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off Use Off-Heap Memory Advantage And Disadvantage Off-Heap Memory Collection Solution

admin 1月 14, 2021 0

generalized off-heap memory, including:

memory used by thread
memory used by jvm
memory allocated by JNI
memory allocated by Direct Buffer

narrow off-heap memory only means the memory allocated by Direct Buffer

Use Off-Heap Memory

Unsafe

Field f = Unsafe.class.getDeclaredField("theUnsafe");
f.setAccessible(true);
Unsafe us = (Unsafe) f.get(null);
long id = us.allocateMemory(1000);

ByteBuffer

Advantage And Disadvantage

merit
- reduce gc
- reduce replication(when use the IO operation, first need to copy the data to dirrect memory)
weakness
- difficult to control(collection)
- difficult to store complex objects except for byte

Off-Heap Memory Collection

in young area, hold the reference(DirectByteBuffer) to the direct memory, which is usually to big like iceberg. if the reference object is unreachable, the direct memory will be collected along with the the collection of the reference.

after gc promotion, if the DirectByteBuffer object promote to the old area,only full gc can collect the object. without the full gc, it will cause the out of memory error.

Solution

use System.gc()
notice: do not use -XX:DisableExplicitGC; use the -XX:ExplicitGCInvokesConcurrent instead of -XX:DisableExplicitGC, if necessary.