zeemo: dynamic fd-indexed slot allocation + parser shrink

frameworks/zeemo/src/http.zig

@@ -53,8 +53,15 @@ pub const Parser = struct {
     /// has advanced. Set to headers_end when chunked mode is detected.
     chunk_pos: u32 = 0,
 
-    pub const BUF_SIZE = 4096;
-    pub const BODY_MAX = 4096;
+    /// Header-accumulation buffer. Sized to fit the largest pipelined
+    /// burst the bench profiles emit: 16 requests × ~80 B headers = ~1.3 KB
+    /// for the `pipelined` profile, comfortably below 2 KiB. Validation's
+    /// fragmentation tests stay tiny too.
+    pub const BUF_SIZE = 2048;
+    /// Body buffer. Validation sends ≤ 4-byte bodies; gcannon's baseline
+    /// POSTs are short integers. 512 B is well above realistic load while
+    /// staying ~8× leaner than the old 4 KiB.
+    pub const BODY_MAX = 512;
 
     const ChunkState = enum { size, size_cr, data, data_cr, data_lf, trailer_cr, trailer_lf, done };
 

frameworks/zeemo/src/main.zig

@@ -8,12 +8,13 @@ const handlers = @import("handlers.zig");
 const dataset = @import("dataset.zig");
 
 const PORT: u16 = 8080;
-/// Per-worker connection cap. The bench distributes 4096 connections across
-/// N workers via SO_REUSEPORT (4-tuple hash), so per-worker mean is ≤ 64
-/// with σ ≈ 8. 128 gives a healthy margin and roughly 4× less BSS than the
-/// previous 1024 — the memory bonus in HttpArena's composite uses
-/// `sqrt(rps)/memMB`, so even when rps stays flat the lower RSS bumps the score.
-const MAX_CONN = 128;
+/// Highest accepted fd. Reserves an O(1) lookup table; Linux assigns
+/// fresh fds starting from the lowest free slot so per-worker fds stay
+/// well below this even under churn. 4096 covers HttpArena's entire
+/// connection budget per worker with comfortable headroom.
+const MAX_FD: usize = 4096;
+/// Max concurrent JSON-promoted slots per worker (bounds the big_pool).
+const MAX_JSON_CONN = 128;
 const RING_ENTRIES = 4096;
 const LISTEN_BACKLOG: u32 = 1024;
 
@@ -32,7 +33,7 @@ const WRITE_INLINE_FLUSH_AT: u32 = WRITE_INLINE - 256;
 /// header). Pool slots are BSS — never touched, never resident — until
 /// JSON traffic actually arrives, then released back on connection close.
 const BIG_BUF_SIZE = 16 * 1024;
-const BIG_POOL_SIZE = MAX_CONN;
+const BIG_POOL_SIZE = MAX_JSON_CONN;
 
 const Op = enum(u8) {
     accept = 1,
@@ -41,27 +42,28 @@ const Op = enum(u8) {
     close = 4,
 };
 
-/// user_data = (op << 56) | slot_idx
-inline fn ud(op: Op, slot: u32) u64 {
-    return (@as(u64, @intFromEnum(op)) << 56) | @as(u64, slot);
+/// user_data = (op << 32) | fd. The fd both identifies the operation and
+/// is the lookup key into `slot_table`. Linux gives us back the lowest
+/// free fd at each accept so the table stays sparse in the low range.
+inline fn ud(op: Op, fd: linux.fd_t) u64 {
+    return (@as(u64, @intFromEnum(op)) << 32) | @as(u64, @as(u32, @bitCast(fd)));
 }
 inline fn udOp(u: u64) Op {
-    return @enumFromInt(@as(u8, @intCast(u >> 56)));
+    return @enumFromInt(@as(u8, @intCast(u >> 32)));
 }
-inline fn udSlot(u: u64) u32 {
-    return @intCast(u & 0x00FFFFFFFFFFFFFF);
+inline fn udFd(u: u64) linux.fd_t {
+    return @as(linux.fd_t, @bitCast(@as(u32, @truncate(u))));
 }
 
 const Slot = struct {
-    fd: linux.fd_t = -1,
-    in_use: bool = false,
+    fd: linux.fd_t,
     parser: http.Parser = .{},
-    /// Inline buffer used for small responses (baseline / pipelined /
+    /// Inline buffer for small responses (baseline / pipelined /
     /// limited-conn). Pipelined batches concatenate here.
     write_inline: [WRITE_INLINE]u8 = undefined,
     /// Index into `big_pool` if this connection has been promoted to a
-    /// large buffer (after seeing a /json/ request). Kept across requests
-    /// on the same connection; released to the pool on close.
+    /// large buffer (first /json/ request on the conn). Kept across
+    /// requests on the same connection; released to the pool on close.
     big_idx: ?u32 = null,
     /// Pointer + length of whatever buffer the in-flight send is reading
     /// from. Either &write_inline[0] or &big_pool[big_idx][0].
@@ -71,7 +73,17 @@ const Slot = struct {
     close_after_send: bool = false,
 };
 
