Multithreading works! Kinda sorta (slows down in chrome, out of bounds accesses on phone)

client/lod_worker.js

@@ -0,0 +1,44 @@
+let thread_id = null;
+let exports = null;
+
+async function init(tid, memory, heap_base) {
+    thread_id = tid;
+
+    const result = await WebAssembly.instantiateStreaming(fetch('wasm/lod.wasm'), {
+        env: { 'memory': memory }
+    });
+
+    exports = result.instance.exports;
+    exports.set_sp(heap_base - thread_id * 16 * 4096); // 64K stack
+
+    postMessage({ 'type': 'init_done' });
+}
+
+function work(indices_base, indices_count, zoom, offsets) {
+    exports.do_lod(
+        indices_base, indices_count, zoom,
+        offsets['coords_from'],
+        offsets['line_threshold'],
+        offsets['xs'],
+        offsets['ys'],
+        offsets['pressures'],
+        offsets['result_buffers'] + thread_id * 4,
+        offsets['result_counts'] + thread_id * 4,
+    );
+
+    postMessage({ 'type': 'lod_done' });
+}
+
+onmessage = (e) => {
+    const d = e.data;
+
+    if (d.type === 'init') {
+        init(d.thread_id, d.memory, d.heap_base);
+    } else if (d.type === 'lod') {
+        work(d.indices_base, d.indices_count, d.zoom, d.offsets);
+    } else {
+        console.error('unexpected worker command:', d.type);
+    }
+}
+
+

client/speed.js

@@ -1,74 +1,65 @@
-async function init_test() {
-    const memory = new WebAssembly.Memory({
-        initial: 32,
-        maximum: 100,
-        shared: true,
+function worker_message(worker, message) {
+    return new Promise((resolve) => {
+        worker.onmessage = (e) => resolve(e.data);
+        worker.postMessage(message);
     });
+}
 
-    const results = await WebAssembly.instantiateStreaming(fetch('wasm/multi.wasm'), {
-        env: { 'memory': memory }
-    });
+function workers_messages(workers, messages) {
+    const promises = [];
 
-    const nworkers = navigator.hardwareConcurrency;
-    const heap_base = results.instance.exports.alloc(0);
-    const buf_offset = results.instance.exports.alloc(1024);
-    const workers = [];
-    const sab = new SharedArrayBuffer(nworkers * 4);
-    const flags = new Int32Array(sab); 
-    let done = 0;
-
-    for (let i = 0; i < nworkers; ++i) {
-        const w = new Worker('wasm_worker.js');
-        workers.push(w);
+    for (let i = 0; i < workers.length; ++i) {
+        promises.push(worker_message(workers[i], messages[i]));
     }
 
-    for (let i = 0; i < nworkers; ++i) {
-        workers[i].onmessage = () => { 
-            ++done; 
-            if (done % nworkers === 0) {
-                console.log('init done');
-
-                for (let j = 0; j < nworkers; ++j) {
-                    workers[j].onmessage = () => {
-                        ++done;
-
-                        if (done % nworkers === 0) {
-                            console.log('work done');
-                            console.log(new Int32Array(memory.buffer, buf_offset, nworkers));
-                        }
-                    }
+    return Promise.all(promises); 
+}
 
-                    workers[j].postMessage({
-                        'type': 1,
-                        'num': 10,
-                    });
-                }
+function workers_thread_message(workers, message, thread_field=null) {
+    const messages = [];
+
+    for (let i = 0; i < workers.length; ++i) {
+        if (thread_field !== null) {
+            const m = structuredClone(message);
+            m[thread_field] = i;
+            messages.push(m);
+        } else {
+            messages.push(message);
         }
     }
 
-        workers[i].postMessage({
-            'type': 0,
-            'thread_id': i,
-            'memory': memory,
-            'stack_base': heap_base,
-            'buffer_offset': buf_offset,
-            'flags': flags
+    return workers_messages(workers, messages);
+}
+
+async function init_wasm(state) {
+    const memory = new WebAssembly.Memory({
+        initial: 32, // 2MiB, 1MiB of which is stack
+        maximum: 16384, // 1GiB
+        shared: true,
     });
-    }
 
