valyala/fasthttp · warning

cannot compress data due to high load

Error message

cannot compress data due to high load

What it means

errHighLoad (unexported, in stackless/writer.go) is returned by the stackless compressor's do function when the system is under high load and it cannot afford to compress data. The stackless package conserves stack/allocations and backs off compression rather than exhausting resources.

Source

Thrown at stackless/writer.go:108

func (w *writer) do(op op) error {
	w.op = op
	if !stacklessWriterFunc(w) {
		return errHighLoad
	}
	err := w.err
	if err != nil {
		return err
	}
	if w.xw.bb != nil && len(w.xw.bb.B) > 0 {
		_, err = w.dstW.Write(w.xw.bb.B)
	}
	w.xw.Reset()

	return err
}

var errHighLoad = errors.New("cannot compress data due to high load")

var (
	stacklessWriterFuncOnce sync.Once
	stacklessWriterFuncFunc func(ctx any) bool
)

func stacklessWriterFunc(ctx any) bool {
	stacklessWriterFuncOnce.Do(func() {
		stacklessWriterFuncFunc = NewFunc(writerFunc)
	})
	return stacklessWriterFuncFunc(ctx)
}

func writerFunc(ctx any) {
	w := ctx.(*writer) //nolint:forcetypeassert
	switch w.op {
	case opWrite:
		w.n, w.err = w.zw.Write(w.p)

View on GitHub (pinned to c96f600972)

Solutions

1. Reduce overall load (throttle requests, add backpressure) so compression can proceed.
2. Disable or lower compression usage when under high load.
3. Retries: the error is transient; retry the write once load drops.

Example fix

// before
if err := sw.Flush(); err != nil { return err } // may be errHighLoad
// after
if err := sw.Flush(); err != nil {
    if errors.Is(err, errHighLoad) { /* shed load or retry later */ }
    return err
}

// no pre-validation possible: errHighLoad is produced internally under load
// mitigate by limiting concurrent compressing writers beforehand

if err := sw.Flush(); err != nil {
    if strings.Contains(err.Error(), "high load") {
        time.Sleep(backoff)
        // retry once load decreases
    }
    return err
}

When it happens

Trigger: Writing compressed output through the stackless writer while its load guard indicates high load; do() returns errHighLoad instead of compressing.

Common situations: Servers under heavy CPU/alloc pressure with compression enabled; small stacks / constrained environments where stackless compression backs off.

Related errors

• brotli: excessive input
• seek is not implemented
• fasthttp: unsupported content-encoding
• cannot create temporary file for %q: %w
• error when compressing file %q to %q: %w

AI-assisted analysis of valyala/fasthttp@c96f600972 (2026-08-31). Data as JSON: /api/errors/55e3b9f7a09ac2c6. Report an issue: GitHub.