Use Async cache to avoid cache contention #7156

priyanshukm · 2024-06-13T14:47:24Z

Several stack trace from our service shows blocked threads on this cache when trying to getRows. Generally these are during service restart as the cache is cold.

As per caffeine documentation:

Some attempted update operations on this cache by other threads may be blocked while the computation is in 
progress, so the computation should be short and simple, and must not attempt to update any other mappings
of this cache.

The async loading cache will have internally store a CompletableFuture in the ConcurrentHashMap to avoid locking inside the ConcurrentHashMap and blocking concurrent accesses to nodes in the same bucket.

Initial capacity impacts the number of buckets in the internal ConcurrentHashMap used by the cache. We want to increase the minimum to reduce contention under heavy concurrent access. This will avoid situations where threads that ask for cache values are blocked due to resizing of the underlying hashMap

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==COMMIT_MSG==
Use Async cache to avoid cache contention
==COMMIT_MSG==

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Several stack trace from our service shows blocked threads on this cache when trying to getRows. Generally these are during service restart as the cache is cold. The async loading cache will have internally store a CompletableFuture in the ConcurrentHashMap to avoid locking inside the ConcurrentHashMap and blocking concurrent accesses to nodes in the same bucket. Initial capacity impacts the number of buckets in the internal ConcurrentHashMap used by the cache. We want to increase the minimum to reduce contention under heavy concurrent access. This will avoid situations where threads that ask for cache values are blocked due to resizing of the underlying hashMap

changelog-app · 2024-06-13T14:47:28Z

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Use Async cache to avoid cache contention

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priyanshukm · 2024-06-13T22:03:22Z

...sdb-client/src/main/java/com/palantir/atlasdb/transaction/impl/ConflictDetectionManager.java

@@ -39,7 +39,11 @@ public class ConflictDetectionManager {
     *  (This has always been the behavior of this class; I'm simply calling it out)
     */
    public ConflictDetectionManager(CacheLoader<TableReference, ConflictHandler> loader) {
-        this.cache = Caffeine.newBuilder().maximumSize(100_000).build(loader);
+        this.cache = Caffeine.newBuilder()


Seems like the tests expects the loader to even load null values while async cache does not allow that, because of which the test is failing.

Throws: NullPointerException – if the specified key is null or if the future returned by the AsyncCacheLoader is null

Having null ConflictHandler is an expected state ?

schlosna · 2024-06-17T16:50:38Z

...sdb-client/src/main/java/com/palantir/atlasdb/transaction/impl/ConflictDetectionManager.java

+        this.cache = Caffeine.newBuilder()
+                .initialCapacity(256)
+                .maximumSize(100_000)
+                .buildAsync(loader)
+                .synchronous();


Can you share the original JFR or other traces indicating we're seeing contention here? I'm a little surprised as I would expect most writes to these caches occur via warmCacheWith in com.palantir.atlasdb.transaction.impl.ConflictDetectionManagers#create(com.palantir.atlasdb.keyvalue.api.KeyValueService, boolean) via a single thread for that ConflictDetectionManager instance, those writes are all serialized so there should not contend.

If we want the async loading, are we concerned that not providing an executor will cause saturation of the default executor ForkJoinPool.commonPool()?

@priyanshukm are you able to provide the above?

Hey provided the JFR in the slack thread as it could be unsafe. Will tag you there too.

@schlosna @priyanshukm @ergo14 - what's the status on this one? Do you believe it valuable to continue reviewing, or is the issue solved some other way?

Main thing I had in mind was to check if things improved after #7224

priyanshukm added 2 commits June 13, 2024 15:48

update initial capacity as there will likely we less unique keys

40ff8bb

fix lint

cd04a63

priyanshukm commented Jun 13, 2024

View reviewed changes

schlosna reviewed Jun 17, 2024

View reviewed changes

priyanshukm mentioned this pull request Aug 6, 2024

Update intial capacity to avoid cache contention #7224

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Use Async cache to avoid cache contention #7156

Use Async cache to avoid cache contention #7156

priyanshukm commented Jun 13, 2024

changelog-app bot commented Jun 13, 2024

priyanshukm Jun 13, 2024

schlosna Jun 17, 2024 •

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priyanshukm Jul 10, 2024

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ergo14 Sep 16, 2024

Use Async cache to avoid cache contention #7156

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Use Async cache to avoid cache contention #7156

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priyanshukm commented Jun 13, 2024

General

Compatibility

Testing and Correctness

Execution

Scale

Development Process

changelog-app bot commented Jun 13, 2024

Generate changelog in changelog/@unreleased

priyanshukm Jun 13, 2024

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schlosna Jun 17, 2024 • edited Loading

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Generate changelog in `changelog/@unreleased`

schlosna Jun 17, 2024 •

edited

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