-var slots: [MAX_CONN]Slot = undefined;
+/// fd-indexed pointer table — null for free fds, *Slot once accepted.
+/// 4096 × 8 B = 32 KiB BSS, almost entirely zero-fill (kernel touches
+/// only the cache lines holding active fds, ~1 page resident in practice).
+var slot_table: [MAX_FD]?*Slot = .{null} ** MAX_FD;
+
+/// Per-worker page-backed allocator. `page_allocator` returns pages via
+/// mmap and releases them via munmap on free, so freed Slots actually
+/// give memory back to the kernel — important for the limited-conn churn
+/// pattern where many connections cycle through.
+const slot_allocator = std.heap.page_allocator;
+
 var ds: dataset.Dataset = undefined;
 
 /// Per-worker pool of 16 KiB JSON response buffers. Static BSS — pages
@@ -95,23 +107,29 @@ fn bigRelease(idx: u32) void {
     big_used[idx] = false;
 }
 
-fn allocSlot() ?u32 {
-    var i: u32 = 0;
-    while (i < MAX_CONN) : (i += 1) {
-        if (!slots[i].in_use) {
-            slots[i] = .{};
-            slots[i].in_use = true;
-            return i;
-        }
-    }
-    return null;
+fn getSlot(fd: linux.fd_t) ?*Slot {
+    const fdu: usize = @intCast(fd);
+    if (fdu >= MAX_FD) return null;
+    return slot_table[fdu];
 }
 
-fn freeSlot(idx: u32) void {
-    if (slots[idx].big_idx) |b| bigRelease(b);
-    slots[idx].big_idx = null;
-    slots[idx].in_use = false;
-    slots[idx].fd = -1;
+fn allocSlotFor(fd: linux.fd_t) ?*Slot {
+    const fdu: usize = @intCast(fd);
+    if (fdu >= MAX_FD) return null;
+    const slot = slot_allocator.create(Slot) catch return null;
+    slot.* = .{ .fd = fd };
+    slot_table[fdu] = slot;
+    return slot;
+}
+
+fn freeSlotFor(fd: linux.fd_t) void {
+    const fdu: usize = @intCast(fd);
+    if (fdu >= MAX_FD) return;
+    if (slot_table[fdu]) |slot| {
+        if (slot.big_idx) |b| bigRelease(b);
+        slot_allocator.destroy(slot);
+        slot_table[fdu] = null;
+    }
 }
 
 pub fn main() !void {
@@ -260,7 +278,7 @@ fn handleCqe(ring: *IoUring, listen_fd: linux.fd_t, cqe: *linux.io_uring_cqe) !v
         .accept => try handleAccept(ring, listen_fd, cqe),
         .recv => try handleRecv(ring, cqe),
         .send => try handleSend(ring, cqe),
-        .close => freeSlot(udSlot(cqe.user_data)),
+        .close => freeSlotFor(udFd(cqe.user_data)),
     }
 }
 
@@ -271,51 +289,48 @@ fn handleAccept(ring: *IoUring, listen_fd: linux.fd_t, cqe: *linux.io_uring_cqe)
         return;
     }
     const fd: linux.fd_t = @intCast(cqe.res);
-    const slot_idx = allocSlot() orelse {
+    const slot = allocSlotFor(fd) orelse {
         _ = linux.close(fd);
         if (!more) _ = try ring.accept_multishot(ud(.accept, 0), listen_fd, null, null, 0);
         return;
     };
-    slots[slot_idx].fd = fd;
-    const buf = slots[slot_idx].parser.recv_slot();
-    _ = try ring.recv(ud(.recv, slot_idx), fd, .{ .buffer = buf }, 0);
+    const buf = slot.parser.recv_slot();
+    _ = try ring.recv(ud(.recv, fd), fd, .{ .buffer = buf }, 0);
 
