RESERVED_MEMORY_ALLOCATION_EXT is recorded when a thread switches to preemptive mode while allocating memory that was already reserved for its query, in other words, drawing down its execution memory grant. The preemptive switch spares the allocation code from checking scheduling quantum exhaustion, the same mechanism as MEMORY_ALLOCATION_EXT, just against granted rather than general memory.
Every sort, hash, and columnstore operation that uses its grant produces some of this, so like its sibling it is everywhere in small doses.
Is It a Problem?
Usually not; the standard profile is a large count of tiny waits. It earns attention in one known pattern: large concurrent columnstore operations, big clustered columnstore index ETL loads especially, where contention on memory allocation from grants has been shown to limit scaling. If your columnstore load throughput plateaus while this wait climbs, you are probably in that territory.
Outside that pattern, treat it as allocation background noise and verify by average duration.
Common Causes
- Normal grant consumption by sorts, hashes, and batch-mode operators.
- Many concurrent memory-hungry operations (columnstore builds and loads) fighting over allocation paths.
- Very large grants being consumed in bursts during ETL windows.
What To Do
- Compute the average wait duration; microseconds means it is noise regardless of the total.
- For columnstore ETL scaling problems: reduce concurrency of simultaneous large loads, or rework the load to use less memory per operation (smaller batches, staged loads).
- Large page support (trace flag 834, with locked pages configured) has been used as a workaround for this specific bottleneck, but it changes buffer pool allocation behaviour globally; test carefully and only for dedicated workloads that demonstrably need it. Note it is not supported when columnstore is combined with certain features, so check current guidance first.
- Confirm memory grants are sane in
sys.dm_exec_query_memory_grants; oversized grants amplify all allocation costs.
How To See It
Rank it against everything else with Get-WaitStatistics, and correlate spikes with your ETL and columnstore maintenance schedule.
Part of the SQL Server Wait Types Library.
Related deep dive: RESOURCE_SEMAPHORE Wait Type.
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