Why is My Entity Framework Async Operation 10x Slower Than Its Synchronous Counterpart?

Entity Framework Async: A Significant Performance Bottleneck

This article details a performance issue encountered while using Entity Framework's asynchronous methods. The author discovered a tenfold increase in execution time for asynchronous queries compared to their synchronous counterparts.

The Problem: Synchronous queries completed within seconds, while their asynchronous equivalents took ten times longer.

Debugging the Issue: SQL Server Profiler confirmed identical SQL queries for both synchronous and asynchronous calls. Further investigation revealed the culprit: the asynchronous operation spawned over two million tasks and incurred significant overhead.

Root Cause: The problem stemmed from a bug in Entity Framework 6's asynchronous implementation. When dealing with tables containing large binary columns, the framework should automatically utilize the CommandBehavior.SequentialAccess flag in asynchronous calls. This crucial optimization was missing.

Resolution: The author suggests bypassing Entity Framework's built-in asynchronous methods. A workaround using TaskCompletionSource allows for manual asynchronous execution, ensuring CommandBehavior.SequentialAccess is applied correctly for tables with large binary data.

Key Observations:

• Ado.Net exhibits similar performance degradation with asynchronous operations on tables containing large binary columns and CommandBehavior.Default.
• The performance difference was most noticeable with large binary columns (256 KB in the author's tests).
• CPU usage patterns were unusual: the synchronous operation used 12% CPU, while the asynchronous operation peaked at 20%, hinting at potential resource contention.

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