-//    const results = await WebAssembly.instantiateStreaming(fetch('wasm/multi.wasm'));
+    // "Master thread" to do maintance on (static allocations, merging results etc)
+    const master_wasm = await WebAssembly.instantiateStreaming(fetch('wasm/lod.wasm'), {
+        env: { 'memory': memory }
+    });
 
-//    state.wasm.exports = results.instance.exports;
-//    state.wasm.exports.memory.grow(4096);
-}
+    const nworkers = navigator.hardwareConcurrency;
 
-async function init_wasm(state) {
-    await init_test();
+    state.wasm.exports = master_wasm.instance.exports;
+    state.wasm.heap_base = state.wasm.exports.alloc_static(0);
+    state.wasm.workers = [];
+    state.wasm.memory = memory;
 
-    const results = await WebAssembly.instantiateStreaming(fetch('wasm/lod.wasm'));
+    for (let i = 0; i < nworkers; ++i) {
+        const w = new Worker('lod_worker.js');
+        state.wasm.workers.push(w);
+    }
 
-    state.wasm.exports = results.instance.exports;
-    state.wasm.exports.memory.grow(4096);
+    await workers_thread_message(state.wasm.workers, {
+        'type': 'init',
+        'heap_base': state.wasm.heap_base,
+        'memory': memory,
+    }, 'thread_id');
 
     state.wasm.stroke_bytes = 4096;
     state.wasm.coords_bytes = 4096;
@@ -95,7 +86,7 @@ async function init_wasm(state) {
         },
     };
 
-    const mem = state.wasm.exports.memory.buffer;
+    const mem = state.wasm.memory.buffer;
 
     state.wasm.buffers['xs'].tv = tv_create_on(Float32Array, state.wasm.coords_bytes / 8, 
         mem, state.wasm.buffers['xs'].offset);
@@ -135,11 +126,14 @@ function wasm_ensure_by(state, nstrokes, ncoords) {
     }
 
     if (realloc) {
-        // TODO: we do memory.grow() somewhere here if needed
+        const current_pages = state.wasm.memory.buffer.byteLength / (4096 * 16);
+        const need_pages = (state.wasm.coords_bytes * 3 + state.wasm.stroke_bytes * 2) / (4096 * 16); // TODO: figure out actual memory requirements
+        const grow_by = need_pages - current_pages;
 
+        state.wasm.memory.grow(grow_by);
         state.wasm.exports.free_static();
 
-        const mem = state.wasm.exports.memory.buffer;
+        const mem = state.wasm.memory.buffer;
         const memv = new Uint8Array(mem);
 
         buffers['xs'].offset = state.wasm.exports.alloc_static(state.wasm.coords_bytes / 2);
@@ -175,35 +169,65 @@ function wasm_ensure_by(state, nstrokes, ncoords) {
     }
 }
 
-function do_lod(state, context) {
+async function do_lod(state, context) {
     state.wasm.exports.free_dynamic();
 
+    const buffers = state.wasm.buffers;
+    const result_buffers = state.wasm.exports.alloc_dynamic(state.wasm.workers.length * 4);
+    const result_counts = state.wasm.exports.alloc_dynamic(state.wasm.workers.length * 4);
     const clipped_indices = state.wasm.exports.alloc_dynamic(context.clipped_indices.size * 4); 
-    const mem = new Uint8Array(state.wasm.exports.memory.buffer);
+    const mem = new Uint8Array(state.wasm.memory.buffer);
     
     // Dynamic input data that should (by design) never be too big
     mem.set(tv_bytes(context.clipped_indices), clipped_indices);
 
