Hadoop Metrics2 should emit Float.MAX_VALUE (instead of Double.MAX_VALUE) to
avoid making Ganglia's gmetad core
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Key: HADOOP-8052
URL: https://issues.apache.org/jira/browse/HADOOP-8052
Project: Hadoop Common
Issue Type: Bug
Components: metrics
Affects Versions: 1.0.0, 0.23.0
Reporter: Varun Kapoor
Assignee: Varun Kapoor
Ganglia's gmetad converts the doubles emitted by Hadoop's Metrics2 system to
strings, and the buffer it uses is 256 bytes wide.
When the SampleStat.MinMax class (in org.apache.hadoop.metrics2.util) emits its
default min value (currently initialized to Double.MAX_VALUE), it ends up
causing a buffer overflow in gmetad, which causes it to core, effectively
rendering Ganglia useless (for some, the core is continuous; for others who are
more fortunate, it's only a one-time Hadoop-startup-time thing).
The fix needed to Ganglia is simple - the buffer needs to be bumped up to be
512 bytes wide, and all will be well - but instead of requiring a minimum
version of Ganglia to work with Hadoop's Metrics2 system, it might be more
prudent to just use Float.MAX_VALUE.
An additional problem caused in librrd (which Ganglia uses beneath-the-covers)
by the use of Double.MIN_VALUE (which functions as the default max value) is an
underflow when librrd runs the received strings through libc's strtod(), but
the librrd code is good enough to check for this, and only emits a warning -
moving to Float.MIN_VALUE fixes that as well.
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