wasm: memory leak

[falconandy]

Originally I met the issue in browser environment. Below is a simplified example with wazero.

tinygo version 0.35.0 linux/amd64 (using go version go1.23.5 and LLVM version 18.1.2)

Test repo: https://github.com/falconandy/tinygo-wasm-leak

Wasm lib:

package main

import (
	_ "embed"
	"fmt"
	"runtime"
)

//go:wasmexport processData
func processData() {
	N := 1 * 1024 * 1024
	data := make([]byte, N)
	fmt.Println(len(data))
}

//go:wasmexport printMemUsage
func printMemUsage() {
	runtime.GC()

	var m runtime.MemStats
	runtime.ReadMemStats(&m)
	fmt.Printf("runtime.MemStats: Alloc = %v MiB, TotalAlloc = %v MiB, Sys = %v MiB\n", bToMb(m.Alloc), bToMb(m.TotalAlloc), bToMb(m.Sys))
}

func bToMb(b uint64) uint64 {
	return b / 1024 / 1024
}

Command to build: tinygo build -target=wasip1 -o ./leak/leak.wasm --no-debug -scheduler=none -buildmode=c-shared ./leak

Test code:

package main

import (
	"context"
	_ "embed"
	"fmt"
	"log"
	"os"

	"github.com/tetratelabs/wazero"
	"github.com/tetratelabs/wazero/api"
	"github.com/tetratelabs/wazero/imports/wasi_snapshot_preview1"
)

//go:embed leak/leak.wasm
var leakWasm []byte

func main() {
	ctx := context.Background()

	r := wazero.NewRuntime(ctx)
	defer r.Close(ctx)

	wasi_snapshot_preview1.MustInstantiate(ctx, r)

	mod, err := r.InstantiateWithConfig(ctx, leakWasm, wazero.NewModuleConfig().WithStartFunctions("_initialize").WithStdout(os.Stdout))
	if err != nil {
		log.Fatal(err)
	}

	processData := mod.ExportedFunction("processData")
	printMemUsage := mod.ExportedFunction("printMemUsage")

	_, err = printMemUsage.Call(ctx)
	if err != nil {
		log.Fatal(err)
	}

	fmt.Println("mod.Memory().Size():", bToMb(mod.Memory().Size()), "MiB")

	for i := range 10 {
		fmt.Println("STEP:", i+1)
		step(ctx, mod, processData, printMemUsage)
	}
}

func step(ctx context.Context, mod api.Module, processData, printMemUsage api.Function) {
	_, err := processData.Call(ctx)
	if err != nil {
		log.Fatal(err)
	}

	_, err = printMemUsage.Call(ctx)
	if err != nil {
		log.Fatal(err)
	}

	fmt.Println("mod.Memory().Size():", bToMb(mod.Memory().Size()), "MiB")
}

func bToMb(b uint32) uint32 {
	return b / 1024 / 1024
}

Output - memory usage only grows. A leak (10 * 1 megabytes)?

runtime.MemStats: Alloc = 0 MiB, TotalAlloc = 0 MiB, Sys = 0 MiB
mod.Memory().Size(): 0 MiB
STEP: 1
1048576
runtime.MemStats: Alloc = 1 MiB, TotalAlloc = 1 MiB, Sys = 1 MiB
mod.Memory().Size(): 2 MiB
STEP: 2
1048576
runtime.MemStats: Alloc = 2 MiB, TotalAlloc = 2 MiB, Sys = 3 MiB
mod.Memory().Size(): 4 MiB
STEP: 3
1048576
runtime.MemStats: Alloc = 3 MiB, TotalAlloc = 3 MiB, Sys = 3 MiB
mod.Memory().Size(): 4 MiB
STEP: 4
1048576
runtime.MemStats: Alloc = 4 MiB, TotalAlloc = 4 MiB, Sys = 7 MiB
mod.Memory().Size(): 8 MiB
STEP: 5
1048576
runtime.MemStats: Alloc = 5 MiB, TotalAlloc = 5 MiB, Sys = 7 MiB
mod.Memory().Size(): 8 MiB
STEP: 6
1048576
runtime.MemStats: Alloc = 6 MiB, TotalAlloc = 6 MiB, Sys = 7 MiB
mod.Memory().Size(): 8 MiB
STEP: 7
1048576
runtime.MemStats: Alloc = 7 MiB, TotalAlloc = 7 MiB, Sys = 7 MiB
mod.Memory().Size(): 8 MiB
STEP: 8
1048576
runtime.MemStats: Alloc = 8 MiB, TotalAlloc = 8 MiB, Sys = 15 MiB
mod.Memory().Size(): 16 MiB
STEP: 9
1048576
runtime.MemStats: Alloc = 9 MiB, TotalAlloc = 9 MiB, Sys = 15 MiB
mod.Memory().Size(): 16 MiB
STEP: 10
1048576
runtime.MemStats: Alloc = 10 MiB, TotalAlloc = 10 MiB, Sys = 15 MiB
mod.Memory().Size(): 16 MiB

