When does java trigger gc

When does java trigger gc

##1. Memory recycling strategies and common concepts

Common memory recycling strategies can be understood from the following dimensions:

1 Serial & Parallel

Serial: Single thread performs memory recycling work . It is very simple and does not need to consider issues such as synchronization, but it takes a long time and is not suitable for multiple CPUs.

Parallel: Multiple threads perform recycling work concurrently. Suitable for multiple CPUs and highly efficient.

2 Concurrency & stop the world

stop the world: The application thread in the jvm will hang, and only the garbage collection thread is working to clean up the garbage. Simple, no need to consider issues such as dirty recycling.

Concurrency: While garbage collection is occurring, the application is also running. Ensure application response time. There may be situations where the recycling is not clean and requires secondary recycling.

3 Compressed & Uncompressed ©

Compression: After garbage collection, the surviving objects will be slid into a continuous space to clean up the fragments and ensure The remaining space is contiguous.

Non-compressed: Keep fragments and do not compress.

copy: Move the surviving objects to the new space and release all the old space. (Requires larger memory.)

A garbage collection algorithm can be considered and designed from the above dimensions, and ultimately produce a garbage collector with different characteristics suitable for different scenarios.

2. JVM’s YGC&FGC

YGC: GC the new generation heap. The frequency is relatively high because most objects have a short lifespan and are recycled in the new generation. The performance cost is small.

FGC: Full heap-wide GC. FGC is triggered when the default heap space usage reaches 80% (adjustable). Taking our production environment as an example, FGC is generally triggered rarely, sometimes once every 10 days or about a week.

3. When will YGC be triggered and when will FGC be triggered?

● Timing of YGC:

Insufficient edn space

● Timing of FGC:

1. Insufficient old space;

2. Insufficient perm space;

3. Display calls to System.gc(), including RMI, etc. Timing trigger;

4.Pessimistic strategy during YGC;

5.Dump live memory information (jmap –dump:live).

For the triggering timing of YGC, it is quite obvious that there is insufficient eden space, and ygc will definitely be triggered at this time.

For the triggering timing of FGC, old Insufficient space, insufficient perm space, and calling system.gc() are relatively obvious. In this case, GC will generally be triggered.

The most complicated one is the so-called pessimistic strategy. The mechanism it triggers is to first calculate the average size of the previous promotion, that is, from the new generation, through ygc to the average size of the new generation, and then If the remaining space in the old generation is less than the promotion size, a FullGC will be triggered. The strategy considered by sdk is that from an average and long-term perspective, the possibility of insufficient promotion space next time is very high. Instead of waiting until then for fullGC, it is better to pessimistically believe that FullGC will definitely be triggered next time and directly execute FullGC first. And from the actual use process, it has also achieved a relatively stable effect.

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