit();
 
     const allocator = slab_alloc.allocator();
@@ -518,7 +539,7 @@ test "slab allocator - various sizes" {
 }
 
 test "slab allocator - exact sizes (no rounding)" {
-    var slab_alloc = TestSlabAllocator.init(testing.allocator);
+    var slab_alloc = TestSlabAllocator.init(testing.allocator, 16);
     defer slab_alloc.deinit();
 
     const allocator = slab_alloc.allocator();
@@ -539,7 +560,7 @@ test "slab allocator - exact sizes (no rounding)" {
 }
 
 test "slab allocator - chunk allocation" {
-    var slab_alloc = TestSlabAllocator.init(testing.allocator);
+    var slab_alloc = TestSlabAllocator.init(testing.allocator, 16);
     defer slab_alloc.deinit();
 
     const allocator = slab_alloc.allocator();
@@ -561,7 +582,7 @@ test "slab allocator - chunk allocation" {
 }
 
 test "slab allocator - reset with retain_capacity" {
-    var slab_alloc = TestSlabAllocator.init(testing.allocator);
+    var slab_alloc = TestSlabAllocator.init(testing.allocator, 16);
     defer slab_alloc.deinit();
 
     const allocator = slab_alloc.allocator();
@@ -588,7 +609,7 @@ test "slab allocator - reset with retain_capacity" {
 }
 
 test "slab allocator - reset with clear" {
-    var slab_alloc = TestSlabAllocator.init(testing.allocator);
+    var slab_alloc = TestSlabAllocator.init(testing.allocator, 16);
     defer slab_alloc.deinit();
 
     const allocator = slab_alloc.allocator();
@@ -610,7 +631,7 @@ test "slab allocator - reset with clear" {
 }
 
 test "slab allocator - stress test" {
-    var slab_alloc = TestSlabAllocator.init(testing.allocator);
+    var slab_alloc = TestSlabAllocator.init(testing.allocator, 16);
     defer slab_alloc.deinit();
 
     const allocator = slab_alloc.allocator();
@@ -647,7 +668,7 @@ test "slab allocator - stress test" {
 }
 
 test "slab allocator - alignment" {
-    var slab_alloc = TestSlabAllocator.init(testing.allocator);
+    var slab_alloc = TestSlabAllocator.init(testing.allocator, 16);
     defer slab_alloc.deinit();
 
     const allocator = slab_alloc.allocator();
@@ -662,7 +683,7 @@ test "slab allocator - alignment" {
 }
 
 test "slab allocator - no resize support" {
-    var slab_alloc = TestSlabAllocator.init(testing.allocator);
+    var slab_alloc = TestSlabAllocator.init(testing.allocator, 16);
     defer slab_alloc.deinit();
 
     const allocator = slab_alloc.allocator();
@@ -678,7 +699,7 @@ test "slab allocator - no resize support" {
 }
 
 test "slab allocator - fragmentation pattern" {
-    var slab_alloc = TestSlabAllocator.init(testing.allocator);
+    var slab_alloc = TestSlabAllocator.init(testing.allocator, 16);
     defer slab_alloc.deinit();
 
     const allocator = slab_alloc.allocator();
@@ -730,7 +751,7 @@ test "slab allocator - fragmentation pattern" {
 }
 
 test "slab allocator - many small allocations" {
-    var slab_alloc = TestSlabAllocator.init(testing.allocator);
+    var slab_alloc = TestSlabAllocator.init(testing.allocator, 16);
     defer slab_alloc.deinit();
 
     const allocator = slab_alloc.allocator();
@@ -752,11 +773,11 @@ test "slab allocator - many small allocations" {
 
     // Should have created multiple chunks
     const slab = slab_alloc.slabs.getPtr(.{ .size = 24, .alignment = Alignment.@"1" }).?;
-    try testing.expect(slab.chunks.items.len > 10);
+    try testing.expect(slab.chunks.items.len > 1);
 }
 
 test "slab allocator - zero waste for exact sizes" {
-    var slab_alloc = TestSlabAllocator.init(testing.allocator);
+    var slab_alloc = TestSlabAllocator.init(testing.allocator, 16);
     defer slab_alloc.deinit();
 
     const allocator = slab_alloc.allocator();
@@ -776,7 +797,7 @@ test "slab allocator - zero waste for exact sizes" {
 }
 
 test "slab allocator - different size classes don't interfere" {
-    var slab_alloc = TestSlabAllocator.init(testing.allocator);
+    var slab_alloc = TestSlabAllocator.init(testing.allocator, 16);
     defer slab_alloc.deinit();
 
     const allocator = slab_alloc.allocator();