-    const buffers = state.wasm.buffers;
-    const segment_count = state.wasm.exports.do_lod(
-        clipped_indices, context.clipped_indices.size, state.canvas.zoom,
-        buffers['coords_from'].offset,
-        buffers['line_threshold'].offset,
-        buffers['xs'].offset,
-        buffers['ys'].offset,
-        buffers['pressures'].offset,
-        buffers['xs'].used / 4,
+    // TODO: this is a very naive and dumb way of distributing work. Better way
+    // would be to distrubute strokes based on total point count, so that
+    // each worker gets approximately the same amout of _points_
+    const indices_per_thread = Math.floor(context.clipped_indices.size / state.wasm.workers.length);
+    const offsets = {
+        'coords_from': buffers['coords_from'].offset,
+        'line_threshold': buffers['line_threshold'].offset,
+        'xs': buffers['xs'].offset,
+        'ys': buffers['ys'].offset,
+        'pressures': buffers['pressures'].offset,
+        'result_buffers': result_buffers,
+        'result_counts': result_counts,
+    };
+
+    const jobs = [];
+
+    for (let i = 0; i < state.wasm.workers.length; ++i) {
+        let count = indices_per_thread;
+
+        if (i === state.wasm.workers.length - 1) {
+            count += context.clipped_indices.size % state.wasm.workers.length;
+        }
+
+        jobs.push({
+            'type': 'lod',
+            'indices_base': clipped_indices + i * 4 * indices_per_thread,
+            'indices_count': count,
+            'zoom': state.canvas.zoom,
+            'offsets': offsets
+        });
+    }
+
+    await workers_messages(state.wasm.workers, jobs);
+    const result_offset = state.wasm.exports.merge_results(
+        result_counts,
+        result_buffers,
+        state.wasm.workers.length
     );
 
-    // Use results without copying from WASM memory 
-    const result_offset = clipped_indices + context.clipped_indices.size * 4
-        + (context.clipped_indices.size + 1) * 4 + buffers['xs'].used;
+    const segment_count = new Int32Array(state.wasm.memory.buffer, result_counts, 1)[0]; // by convention
 
-    const wasm_points = new Float32Array(state.wasm.exports.memory.buffer, 
+    // Use results without copying from WASM memory 
+    const wasm_points = new Float32Array(state.wasm.memory.buffer, 
         result_offset, segment_count * 2);
-    const wasm_ids = new Uint32Array(state.wasm.exports.memory.buffer, 
+    const wasm_ids = new Uint32Array(state.wasm.memory.buffer, 
         result_offset + segment_count * 2 * 4, segment_count);
-    const wasm_pressures = new Uint8Array(state.wasm.exports.memory.buffer,
+    const wasm_pressures = new Uint8Array(state.wasm.memory.buffer,
         result_offset + segment_count * 2 * 4 + segment_count * 4, segment_count);
 
     context.instance_data_points.data = wasm_points;

client/wasm/compile_command

@@ -1 +1 @@
-clang -Oz --target=wasm32 -nostdlib -msimd128 -mbulk-memory -matomics -Wl,--no-entry,--import-memory,--shared-memory,--max-memory=$((1024 * 1024 * 1024)) -z stack-size=$((1024 * 1024)) multi.c -o multi.wasm  
+clang -Oz --target=wasm32 -nostdlib -msimd128 -mbulk-memory -matomics -Wl,--no-entry,--import-memory,--shared-memory,--export-all,--max-memory=$((1024 * 1024 * 1024)) -z stack-size=$((1024 * 1024)) lod.c -o lod.wasm  

client/wasm/lod.c

@@ -8,10 +8,11 @@ static int allocated_static;
 static int allocated_dynamic;
 
 void
-set_sp(void *sp)
+set_sp(char *sp)
 {
-    __asm__ volatile(
-        "local.get 0\n"
+    __asm__ __volatile__(
+        ".globaltype __stack_pointer, i32\n"
+        "local.get %0\n"
         "global.set __stack_pointer\n"
         : : "r"(sp)
     );
@@ -32,16 +33,26 @@ free_dynamic(void)
 void *
 alloc_static(int size)
 {
+    // This IS NOT thread-safe
     void *result = &__heap_base + allocated_static;
     allocated_static += size;
     return(result);
 }
 
+static int
+round_to_pow2(int value, int multiple)
+{
+    return((value + multiple - 1) & -multiple);
+}
+
 void *
 alloc_dynamic(int size)
 {
-    void *result = &__heap_base + allocated_static + allocated_dynamic;
-    allocated_dynamic += size;
+    // Very ad-van-ced thread-safe allocator
+    // CAN be called from multiple threads
+    size = round_to_pow2(size, 4);
+    int original_allocated_dynamic = __atomic_fetch_add(&allocated_dynamic, size, __ATOMIC_SEQ_CST);
+    void *result = &__heap_base + allocated_static + original_allocated_dynamic;
     return(result);
 }
 
