Use actual number of input splits when calculating target throughput per task

[anmsf]

Right now, it looks like the lib uses the max number of possible tasks for calculating target throughput per task:

emr-dynamodb-connector/emr-dynamodb-hadoop/src/main/java/org/apache/hadoop/dynamodb/write/WriteIopsCalculator.java

Line 82 in ee52fdf

long throughputPerTask = Math.max((long) (calculatedThroughput / maxParallelTasks), 1);

maxParallelTasks = Math.min(calculateMaxMapTasks(totalMapTasks), totalMapTasks);

But for provisioned mode with autoscaling and low table minimum throughput [say, 100] it ends up significantly underestimating per task throughput for smaller inputs. This works out okay when there's a large enough input so actual mappers is similar to max mappers, but for smaller inputs it can get stuck at a low throughput and never trigger autoscale.

We've seen larger inputs trigger autoscaling all the way up to 500k which is what we want. But for smaller inputs we saw a case where it kept on writing at 1 task/minute since there were just 8 tasks instead of the max 48 the library assumes so we never hit the autoscale threshold.

Is it possible to look at the actual number of splits instead?