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https://issues.apache.org/jira/browse/CASSANDRA-20226?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=18036651#comment-18036651
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Dmitry Konstantinov edited comment on CASSANDRA-20226 at 11/10/25 12:09 AM:
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Note: the dips on the graphs for the new version are related to facing of 2 new
bottlenecks once we've got the ability to send a higher load:
# Default 2 flushing threads cannot keep the write pace and we have writer
threads are blocked periodically by a backpressure logic on allocating in
org.apache.cassandra.utils.memory.MemtableAllocator.SubAllocator#allocate
# Heap allocation rate is higher and concurrent GC threads consume more CPU,
in combination with high CPU usage in total (near 90-100%) and the fact that we
have by default number of concurrent GC threads equal to number of cpu cores in
Cassandra settings - concurrent GC threads steal too much CPU from processing
logic and heavily impact it.
To check I've configured 4 flushing threads + reduced -XX:ConcGCThreads to 4
(-XX:ParallelGCThreads was not changed and still is 16), with these changes the
graph is smoother:
!image-2025-11-10-00-04-57-497.png|width=1000!
was (Author: dnk):
Note: the dips on the graphs for the new version are related to facing of 2 new
bottlenecks once we've got the ability to send a higher load:
# Default 2 flushing threads cannot keep the write pace and we have writer
threads are blocked periodically by a backpressure logic on allocating in
org.apache.cassandra.utils.memory.MemtableAllocator.SubAllocator#allocate
# Heap allocation rate is higher and concurrent GC threads consume more CPU,
in combination with high CPU usage in total (near 90-100%) and the fact that we
have by default number of concurrent GC threads equal to number of cpu cores -
concurrent GC threads steal too much CPU from processing logic and heavily
impact it.
To check I've configured 4 flushing threads + reduced -XX:ConcGCThreads to 4
(-XX:ParallelGCThreads was not changed and still is 16), with these changes the
graph is smoother:
!image-2025-11-10-00-04-57-497.png|width=1000!
> Reduce contention in MemtableAllocator.allocate
> -----------------------------------------------
>
> Key: CASSANDRA-20226
> URL: https://issues.apache.org/jira/browse/CASSANDRA-20226
> Project: Apache Cassandra
> Issue Type: Improvement
> Components: Local/Memtable
> Reporter: Dmitry Konstantinov
> Assignee: Dmitry Konstantinov
> Priority: Normal
> Fix For: 5.x
>
> Attachments: 5.1_batch_LongAdder.html, 5.1_batch_addAndGet.html,
> 5.1_batch_alloc_batching.html, 5.1_batch_baseline.html,
> 5.1_batch_pad_allocated.html, CASSANDRA-20226_ci_summary.htm,
> CASSANDRA-20226_results_details.tar.xz,
> ci_summary_netudima_CASSANDRA-20226-trunk_52.html, cpu_profile_batch.html,
> image-2025-01-20-23-38-58-896.png, image-2025-11-10-00-04-57-497.png,
> profile.yaml, results_details_netudima_CASSANDRA-20226-trunk_52.tar.xz,
> test_results_m8i.4xlarge_heap_buffers.html,
> test_results_m8i.4xlarge_heap_buffers.png,
> test_results_m8i.4xlarge_offheap_objects.html,
> test_results_m8i.4xlarge_offheap_objects.png
>
> Time Spent: 0.5h
> Remaining Estimate: 0h
>
> For a high insert batch rate it looks like we have a bottleneck in
> NativeAllocator.allocate probably caused by contention within the logic.
> !image-2025-01-20-23-38-58-896.png|width=300!
> [^cpu_profile_batch.html]
> The logic has at least the following 2 potential places to assess:
> # allocation cycle in MemtablePool.SubPool#tryAllocate. This logic has a
> while loop with a CAS, which can be non-efficient under a high contention,
> similar to CASSANDRA-15922 we can try to replace it with addAndGet (need to
> check if it does not break the allocator logic)
> # swap region logic in NativeAllocator.trySwapRegion (under a high insert
> rate 1MiB regions can be swapped quite frequently)
> Reproducing test details:
> * test logic
> {code:java}
> ./tools/bin/cassandra-stress "user profile=./profile.yaml no-warmup
> ops(insert=1) n=10m" -rate threads=100 -node somenode
> {code}
> * Cassandra version: 5.0.3
> * configuration changes compared to default:
> {code:java}
> memtable_allocation_type: offheap_objects
> memtable:
> configurations:
> skiplist:
> class_name: SkipListMemtable
> trie:
> class_name: TrieMemtable
> parameters:
> shards: 32
> default:
> inherits: trie
> {code}
> * 1 node cluster
> * OpenJDK jdk-17.0.12+7
> * Linux kernel: 4.18.0-240.el8.x86_64
> * CPU: 16 cores, Intel(R) Xeon(R) CPU E5-2680 v4 @ 2.40GHz
> * RAM: 46GiB
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