MEMORY_ALLOCATION_EXT is recorded when a thread switches to preemptive mode while allocating memory from the OS or from SQL Server’s internal allocators. The switch exists so the allocation code does not have to keep checking whether the thread’s scheduling quantum has run out; the thread steps outside cooperative scheduling, allocates, and steps back in.
Memory allocations happen constantly, so this wait appears on essentially every SQL Server instance, usually with a huge count of very short waits.
Is It a Problem?
Almost never. Its normal profile is millions of occurrences averaging microseconds, which is background hum, not contention. Wait counts scare people here; look at the average duration instead.
The known exception is niche: workloads hammering specific allocation-heavy code paths, for example heavy HASHBYTES usage on SQL Server 2017, have shown this wait as a measurable bottleneck. If average wait times climb and total time reaches your top waits, some code path is allocating far too often.
Common Causes
- Normal engine operation: query execution, caching, and internal structures allocating memory continuously.
- Allocation-heavy functions called at extreme frequency (the SQL Server 2017
HASHBYTEScase is the documented example). - Severe memory pressure making allocations slower and more frequent as caches shrink and rebuild.
What To Do
- Check the average: total wait time divided by count. Microseconds means ignore it and move on.
- If it genuinely ranks with meaningful average durations, look for the pattern: which queries or functions run at very high frequency, and whether memory pressure indicators (page life expectancy dips,
RESOURCE_SEMAPHOREwaits) accompany it. - Address the source: cache results of expensive deterministic functions, reduce the calling frequency, or fix the memory pressure with proper max server memory sizing.
- Patch level matters for the known edge cases; check release notes if a specific function correlates.
How To See It
Rank it against everything else with Get-WaitStatistics, and judge it strictly on avg_wait_ms, not the eye-catching count.
Part of the SQL Server Wait Types Library.
Related deep dive: RESOURCE_SEMAPHORE Wait Type.
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