@@ -183,21 +194,27 @@ rdp_find_max(float *xs, float *ys, unsigned char *pressures, float zoom, int coo
     return(result);
 }
 
-int
+void
 do_lod(int *clipped_indices, int clipped_count, float zoom, 
        int *stroke_coords_from,
        float *line_threshold, 
        float *xs,
        float *ys,
        unsigned char *pressures,
-       int coordinates_count)
+       char **result_buffer,
+       int *result_count)
 {
     if (clipped_count == 0) {
-        return(0);
+        result_count[0] = 0;
+        return;
     }
 
+    int first_stroke = clipped_indices[0];
+    int last_stroke = clipped_indices[clipped_count - 1];
+    int total_points = stroke_coords_from[last_stroke + 1] - stroke_coords_from[first_stroke];
+
     int *segments_from = alloc_dynamic((clipped_count + 1) * 4);
-    int *segments = alloc_dynamic(coordinates_count * 4);
+    int *segments = alloc_dynamic(total_points * 4); // TODO: this is a very conservative estimate, we can lower memory usage if we get this tighter
     
     int segments_head = 0;
     int stack[4096]; // TODO: what's a reasonable max size for this?
@@ -267,9 +284,11 @@ do_lod(int *clipped_indices, int clipped_count, float zoom,
     segments_from[clipped_count] = segments_head;
 
     // Write actual coordinates (points) and stroke ids
-    float *points = alloc_dynamic(segments_head * 2 * 4);
-    int *ids = alloc_dynamic(segments_head * 4);
-    unsigned char *pressures_res = alloc_dynamic(segments_head);
+    // Do this in one allocation so that they're not interleaved between threads
+    char *output = alloc_dynamic(segments_head * (3 * 4 + 1));
+    float *points = (float *) output;
+    int *ids = (int *) (output + segments_head * 4 * 2);
+    unsigned char *pressures_res = (unsigned char *) (output + segments_head * 4 * 3);
 
     int phead = 0;
     int ihead = 0;
@@ -300,5 +319,40 @@ do_lod(int *clipped_indices, int clipped_count, float zoom,
         }
     }
     
-    return(segments_head);
+    result_buffer[0] = output;
+    result_count[0] = segments_head;
+}
+
+// NOT thread-safe, only call from one thread
+char *
+merge_results(int *segment_counts, char **buffers, int nthreads)
+{
+    int total_segments = 0;
+
+    for (int i = 0; i < nthreads; ++i) {
+        total_segments += segment_counts[i];
+    }
+
+    char *merged = alloc_dynamic(total_segments * (3 * 4 + 1));
+    
+    float *points = (float *) merged;
+    int *ids = (int *) (merged + total_segments * 4 * 2);
+    unsigned char *pressures = (unsigned char *) (merged + total_segments * 4 * 3);
+
+    for (int i = 0; i < nthreads; ++i) {
+        int segments = segment_counts[i];
+        if (segments > 0) {
+            __builtin_memcpy(points, buffers[i], segments * 4 * 2);
+            __builtin_memcpy(ids, buffers[i] + segments * 4 * 2, segments * 4);
+            __builtin_memcpy(pressures, buffers[i] + segments * 4 * 3, segments);
+
+            points += segments * 2;
+            ids += segments;
+            pressures += segments;
+        }
+    }
+
+    segment_counts[0] = total_segments;
+
+    return(merged);
 }

client/wasm/multi.c

@@ -1,37 +0,0 @@
-#include <wasm_simd128.h>
-
-extern char __heap_base;
-
-static int allocated;
-
-void
-set_sp(char *sp)
-{
-    __asm__ __volatile__(
-        ".globaltype __stack_pointer, i32\n"
-        "local.get %0\n"
-        "global.set __stack_pointer\n"
-        : : "r"(sp)
-    );
-}
-
-void *
-alloc(int size)
-{
-    void *result = &__heap_base + allocated;
-    allocated += size;
-    return(result);
-}
-
-static void
-impl(int *buffer, int index, int number)
-{
-    buffer[index] = number;
-}
-
-void
-write_a_number(int *buffer, int index, int number)
-{
-    int n = number * 2;
-    impl(buffer, index, n);
-}