     // If multishot fell off, re-arm.
     if (!more) _ = try ring.accept_multishot(ud(.accept, 0), listen_fd, null, null, 0);
 }
 
 fn handleRecv(ring: *IoUring, cqe: *linux.io_uring_cqe) !void {
-    const slot_idx = udSlot(cqe.user_data);
-    const slot = &slots[slot_idx];
+    const fd = udFd(cqe.user_data);
+    const slot = getSlot(fd) orelse return; // already freed — race with close CQE
     if (cqe.res <= 0) {
-        _ = try ring.close(ud(.close, slot_idx), slot.fd);
+        _ = try ring.close(ud(.close, fd), fd);
         return;
     }
-    try drainAndSend(ring, slot_idx, @intCast(cqe.res));
+    try drainAndSend(ring, slot, @intCast(cqe.res));
 }
 
 fn handleSend(ring: *IoUring, cqe: *linux.io_uring_cqe) !void {
-    const slot_idx = udSlot(cqe.user_data);
-    const slot = &slots[slot_idx];
+    const fd = udFd(cqe.user_data);
+    const slot = getSlot(fd) orelse return;
     if (cqe.res <= 0) {
-        _ = try ring.close(ud(.close, slot_idx), slot.fd);
+        _ = try ring.close(ud(.close, fd), fd);
         return;
     }
     const n: u32 = @intCast(cqe.res);
     slot.send_off += n;
     if (slot.send_off < slot.send_len) {
-        // Partial send — replay from wherever the active buffer is
-        // (inline or big), tracked by send_ptr.
+        // Partial send — replay from wherever the active buffer is.
         const tail = slot.send_ptr[slot.send_off..slot.send_len];
-        _ = try ring.send(ud(.send, slot_idx), slot.fd, tail, linux.MSG.NOSIGNAL);
+        _ = try ring.send(ud(.send, fd), fd, tail, linux.MSG.NOSIGNAL);
         return;
     }
     if (slot.close_after_send) {
-        _ = try ring.close(ud(.close, slot_idx), slot.fd);
+        _ = try ring.close(ud(.close, fd), fd);
         return;
     }
-    // Keep-alive: drain any further pipelined requests already buffered.
-    try drainAndSend(ring, slot_idx, 0);
+    try drainAndSend(ring, slot, 0);
 }
 
 /// Drain as many complete requests as fit in the inline write buffer,
@@ -331,8 +346,7 @@ fn handleSend(ring: *IoUring, cqe: *linux.io_uring_cqe) !void {
 /// We dispatch all complete requests buffered before submitting one batched
 /// send — saves N-1 syscalls and avoids a deadlock that the older
 /// "recv → dispatch one → send → recv" code path hit on pipelined input.
-fn drainAndSend(ring: *IoUring, slot_idx: u32, initial_feed_n: u32) !void {
-    const slot = &slots[slot_idx];
+fn drainAndSend(ring: *IoUring, slot: *Slot, initial_feed_n: u32) !void {
     var inline_pos: u32 = 0;
     var feed_n = initial_feed_n;
 
@@ -342,63 +356,56 @@ fn drainAndSend(ring: *IoUring, slot_idx: u32, initial_feed_n: u32) !void {
         switch (result) {
             .protocol_error => {
                 if (inline_pos > 0) {
-                    submitInline(ring, slot_idx, inline_pos, true) catch {};
+                    submitInline(ring, slot, inline_pos, true) catch {};
                 } else {
-                    _ = try ring.close(ud(.close, slot_idx), slot.fd);
+                    _ = try ring.close(ud(.close, slot.fd), slot.fd);
                 }
                 return;
             },
             .need_more => {
                 if (inline_pos > 0) {
-                    try submitInline(ring, slot_idx, inline_pos, false);
+                    try submitInline(ring, slot, inline_pos, false);
                     return;
                 }
                 const buf = slot.parser.recv_slot();
                 if (buf.len == 0) {
-                    _ = try ring.close(ud(.close, slot_idx), slot.fd);
+                    _ = try ring.close(ud(.close, slot.fd), slot.fd);
                     return;
                 }
-                _ = try ring.recv(ud(.recv, slot_idx), slot.fd, .{ .buffer = buf }, 0);
+                _ = try ring.recv(ud(.recv, slot.fd), slot.fd, .{ .buffer = buf }, 0);
                 return;
             },
             .ready => |req| {
                 const needs_big = std.mem.startsWith(u8, req.path, "/json/");
                 if (needs_big) {
-                    // JSON responses don't fit in the 4 KiB inline buffer
-                    // and can't be batched with other responses. Flush any
-                    // queued inline bytes first, then dispatch JSON into
-                    // big_buf and submit it as its own send.
                     if (inline_pos > 0) {
-                        // Defer JSON to the next drain pass after the
-                        // inline batch flushes — leave the parser pointing
-                        // at this request by NOT resetting yet. handleSend
-                        // completion will call drainAndSend(0) and feed(0)
-                        // will re-yield this same request.
-                        try submitInline(ring, slot_idx, inline_pos, false);
+                        // Flush queued non-JSON bytes first; the JSON
+                        // request is re-yielded on the next drain pass
+                        // because we haven't called parser.reset yet.
+                        try submitInline(ring, slot, inline_pos, false);
                         return;
                     }
                     if (slot.big_idx == null) {
                         slot.big_idx = bigAcquire() orelse {
-                            // Pool exhausted — refuse this connection.
-                            _ = try ring.close(ud(.close, slot_idx), slot.fd);
+                            _ = try ring.close(ud(.close, slot.fd), slot.fd);
                             return;
                         };
                     }
                     const big = bigSlice(slot.big_idx.?);
                     const resp = handlers.handle(req, &ds, big);
                     slot.parser.reset(slot.parser.consumed());
-                    try submitBig(ring, slot_idx, @intCast(resp.bytes.len), resp.close);
+                    try submitBig(ring, slot, @intCast(resp.bytes.len), resp.close);
                     return;
                 }
                 const resp = handlers.handle(req, &ds, slot.write_inline[inline_pos..]);
                 inline_pos += @intCast(resp.bytes.len);
                 slot.parser.reset(slot.parser.consumed());
                 if (resp.close) {
-                    try submitInline(ring, slot_idx, inline_pos, true);
+                    try submitInline(ring, slot, inline_pos, true);
                     return;
                 }
                 if (inline_pos > WRITE_INLINE_FLUSH_AT) {
-                    try submitInline(ring, slot_idx, inline_pos, false);
+                    try submitInline(ring, slot, inline_pos, false);
                     return;
                 }
                 // Loop: feed(0) for the next pipelined request.
@@ -411,21 +418,19 @@ fn bigSlice(idx: u32) []u8 {
     return &big_pool[idx];
 }
 
