ValueTask may seem like a micro-optimization, but it is very important on hot paths, and easy to miss. Last week I’ve learned this the hard way.

Async on Hot Path

While working on Data Streamer for Apache Ignite 3, I’ve added AddWithRetryUnmapped wrapper around synchronous Add method to deal with schema updates:

await foreach (var item in data)
{    
    await AddWithRetryUnmapped(item); // was: Add(item);
}

async Task<(Batch<T> Batch, string Partition)> AddWithRetryUnmapped(T item)
{
    try
    {
        return Add(item);
    }
    catch (Exception e) when (e.CausedByUnmappedColumns())
    {
        schema = await schemaProvider(Table.SchemaVersionForceLatest).ConfigureAwait(false);
        return Add(item);
    }
}

This fairly simple change resulted in a 3x memory allocation increase in a basic streamer benchmark (100_000 items).

|                    Method | ServerCount |      Mean | Allocated |
|     DataStreamer (before) |           4 |  32.64 ms |      4 MB |
|     DataStreamer (after)  |           4 |  36.41 ms |     12 MB |

Luckily, the fix is just as simple: replacing Task with ValueTask in the method signature brings the performance back to normal.

• Schema change is a rare scenario, so AddWithRetryUnmapped will complete synchronously most of the time
• Add is being called in a loop for thousands of items, so even a small allocation adds up quickly

ValueTask has almost no overhead when the method completes synchronously - it is just a small struct, so the addition of AddWithRetryUnmapped does not affect performance when there are no schema changes.

Conclusion

• Benchmark every change to catch performance regressions
• Use ValueTask when there is any chance that the method will complete synchronously

Links

• Understanding the Whys, Whats, and Whens of ValueTask
• The performance characteristics of async methods in C#