Apache Geode Offheap Storage
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Motivation and goals for off-heap storage Off-heap features and usage Implementation overview Preliminary benchmarks: off-heap vs. heap Tips and best practices
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Apache Geode Offheap Storage
- 2. Agenda • Motivation and goals for off-heap storage • Off-heap features and usage • Implementation overview • Preliminary benchmarks: off-heap vs. heap • Tips and best practices
- 3. Motivation and goals for off-heap storage
- 4. Why Off-heap • • Increase data density and reduce memory overhead • 50+ GB user data in one JVM • 10+ TB user data in one cluster • Usable out-of-box without extensive GC tuning of JVM • Maintain existing throughput performance
- 6. Off-heap: How Do I Use It? • Set the off-heap memory size for the process – Using the new property: off-heap-memory-size • Mark regions whose entry values should be stored off-heap – Using the new region attribute: off-heap (false | true) • Adjust the JVM heap memory size down accordingly – The smaller the better; at least try to keep it below 32G • Optionally – Configure Resource Manager
- 7. Off-heap Features • Startup options • Interaction with other features • Resource Manager • Monitoring & Management • Limitations
- 8. Startup Options • --off-heap-memory-size – specifies amount of off-heap memory to allocate • -lock-memory – specifies to lock memory from the OS • Example: gfsh start server –initial-heap=10G –max-heap=10G –off-heap- memory-size=200G –lock-memory=true
- 9. Off-heap Interaction with Other Features • PDX – Values currently copied from off-heap to create a PDXInstance • Deltas: expensive • Compression: compatible with off-heap • Querying: more expensive with off-heap • EntryEvents – Limited availability of oldValue, newValue • Indexes – Functional range indexes not supported (too expensive)
- 10. Off-heap and Resource Manager • Out of Memory Semantics • Eviction and Critical Thresholds • Resource Manager API
- 11. Out of Memory occurs when... • Java heap runs out of memory – Threads start throwing OutOfMemoryError • Off-heap runs out of memory – Threads start throwing OutOfOffHeapMemoryException • => causing the Geode member to close and disconnect – Closes the Cache to prevent reading inconsistent data – Disconnects from the Geode cluster to prevent distribution problems or hangs
- 12. Eviction and Critical Thresholds for Java Heap • CriticalHeapPercentage – triggers LowMemoryException for puts into heap regions – default is 90% – critical member informs other members that it is critical • EvictionHeapPercentage – triggers eviction of entries in heap regions configured with LRU_HEAP – default is 90% of CriticalHeapPercentage
- 13. Eviction and Critical Thresholds for Off-heap • CriticalOffHeapPercentage – triggers LowMemoryException for puts into off-heap regions – default is 90% if –off-heap-memory-size is specified – critical member informs other members that it is critical • EvictionOffHeapPercentage – triggers eviction of entries in off-heap regions configured with LRU_HEAP – default is 90% of CriticalOffHeapPercentage if –off-heap-memory- size is specified
- 14. Startup Options • Existing: • -critical-heap-percentage • -eviction-heap-percentage • New: • -critical-off-heap-percentage • -eviction-off-heap-percentage • Example: start server –initial-heap=10G –max-heap=10G –off-heap-memory- size=200G –lock-memory=true –critical-off-heap-percentage=99
- 15. ResourceManager API • GemFireCache#getResourceManager() • com.gemstone.gemfire.cache.control.ResourceManager – exposes getters/setters for all of the heap and off-heap threshold percentages – Examples: ▪ public void setCriticalOffHeapPercentage(float offHeapPercentage); ▪ public float getCriticalOffHeapPercentage();
- 16. Monitoring & Management • Statistics • Mbeans • gfsh
- 17. Statistics name description compactions The total number of times off-heap memory has been compacted. compactionTime The total time spent compacting off-heap memory. fragmentation The percentage of off-heap memory fragmentation. Updated every time a compaction is performed. fragments The number of fragments of free off-heap memory. Updated every time a compaction is done. freeMemory The amount of off-heap memory, in bytes, that is not being used. largestFragment The largest fragment of memory found by the last compaction of off heap memory. Updated every time a compaction is done. maxMemory The maximum amount of off-heap memory, in bytes. This is the amount of memory allocated at startup and does not change. objects The number of objects stored in off-heap memory. reads The total number of reads of off-heap memory. usedMemory The amount of off-heap memory, in bytes, that is being used to store data.