If I run the code locally (a unit test, with removed go:wasmexport directives) - no leaks (TotalAlloc is 0 for some reason).

func TestNativeLeak(t *testing.T) {
	printMemUsage()

	for i := range 10 {
		fmt.Println("STEP:", i+1)
		processData()
		printMemUsage()
	}
}

tinygo test -v ./leak/

runtime.MemStats: Alloc = 0 MiB, TotalAlloc = 0 MiB, Sys = 0 MiB
STEP: 1
1048576
runtime.MemStats: Alloc = 0 MiB, TotalAlloc = 0 MiB, Sys = 0 MiB
STEP: 2
1048576
runtime.MemStats: Alloc = 0 MiB, TotalAlloc = 0 MiB, Sys = 0 MiB
STEP: 3
1048576
runtime.MemStats: Alloc = 0 MiB, TotalAlloc = 0 MiB, Sys = 0 MiB
STEP: 4
1048576
runtime.MemStats: Alloc = 0 MiB, TotalAlloc = 0 MiB, Sys = 0 MiB
STEP: 5
1048576
runtime.MemStats: Alloc = 0 MiB, TotalAlloc = 0 MiB, Sys = 0 MiB
STEP: 6
1048576
runtime.MemStats: Alloc = 0 MiB, TotalAlloc = 0 MiB, Sys = 0 MiB
STEP: 7
1048576
runtime.MemStats: Alloc = 0 MiB, TotalAlloc = 0 MiB, Sys = 0 MiB
STEP: 8
1048576
runtime.MemStats: Alloc = 0 MiB, TotalAlloc = 0 MiB, Sys = 0 MiB
STEP: 9
1048576
runtime.MemStats: Alloc = 0 MiB, TotalAlloc = 0 MiB, Sys = 0 MiB
STEP: 10
1048576
runtime.MemStats: Alloc = 0 MiB, TotalAlloc = 0 MiB, Sys = 0 MiB

[ydnar]

If you run it 100x or 500x, you’ll see the memory usage top out. The GC is working.

…
STEP: 497
1048576
runtime.MemStats: Alloc = 24 MiB, TotalAlloc = 497 MiB, Sys = 31 MiB
mod.Memory().Size(): 32 MiB
STEP: 498
1048576
runtime.MemStats: Alloc = 23 MiB, TotalAlloc = 498 MiB, Sys = 31 MiB
mod.Memory().Size(): 32 MiB
STEP: 499
1048576
runtime.MemStats: Alloc = 23 MiB, TotalAlloc = 499 MiB, Sys = 31 MiB
mod.Memory().Size(): 32 MiB
STEP: 500
1048576
runtime.MemStats: Alloc = 23 MiB, TotalAlloc = 500 MiB, Sys = 31 MiB
mod.Memory().Size(): 32 MiB

[falconandy]

@ydnar

If you run it 100x or 500x, you’ll see the memory usage top out. The GC is working.

Probably it depends on allocated size. If I increase N to 10*1024*1024, the wasm version still leaks (native 'tinygo test' - no leaks):

...
STEP: 201
10485760
runtime.MemStats: Alloc = 2010 MiB, TotalAlloc = 2010 MiB, Sys = 2047 MiB
mod.Memory().Size(): 2048 MiB
STEP: 202
2025/03/10 09:19:47 wasm error: out of bounds memory access
wasm stack trace:
        main.runtime.growHeap() i32
        main.runtime.alloc(i32,i32) i32
        main.main.processData#wasmexport()
exit status 1

tinygo version 0.36.0 linux/amd64 (using go version go1.24.1 and LLVM version 19.1.2)