## What changes were proposed in this pull request?
* Implement (optional) use of KryoPool in KryoSerializer, an alternative to the existing implementation of caching a Kryo instance inside KryoSerializerInstance
* Add config key & documentation of spark.kryo.pool in order to turn this on
* Add benchmark KryoSerializerBenchmark to compare new and old implementation
* Add results of benchmark
## How was this patch tested?
Added new tests inside KryoSerializerSuite to test the pool implementation as well as added the pool option to the existing regression testing for SPARK-7766
This is my original work and I license the work to the project under the project’s open source license.
Closes#22855 from patrickbrownsync/kryo-pool.
Authored-by: Patrick Brown <patrick.brown@blyncsy.com>
Signed-off-by: Sean Owen <sean.owen@databricks.com>
## What changes were proposed in this pull request?
Before the code changes, I tried to run it with 8G memory:
```
build/sbt -mem 8000 "core/testOnly org.apache.spark.serializer.KryoBenchmark"
```
Still I got got OOM.
This is because the lengths of the arrays are random
669ade3a8e/core/src/test/scala/org/apache/spark/serializer/KryoBenchmark.scala (L90-L91)
And the 2D array is usually large: `10000 * Random.nextInt(0, 10000)`
This PR is to fix it and refactor it to use main method.
The benchmark result is also reason compared to the original one.
## How was this patch tested?
Run with
```
bin/spark-submit --class org.apache.spark.serializer.KryoBenchmark core/target/scala-2.11/spark-core_2.11-3.0.0-SNAPSHOT-tests.jar
```
and
```
SPARK_GENERATE_BENCHMARK_FILES=1 build/sbt "core/test:runMain org.apache.spark.serializer.KryoBenchmark"
Closes#22663 from gengliangwang/kyroBenchmark.
Authored-by: Gengliang Wang <gengliang.wang@databricks.com>
Signed-off-by: Sean Owen <sean.owen@databricks.com>