- 18. MBeans MemberMXBean getOffHeapCompactionTime -- provides the value of the compactionTime statistic getOffHeapFragmentation -- provides the value of the fragmentation statistic getOffHeapFreeMemory -- provides the value of the freeMemory statistic getOffHeapObjects -- provides the value of the objects statistic getOffHeapUsedMemory -- provides the value of the usedMemory statistic getOffHeapMaxMemory -- provides the value of freeMemory + usedMemory RegionMXBean listRegionAttributes (operation) enableOffHeapMemory (true | false)
- 19. Gfsh Support for Off-heap Memory • alter disk-store: new option "--off-heap" for setting off-heap for each region in the disk-store • create region: new option "--off-heap" for setting off-heap • describe member: now displays the off-heap size • describe offline-disk-store: now shows if a region is off-heap • describe region: now displays the off-heap region attribute • show metrics: Now has an offheap category. The offheap metrics are: maxMemory, freeMemory, usedMemory, objects, fragmentation, and compactionTime • start server: added --lock-memory, --off-heap-memory-size, --critical- off-heap-percentage, and --eviction-off-heap-perentage
- 20. Off-heap Limitations • Maximum object size limited to slightly less than 2 GB • All data nodes must consistently configure a region to be off- heap • Functional Range Indexes not supported • Keys, subscription queue entries not stored off-heap • Fragmentation statistic is only updated during off-heap compactions
- 22. Off-heap: How are We Doing It? • Using memory that is separate from the Java heap – Build our own Memory Manager – Memory Manager is very finely tuned and specific to our usage – Avoid GC overhead ▪ Avoid copying of objects for promotion between generations ▪ Garbage Collector is a major performance killer – Use sun.misc.Unsafe API for performance • Optimizing code to minimize usage of heap memory • Using off-heap as primary store instead of overflowing to it
- 24. Off-heap Implementation • Memory allocated in 2GB slabs – Max data value size: ~2GB – Object values stored serialized; blobs stored as byte arrays – Allocation faster for values < 128KB ▪ Controlled by a system property: gemfire.OFF_HEAP_FREE_LIST_COUNT ▪ First try to allocate from the free list; if that fails, allocate from unused memory ▪ Small values (< 8B) inlined (not using any off-heap space) • Compaction consolidates free memory to minimize fragmentation – Blocks writes; best to avoid by minimizing fragmentation
- 25. Off-heap Implementation (cont’d) • Allocated chunks – Header ▪ isSerialized ▪ isCompressed ▪ Size ▪ Padding size • Free chunks – Header ▪ Size ▪ Address of next chunk in the list
- 26. What is Stored On-heap vs. Off-heap Stored On-heap Stored Off-heap Region Meta-Data Values Entry Meta-Data Reference Counts Off-Heap Addresses Lists of Free Memory Blocks Keys WAN Queue Elements Indexes Subscription Queue Elements
- 27. Preliminary Benchmarks: Off-heap vs. Heap
- 28. Off-heap: Initial Testing Results • 256 GB user data per node across 8 nodes for total of 2 TB of user data • Heap-only test worked twice as hard to produce 1/3 the updates as test using Off-Heap – Details on the next slide • Succeeded in scaling up to much larger in-memory data • Increased throughput of operations for large data sets
- 29. Heap vs. Off-Heap Comparison Java Heap Off-Heap creates/sec 30,000 45,000 updates/sec 17,000 (std dev: 2130) 51,000 (std dev: 737) Java RSS size 50 GB 32 GB CPU load 70% (load avg 10 cpus) 32% (load avg 5 cpus) JVM GC ConcurrentMarkSweep ConcurrentMarkSweep GC ms/sec 777 ms 24 ms GC marks (GC pauses) 1 per 30 sec never
- 31. Off-heap Rules of Thumb • Avoid fragmentation – In order to avoid compaction – Avoid usage patterns that lead to fragmentation – Many updates of varying value size • Avoid “unfriendly” features – Deltas – Functional Range Indexes – Querying
- 32. Off-heap Recommendations • Do use when – The values are relatively uniform in size – The values are mostly less than 128K in size – The usage patterns involve cycles of many creates followed by destroys or clear – The values do not need to be frequently deserialized • Configure all data nodes with the same off-heap-memory- size
- 34. We’d appreciate your thoughts... • Would you like an API to invoke a compaction? • Would you like to be able to configure the slab size? • Would you like to configure the max value size for the most efficient off-heap allocation, or maybe the size increment? • Anything else? • Full spec at: https://cwiki.apache.org/confluence/display/GEODE/Off- Heap+Memory+Spec
- 35. Thank You!