client/wasm_worker.js

@@ -1,40 +0,0 @@
-let thread_id = null;
-let buf_offset = null;
-let exports = null;
-let flags = null;
-
-function done() {
-    postMessage('done');
-}
-
-async function init_wasm(tid, memory, offset, notify_flags, stack_base) {
-    thread_id = tid;
-    buf_offset = offset;
-
-    const result = await WebAssembly.instantiateStreaming(fetch('wasm/multi.wasm'), {
-        env: { 'memory': memory }
-    });
-
-    exports = result.instance.exports;
-    //console.log(tid, 'init');
-    exports.set_sp(stack_base - thread_id * 4096);
-
-    flags = notify_flags;
-    done();
-}
-
-function do_work(num, callback) {
-    //console.log(thread_id, 'work');
-    exports.write_a_number(buf_offset, thread_id, thread_id * num); 
-    done();
-}
-
-onmessage = (e) => {
-    if (e.data.type === 0) {
-        init_wasm(e.data.thread_id, e.data.memory, e.data.buffer_offset, e.data.flags, e.data.stack_base);
-    } else if (e.data.type === 1) {
-        do_work(e.data.num);
-    }
-}
-
-

client/webgl_draw.js

@@ -1,7 +1,7 @@
 function schedule_draw(state, context) {
     if (!state.timers.raf) {
-        window.requestAnimationFrame(() => {
-            draw(state, context);
+        window.requestAnimationFrame(async () => {
+            await draw(state, context);
         });
         state.timers.raf = true;
     }
@@ -73,7 +73,7 @@ function draw_html(state) {
     }
 }
 
-function draw(state, context) {
+async function draw(state, context) {
     const cpu_before = performance.now();
 
     state.timers.raf = false;
@@ -89,22 +89,23 @@ function draw(state, context) {
         gl.beginQuery(context.gpu_timer_ext.TIME_ELAPSED_EXT, query);
     }
        
-    gl.viewport(0, 0, context.canvas.width, context.canvas.height);
-    gl.clearColor(context.bgcolor.r, context.bgcolor.g, context.bgcolor.b, 1);
-    gl.clearDepth(0.0);
-    gl.clear(gl.COLOR_BUFFER_BIT | gl.DEPTH_BUFFER_BIT);
-
     locations = context.locations['sdf'].main;
     buffers = context.buffers['sdf'];
 
-    gl.useProgram(context.programs['sdf'].main);
-
     bvh_clip(state, context); 
     
-    const segment_count = geometry_write_instances(state, context);
+    const segment_count = await geometry_write_instances(state, context);
     const dynamic_segment_count = context.dynamic_segment_count;
     const dynamic_stroke_count = context.dynamic_stroke_count;
 
+    // Only clear once we have the data, this might not always be on the same frame?
+    gl.viewport(0, 0, context.canvas.width, context.canvas.height);
+    gl.clearColor(context.bgcolor.r, context.bgcolor.g, context.bgcolor.b, 1);
+    gl.clearDepth(0.0);
+    gl.clear(gl.COLOR_BUFFER_BIT | gl.DEPTH_BUFFER_BIT);
+
+    gl.useProgram(context.programs['sdf'].main);
+
     // "Static" data upload
     if (segment_count > 0) {
         const total_static_size = context.instance_data_points.size * 4 +

client/webgl_geometry.js

@@ -37,11 +37,11 @@ function geometry_prepare_stroke(state) {
 }
 
 
-function geometry_write_instances(state, context) {
+async function geometry_write_instances(state, context, callback) {
     state.stats.rdp_max_count = 0;
     state.stats.rdp_segments = 0;
 
-    const segment_count = do_lod(state, context);
+    const segment_count = await do_lod(state, context);
 
     if (config.debug_print) console.debug('instances:', segment_count, 'rdp max:', state.stats.rdp_max_count, 'rdp segments:', state.stats.rdp_segments);