Tim Armstrong created IMPALA-8203:
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Summary: 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
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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