Off Heap Cache - OHCache - Simple Example

Reference

https://github.com/snazy/ohc

https://docs.oracle.com/javase/8/docs/api/java/nio/MappedByteBuffer.html
https://github.com/shooeugenesea/study-practice/tree/ohc

What

OHC 是一個 off heap 的 cache library

Why

如果是用 map 做 cache, 資料都在 heap 裡面, heap 裡面的問題就是

1. 會被 GC, GC 就會週期性的對 JVM 效能產生影響
2. 如果要 persist 需要另外實作

如果是放在 off heap, 比方說用 mmap, 或是用 MappedByteBuffer access 的記憶體, 就是放在 heap memory 之外, 由作業系統控制著. 

好處是記憶體量不用被 heap size 限制, 缺點是要自己小心控制, 不然會造成系統問題.'

How

Maven

<dependency>
 <groupId>org.caffinitas.ohc</groupId>
 <artifactId>ohc-core</artifactId>
 <version>0.4.4</version>
</dependency>

Codes

public class SimpleMain {

    public static void main(String[] params) {
        Scanner scanner = new Scanner(System.in);
        while(scanner.hasNextLine()) {
            if (scanner.nextLine().trim().equals("GO")) {
                System.out.println(Runtime.getRuntime().freeMemory()/1024/1024 + "/" + Runtime.getRuntime().totalMemory()/1024/1024);
                OHCache<String,String> c = OHCacheBuilder.<String,String>newBuilder()
                        .keySerializer(new StringSerializer())
                        .valueSerializer(new StringSerializer())
                        .build();
                c.put("A", "A");
                System.out.println(c.get("A"));
                IntStream.range(0, 1000_000).forEach(idx -> {
                    c.put(UUID.randomUUID().toString(), UUID.randomUUID().toString());
                });
                System.out.println(Runtime.getRuntime().freeMemory()/1024/1024 + "/" + Runtime.getRuntime().totalMemory()/1024/1024);
            } else {
                System.out.println("input 'GO' then click enter");
            }
        }
    }

    private static class StringSerializer implements CacheSerializer<String> {

        @Override
        public void serialize(String s, ByteBuffer byteBuffer) {
            byteBuffer.put(s.getBytes());
        }

        @Override
        public String deserialize(ByteBuffer byteBuffer) {
            return StandardCharsets.UTF_8.decode(byteBuffer).toString();
        }

        @Override
        public int serializedSize(String s) {
            return s.getBytes().length;
        }
    }

}

Output

這張是輸入了好幾次 GO, 每次都會輸入一百萬次 key value 到 cache.

jconsole 與系統資源使用圖可以看到 heap 只用不到 100MB, 但實際上已經用了 600MB.
就是都放在 off heap 中