[GitHub] [spark] kiszk commented on a change in pull request #24028: [SPARK-26917][SQL] Further reduce locks in CacheManager

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kiszk commented on a change in pull request #24028: [SPARK-26917][SQL] Further reduce locks in CacheManager
URL: https://github.com/apache/spark/pull/24028#discussion_r268476368
 
 

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 File path: sql/core/src/main/scala/org/apache/spark/sql/execution/CacheManager.scala
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 @@ -194,46 +167,36 @@ class CacheManager extends Logging {
   private def recacheByCondition(
       spark: SparkSession,
       condition: CachedData => Boolean): Unit = {
-    val needToRecache = scala.collection.mutable.ArrayBuffer.empty[CachedData]
-    readLock {
-      val it = cachedData.iterator()
-      while (it.hasNext) {
-        val cd = it.next()
-        if (condition(cd)) {
-          needToRecache += cd
-        }
-      }
+    val needToRecache = cachedData.filter(condition)
+    this.synchronized {
+      // Remove the cache entry before creating a new ones.
+      cachedData = cachedData.filterNot(cd => needToRecache.exists(_ eq cd))
 
 Review comment:
   I talked with @maropu. Since write to a `volatile` reference is [atomic](https://docs.oracle.com/javase/specs/jls/se8/html/jls-17.html#jls-17.7), I think that we could remove `this.sychronization` if we would allow this operation `cachedData.filterNot(cd => needToRecache.exists(_ eq cd))` to run on multiple threads.
   
   Since the performance bottleneck was in read lock, I do not recommend to apply this change very soon.
   

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