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https://issues.apache.org/jira/browse/IMPALA-8203?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Work on IMPALA-8203 started by Alex Rodoni.
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> Revisit disable_codegen docs
> ----------------------------
>
> Key: IMPALA-8203
> URL: https://issues.apache.org/jira/browse/IMPALA-8203
> Project: IMPALA
> Issue Type: Improvement
> Components: Docs
> Affects Versions: Impala 3.1.0
> Reporter: Tim Armstrong
> Assignee: Alex Rodoni
> Priority: Minor
>
> This text is confusing in various ways, someone pointed out to me.
> * We don't need to specifically reference crashes - mostly it's used as a
> workaround for performance problems. I think we should just say that it's
> used to work around any issues with Impala's runtime code generation.
> * We should say clearly that most queries will run significantly slower with
> DISABLE_CODEGEN=true. The current text says this but it's hard to parse
> because of the implied double negative "do not run with the setting turned
> on", where "turned on" means "not disabled".
> * It's still worth mentioning that some short-running queries may run faster
> with DISABLE_CODEGEN=true because of the overhead, but we shouldn't actively
> suggest users to tweak it in general. In 2.10+, the
> DISABLE_CODEGEN_ROWS_THRESHOLD optimisation automatically disables codegen
> for small queries to solve this automatically.
> {noformat}
> DISABLE_CODEGEN Query Option
> This is a debug option, intended for diagnosing and working around issues
> that cause crashes. If a query fails with an "illegal instruction" or other
> hardware-specific message, try setting DISABLE_CODEGEN=true and running the
> query again. If the query succeeds only when the DISABLE_CODEGEN option is
> turned on, submit the problem to <support channel> and include that detail in
> the problem report. Do not otherwise run with this setting turned on, because
> it results in lower overall performance.
> Because the code generation phase adds a small amount of overhead for each
> query, you might turn on the DISABLE_CODEGEN option to achieve maximum
> throughput when running many short-lived queries against small tables.
> Type: Boolean; recognized values are 1 and 0, or true and false; any other
> value interpreted as false
> Default: false (shown as 0 in output of SET statement)
> {noformat}
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