add rate limit for local backend, parse limit args

[recall704]

What problem does this PR solve?

add rate limit for local backend

What is changed and how it works?

set rate limit for io.Writer

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[recall704]

pingcap/kvproto#756 PR required

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[kennytm]

Instead of adding a rate_limit field to local storage only, I suggest adapting to the existing --ratelimit parameter.

• Not good: -o 'local:///data/output?rate_limit=100'
• Good: --ratelimit 100 -o 'local:///data/output'

The rate limiter should introduce a wrapper of ExternalStorage similar to the withCompression type.

[recall704]

Instead of adding a rate_limit field to local storage only, I suggest adapting to the existing --ratelimit parameter.

• Not good: -o 'local:///data/output?rate_limit=100'
• Good: --ratelimit 100 -o 'local:///data/output'

The rate limiter should introduce a wrapper of ExternalStorage similar to the withCompression type.

type withRatelimit struct {
	ExternalStorage
	Ratelimit uint64 // Byte/sec
}

// WithRatelimit
func WithRatelimit(inner ExternalStorage, ratelimit uint64) ExternalStorage {
	if ratelimit == 0 {
		return inner
	}
	return &withRatelimit{ExternalStorage: inner, Ratelimit: ratelimit}
}

func (r *withRatelimit) Create(ctx context.Context, name string) (ExternalFileWriter, error) {
	var (
		writer ExternalFileWriter
		err    error
	)
	if localStorage, ok := r.ExternalStorage.(*LocalStorage); ok {
		file, err := os.Create(filepath.Join(localStorage.base, name))
		if err != nil {
			return nil, errors.Trace(err)
		}
		buf := bufio.NewWriter(file)
		if r.Ratelimit > 0 {
			w := shapeio.NewWriterWithContext(buf, ctx)
			w.SetRateLimit(float64(r.Ratelimit))
			return newFlushStorageWriter(w, buf, file), nil
		}
		return newFlushStorageWriter(buf, buf, file), nil
	}
	writer, err = r.ExternalStorage.Create(ctx, name)
	if err != nil {
		return nil, errors.Trace(err)
	}
	return writer, nil
}

I try to use withRatelimit to set rate limit, but I have to overwrite Create method, any idea?

thanks

[kennytm]

@recall704 sorry for late reply, there's a week-long holiday in China and Japan 🙃.

I try to use withRatelimit to set rate limit, but I have to overwrite Create method, any idea?

you could just return a wrapper type that implements ExternalFileWriter:

type withRateLimitWriter struct {
    ExternalFileWriter
    Ratelimit uint64
}

func (r *withRatelimit) Create(ctx context.Context, name string) (ExternalFileWriter, error) {
    inner, err := r.ExternalStorage.Create(ctx, name)
    if err != nil {
        return nil, err
    }
    return &withRateLimitWriter{
        ExternalFileWriter: inner,
        Ratelimit:          r.Ratelimit,
    }, nil
}

and then apply the Ratelimit in its Write method.

func (rw *withRateLimitWriter) Write(ctx context.Context, p []byte) (int, error) {
    // do rate limiting here.
}

There's no need to figure out how to fit this into shapeio's Reader/Writer API, since that package is just using golang.org/x/time/rate anyway. I suggest you use rate.Limiter directly instead of shapeio (allowing multiple writers from Create sharing the same limiter as a global rate limit).

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