gianm commented on a change in pull request #8116: remove unnecessary lock in
ForegroundCachePopulator leading to a lot of contention
URL: https://github.com/apache/incubator-druid/pull/8116#discussion_r306044913
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File path:
server/src/main/java/org/apache/druid/client/cache/BackgroundCachePopulator.java
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@@ -38,6 +38,14 @@
import java.util.concurrent.ExecutorService;
import java.util.function.Function;
+/**
+ * {@link CachePopulator} implementation that uses a {@link ExecutorService}
thread pool to populate a cache in the
+ * background. Used if config "druid.*.cache.numBackgroundThreads" is greater
than 0. If maximum cache entry size,
+ * specified by config "druid.*.cache.maxEntrySize", is exceeded, this {@link
CachePopulator} implementation will
+ * be a bit less efficient than {@link ForegroundCachePopulator} which can
stop retaining values early, in exchange
Review comment:
This comment has some details -
https://github.com/apache/incubator-druid/pull/8116#discussion_r306041975
I do think it's too inside-baseball for user-facing docs, and maybe even for
javadocs. It's referring to the fact that the foreground populator avoids some
work, relative to the background populator, in the case that maxEntrySize is
exceeded. In particular, when building a cache entry from a `Sequence<T>`, the
background populator will run `cacheFn` on all `T` first, and then when they're
all done it will start serializing them one by one. At that point, if
maxEntrySize is exceeded, it will throw the rest away and exit early without
populating the cache. But it still went through the effort of computing
`cacheFn` on the `T`s that it threw away. The foreground populator, on the
other hand, does not do that.
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