This adds a thread-level collector of IOStatistics, IOStatisticsContext,
which can be:
* Retrieved for a thread and cached for access from other
threads.
* reset() to record new statistics.
* Queried for live statistics through the
IOStatisticsSource.getIOStatistics() method.
* Queries for a statistics aggregator for use in instrumented
classes.
* Asked to create a serializable copy in snapshot()
The goal is to make it possible for applications with multiple
threads performing different work items simultaneously
to be able to collect statistics on the individual threads,
and so generate aggregate reports on the total work performed
for a specific job, query or similar unit of work.
Some changes in IOStatistics-gathering classes are needed for
this feature
* Caching the active context's aggregator in the object's
constructor
* Updating it in close()
Slightly more work is needed in multithreaded code,
such as the S3A committers, which collect statistics across
all threads used in task and job commit operations.
Currently the IOStatisticsContext-aware classes are:
* The S3A input stream, output stream and list iterators.
* RawLocalFileSystem's input and output streams.
* The S3A committers.
* The TaskPool class in hadoop-common, which propagates
the active context into scheduled worker threads.
Collection of statistics in the IOStatisticsContext
is disabled process-wide by default until the feature
is considered stable.
To enable the collection, set the option
fs.thread.level.iostatistics.enabled
to "true" in core-site.xml;
Contributed by Mehakmeet Singh and Steve Loughran
363f8138d2