Troubleshooting RESOURCE_SEMAPHORE_QUERY_COMPILE Waits In SQL Server

Posted on by Erik Darling

Unfortunate

RESOURCE_SEMAPHORE_QUERY_COMPILE happens, in a nutshell, when SQL Server has allocated all the memory it’s willing to give out to compile query plans of a certain size and, throttles itself by making other queries wait to compile. For more details, head over here.

Now, this of course gets easier if you’re using Query Store. You can get heaps of information about query compilation from query_store_query. For everyone else, you’re left dipping into the plan cache to try to find queries with “high” compile memory. That can be hit or miss, of course.

But if it’s something you really find yourself needing to track down, here’s one way to do it:

WITH XMLNAMESPACES('http://schemas.microsoft.com/sqlserver/2004/07/showplan' AS p)
SELECT TOP (10) 
    x.compile_time_ms,
    x.compile_cpu_ms,
    x.compile_memory_kb,
    x.max_compile_memory_kb,
    x.is_memory_exceeded,
    x.query_plan
FROM
(
    SELECT
        c.x.value('@CompileTime', 'BIGINT') AS compile_time_ms,
        c.x.value('@CompileCPU', 'BIGINT') AS compile_cpu_ms,
        c.x.value('@CompileMemory', 'BIGINT') AS compile_memory_kb,
        c.x.value('(//p:OptimizerHardwareDependentProperties/@MaxCompileMemory)[1]', 'BIGINT') AS max_compile_memory_kb,
        c.x.exist('//p:StmtSimple/@StatementOptmEarlyAbortReason[.="MemoryLimitExceeded"]') AS is_memory_exceeded,
        deqp.query_plan
    FROM sys.dm_exec_cached_plans AS decp
    CROSS APPLY sys.dm_exec_query_plan(decp.plan_handle) AS deqp
    CROSS APPLY deqp.query_plan.nodes('/p:ShowPlanXML/p:BatchSequence/p:Batch/p:Statements/p:StmtSimple/p:QueryPlan') AS c(x)
    WHERE c.x.exist('@CompileMemory[. > 5120]') = 1
) AS x
ORDER BY x.compile_memory_kb DESC;

This query is filtering for plans with compile memory over 5MB. I set the bar pretty low there, but feel free to raise it up.

If you want to look at gateway info, and you’re on SQL Server 2016 or newer, you can use this DMV:

SELECT *
FROM sys.dm_exec_query_optimizer_memory_gateways AS deqomg
WHERE deqomg.pool_id > 1;

Scoping It Out

It only makes sense to run that query if you’re hitting RESOURCE_SEMAPHORE_QUERY_COMPILE wait with some frequency.

If you are, you just may be lucky enough to find the culprit, if your plan cache has been around long enough.

Thanks for reading!

Going Further

If this is the kind of SQL Server stuff you love learning about, you’ll love my training. I’m offering a 75% discount to my blog readers if you click from here. I’m also available for consulting if you just don’t have time for that and need to solve performance problems quickly.

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