Hey Nathan,
Thanks for sharing, this is a very interesting post :) My comments are
inlined below.
Cheng
On 1/7/15 11:53 AM, Nathan McCarthy wrote:
Hi,
I’m trying to use a combination of SparkSQL and ‘normal' Spark/Scala
via rdd.mapPartitions(…). Using the latest release 1.2.0.
Simple example; load up some sample data from parquet on HDFS (about
380m rows, 10 columns) on a 7 node cluster.
val t = sqlC.parquetFile("/user/n/sales-tran12m.parquet”)
t.registerTempTable("test1”)
sqlC.cacheTable("test1”)
Now lets do some operations on it; I want the total sales & quantities
sold for each hour in the day so I choose 3 out of the 10 possible
columns...
sqlC.sql("select Hour, sum(ItemQty), sum(Sales) from test1 group by
Hour").collect().foreach(println)
After the table has been 100% cached in memory, this takes around 11
seconds.
Lets do the same thing but via a MapPartitions call (this isn’t
production ready code but gets the job done).
val try2 = sqlC.sql("select Hour, ItemQty, Sales from test1”)
rddPC.mapPartitions { case hrs =>
val qtySum = new Array[Double](24)
val salesSum = new Array[Double](24)
for(r <- hrs) {
val hr = r.getInt(0)
qtySum(hr) += r.getDouble(1)
salesSum(hr) += r.getDouble(2)
}
(salesSum zip qtySum).zipWithIndex.map(_.swap).iterator
}.reduceByKey((a,b) => (a._1 + b._1, a._2 +
b._2)).collect().foreach(println)
I believe the evil thing that makes this snippet much slower is the
for-loop. According to my early benchmark done with Scala 2.9, for-loop
can be orders of magnitude slower than a simple while-loop, especially
when the body of the loop only does something as trivial as this case.
The reason is that Scala for-loop is translated into corresponding
foreach/map/flatMap/withFilter function calls. And that's exactly why
Spark SQL tries to avoid for-loop or any other functional style code in
critical paths (where every row is touched), we also uses reusable
mutable row objects instead of the immutable version to improve
performance. You may check HiveTableScan, ParquetTableScan,
InMemoryColumnarTableScan etc. for reference. Also, the `sum` function
calls in your SQL code are translated into `o.a.s.s.execution.Aggregate`
operators, which also use imperative while-loop and reusable mutable rows.
Another thing to notice is that the `hrs` iterator physically points to
underlying in-memory columnar byte buffers, and the `for (r <- hrs) {
... }` loop actually decompresses and extracts values from required byte
buffers (this is the "unwrapping" processes you mentioned below).
Now this takes around ~49 seconds… Even though test1 table is 100%
cached. The number of partitions remains the same…
Now if I create a simple RDD of a case class HourSum(hour: Int, qty:
Double, sales: Double)
Convert the SchemaRDD;
val rdd = sqlC.sql("select * from test1").map{ r =>
HourSum(r.getInt(1), r.getDouble(7), r.getDouble(8)) }.cache()
//cache all the data
rdd.count()
Then run basically the same MapPartitions query;
rdd.mapPartitions { case hrs =>
val qtySum = new Array[Double](24)
val salesSum = new Array[Double](24)
for(r <- hrs) {
val hr = r.hour
qtySum(hr) += r.qty
salesSum(hr) += r.sales
}
(salesSum zip qtySum).zipWithIndex.map(_.swap).iterator
}.reduceByKey((a,b) => (a._1 + b._1, a._2 +
b._2)).collect().foreach(println)
This takes around 1.5 seconds! Albeit the memory footprint is much larger.
I guess this 1.5 seconds doesn't include the time spent on caching the
simple RDD? As I've explained above, in the first `mapPartitions` style
snippet, columnar byte buffer unwrapping happens within the
`mapPartitions` call. However, in this version, the unwrapping process
happens when the `rdd.count()` action is performed. At that point, all
values of all columns are extracted from underlying byte buffers, and
the portion of data you need are then manually selected and transformed
into the simple case class RDD via the `map` call.
If you include time spent on caching the simple case class RDD, it
should be even slower than the first `mapPartitions` version.
My thinking is that because SparkSQL does store things in a columnar
format, there is some unwrapping to be done out of the column array
buffers which takes time and for some reason this just takes longer
when I switch out to map partitions (maybe its unwrapping the entire
row, even though I’m using just a subset of columns, or maybe there is
some object creation/autoboxing going on when calling getInt or
getDouble)…
I’ve tried simpler cases too, like just summing sales. Running sum via
SQL is fast (4.7 seconds), running a mapPartition sum on a double RDD
is even faster (2.6 seconds). But MapPartitions on the SchemaRDD;
/sqlC.sql("select SalesInclGST from test1").mapPartitions(iter =>
Iterator(iter.foldLeft(0.0)((t,r) => t+r.getDouble(0)))).sum/
takes a long time (33 seconds). In all these examples everything is
fully cached in memory. And yes for these kinds of operations I can
use SQL, but for more complex queries I’d much rather be using a combo
of SparkSQL to select the data (so I get nice things like Parquet
pushdowns etc.) & functional Scala!
Again, unfortunately, functional style code like `Iterator.sum` and
`Iterator.foldLeft` can be really slow on critical paths.
I think I’m doing something dumb… Is there something I should be doing
to get faster performance on MapPartitions on SchemaRDDs? Is there
some unwrapping going on in the background that catalyst does in a
smart way that I’m missing?
It makes sense that people use both Spark SQL and Spark core, especially
when Spark SQL lacks features users need (like window function, for
now). The suggestion here is, if you really care about performance (more
than code readability and maintenance cost), then avoid immutable,
functional code whenever possible on any critical paths...
Cheers,
~N
Nathan McCarthy
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