堆内缓存：
- LinkedHashMap：Java自带类，内置LRU驱逐策略的实现（access-order）；多线程访问需要自己实现同步。
- Guava Cache：Google Guava工具包中的缓存实现，支持LRU驱逐策略；支持多线程并发访问，支持按时间过期，但只有在访问时才清除过期数据。
- Ehcache：支持多种驱逐策略：LFU、LRU、FIFO，支持持久化和集群。性能跟Guava Cache比相当。
- Caffeine：支持W-TinyLFU驱逐策略，Benchmark测试读写性能是Guava Cache的6倍左右。
堆外缓存：
- OHCache：支持缓存驱逐和过期（Cassandra使用的缓存库）
- ChronicleMap：支持Hash结构，性能好，不支持缓存驱逐
- MapDB：支持Tree结构，可顺序扫描，不支持缓存驱逐
- Ehcache3：BigMemory收费
当缓存数据量非常大时，GC压力过大会导致服务响应慢甚至崩溃，在HugeGraph图数据库中为了缓解该问题，需引入堆外缓存，选择了使用OHCache。
OHCache对外缓存总结
开源：https://github.com/snazy/ohc
特性：
- 底层有2种实现：linked实现，适用于中大entry场景，默认实现，大部分feature只在该实现中才支持；chunked实现，适用于小entry场景，通过设置chunkSize或者fixedEntrySize启用。
- 通过eviction()来设置缓存驱逐策略，支持：LRU, W_TINY_LFU, NONE共3种，后两种仅在linked实现中支持。
- 通过capacity()来指定缓存的容量，单位是字节，注意不是条数。
- 通过hashTableSize()来设置每个hash_table最大存放的entry个数数量（默认是0.75的factor）。
- 必须指定自定义的keySerializer、valueSerializer系列化器，重载实现系列化serialize()方法与反序列化deserialize()方法；需要注意的是还需要重载serializedSize()方法，且该方法返回值必须和实际写入的内容大小完全一样，否则会报错或数据不对。
- 支持缓存条目过期机制，通过timeouts()开启；允许通过defaultTTLmillis()设置一个默认过期时间，也可以在put()的时候单独设置expireAt过期时间。注意：过期机制仅linked实现才支持，因为chunked实现是以chunk为单位进行缓存驱逐的，无法精确到entry粒度；而且并没有独立的线程来回收过期条目，只会在get或put操作时进行检查（见get()或者ensureFreeSpaceForNewEntry()），若过期则删除，详细代码见Timeouts.removeExpired()。
- 可以通过getWithLoader()来利用线程池异步加载数据。
- 建议使用jemalloc来减少内存碎片。
- 实际的内存占用：daa_capacity + segment_count * hash_table_size * 8，一个cache包括segment_count个hash_table，segment_count默认是2 * CPUs；一个hash_table的存放8字节*表大小个的entry地址信息。每个entry都带着头部（linked实现占64字节，chunked实现若定长KV则占16字节否则占24字节），头部后面是键值数据，头部包括：前/后指针、键/值大小、引用数等。
Configures and builds OHC instance:
| Field | Meaning | Default |
|---|---|---|
| keySerializer | Serializer implementation used for keys | Must be configured |
| valueSerializer | Serializer implementation used for values | Must be configured |
| executorService | Executor service required for get operations using a cache loader. E.g. OHCache.getWithLoaderAsync(Object, CacheLoader) | (Not configured by default meaning get operations with cache loader not supported by default) |
| segmentCount | Number of segments | 2 * number of CPUs (java.lang.Runtime.availableProcessors()) |
| hashTableSize | Initial size of each segment’s hash table | 8192 |
| loadFactor | Hash table load factor. I.e. determines when rehashing occurs. | .75f |
| capacity | Capacity of the cache in bytes | 16 MB * number of CPUs (java.lang.Runtime.availableProcessors()), minimum 64 MB |
| chunkSize | If set and positive, the chunked implementation will be used and each segment will be divided into this amount of chunks. | 0 - i.e. linked implementation will be used |
| fixedEntrySize | If set and positive, the chunked implementation with fixed sized entries will be used. The parameter chunkSize must be set for fixed-sized entries. | 0 - i.e. linked implementation will be used, if chunkSize is also 0 |
| maxEntrySize | Maximum size of a hash entry (including header, serialized key + serialized value) | (not set, defaults to capacity divided by number of segments) |
| throwOOME | Throw OutOfMemoryError if off-heap allocation fails | false |
| hashAlgorighm | Hash algorithm to use internally. Valid options are: XX for xx-hash, MURMUR3 or CRC32 Note: this setting does may only help to improve throughput in rare situations - i.e. if the key is very long and you’ve proven that it really improves performace | MURMUR3 |
| unlocked | If set to true, implementations will not perform any locking. The calling code has to take care of synchronized access. In order to create an instance for a thread-per-core implementation, set segmentCount=1, too. | false |
| defaultTTLmillis | If set to a value > 0, implementations supporting TTLs will tag all entries with the given TTL in milliseconds. | 0 |
| timeoutsSlots | The number of timeouts slots for each segment - compare with hashed wheel timer. | 64 |
| timeoutsPrecision | The amount of time in milliseconds for each timeouts-slot. | 128 |
| ticker | Indirection for current time - used for unit tests. | Default ticker using System.nanoTime() and System.currentTimeMillis() |
| Expected number of elements in the cache | No default value, recommended to provide a default value. | |
| eviction | Choose the eviction algorithm to use. Available are:LRU: Plain LRU - least used entry is subject to evictionW-WinyLFU: Enable use of Window Tiny-LFU. The size of the frequency sketch (“admission filter”) is set to the value of hashTableSize. See this article for a description.None: No entries will be evicted - this effectively provides a capacity-bounded off-heap map. | LRU |
| frequencySketchSize | Size of the frequency sketch used by W-WinyLFU | Defaults to hashTableSize. |
| edenSize | Size of the eden generation used by W-WinyLFU relative to a segment’s size | 0.2 |
文档地址：https://javadoc.io/static/org.caffinitas.ohc/ohc-core/0.7.0/org/caffinitas/ohc/OHCacheBuilder.html