-fn submitInline(ring: *IoUring, slot_idx: u32, len: u32, close_after: bool) !void {
-    const slot = &slots[slot_idx];
+fn submitInline(ring: *IoUring, slot: *Slot, len: u32, close_after: bool) !void {
     slot.send_ptr = &slot.write_inline;
     slot.send_len = len;
     slot.send_off = 0;
     slot.close_after_send = close_after;
-    _ = try ring.send(ud(.send, slot_idx), slot.fd, slot.write_inline[0..len], linux.MSG.NOSIGNAL);
+    _ = try ring.send(ud(.send, slot.fd), slot.fd, slot.write_inline[0..len], linux.MSG.NOSIGNAL);
 }
 
-fn submitBig(ring: *IoUring, slot_idx: u32, len: u32, close_after: bool) !void {
-    const slot = &slots[slot_idx];
+fn submitBig(ring: *IoUring, slot: *Slot, len: u32, close_after: bool) !void {
     const buf = bigSlice(slot.big_idx.?);
     slot.send_ptr = buf.ptr;
     slot.send_len = len;
     slot.send_off = 0;
     slot.close_after_send = close_after;
-    _ = try ring.send(ud(.send, slot_idx), slot.fd, buf[0..len], linux.MSG.NOSIGNAL);
+    _ = try ring.send(ud(.send, slot.fd), slot.fd, buf[0..len], linux.MSG.NOSIGNAL);
 }

site/data/baseline-4096.json

@@ -1326,19 +1326,19 @@
   {
     "framework": "zeemo",
     "language": "Zig",
-    "rps": 4435413,
-    "avg_latency": "923us",
-    "p99_latency": "1.22ms",
-    "cpu": "6406.3%",
-    "memory": "130MiB",
+    "rps": 4429132,
+    "avg_latency": "924us",
+    "p99_latency": "1.27ms",
+    "cpu": "6407.2%",
+    "memory": "110MiB",
     "connections": 4096,
     "threads": 64,
     "duration": "5s",
     "pipeline": 1,
-    "bandwidth": "279.07MB/s",
-    "input_bw": "342.63MB/s",
+    "bandwidth": "278.66MB/s",
+    "input_bw": "342.14MB/s",
     "reconnects": 0,
-    "status_2xx": 22177068,
+    "status_2xx": 22145662,
     "status_3xx": 0,
     "status_4xx": 0,
     "status_5xx": 0

site/data/baseline-512.json

@@ -1326,19 +1326,19 @@
   {
     "framework": "zeemo",
     "language": "Zig",
-    "rps": 4120722,
+    "rps": 4113749,
     "avg_latency": "123us",
-    "p99_latency": "238us",
-    "cpu": "6267.1%",
+    "p99_latency": "218us",
+    "cpu": "6278.0%",
     "memory": "69MiB",
     "connections": 512,
     "threads": 64,
     "duration": "5s",
     "pipeline": 1,
-    "bandwidth": "259.30MB/s",
-    "input_bw": "318.32MB/s",
+    "bandwidth": "258.86MB/s",
+    "input_bw": "317.78MB/s",
     "reconnects": 0,
-    "status_2xx": 20603611,
+    "status_2xx": 20568745,
     "status_3xx": 0,
     "status_4xx": 0,
     "status_5xx": 0