Why Major Garbage collection is slower than Minor?

Question

Gone thru this link but still has confusion what actually happens in minor and major GC collection.

Say i have 100 objects in younger generation out of which 85 object are unreachabe objects. Now when Minor GC runs, it will reclaim the memory of 85 objects and move 15 objects to older(tenured) generation.

Now 15 live objects exists in older generation out of which 3 are unreachable. Say Major GC takes places. It will keep 15 objects as it is and reclaim the memory for 3 unreachable object. Major GC is said to be slower than minor GC. My question is why ? Is it because of major GC happens on generally greater number of objects than minor as minor gc occurs more frequently than major?

As per understanding major GC should be faster as it needs to do less work i.e reclaiming memory from unreachable objects than minor GC because high mortality rate in young generation.

Answer 1

1) Minor GC will first move 15 objects to one of survivor spaces, eg SS1, next GC will move those who are still alive to SS2, next GC will move those who survived back to SS1 and so forth. Only those who survived several (eg 8) relocations (minor GCs) will finally go to old generation.

2) Major GC happens only when JVM cannot allocate an object in old generation because there is no free space in it. To clean memory from dead objects GC goes over all objects in old generation, since old generation is several times larger than new generation, it may hold several times more objects, so GC processing will take several times longer

Answer 2

My question is why? Is it because of major GC happens on generally greater number of objects than minor as minor gc occurs more frequently than major?

You pretty much hit the nail on its head. From the Oracle article, emphasis mine:

Often a major collection is much slower because it involves all live objects.

So not only does a major GC analyze those 15 objects in the old generation, it also goes through the young generation (again) and permgen and GCs those areas of the heap. Minor GC only analyzes the young generation, so there generally wouldn't be as many objects to look at.

As per understanding major GC should be faster as it needs to do less work (i.e reclaiming memory from unreachable objects) than minor GC because high mortality rate in young generation.

I think I understand why you think that. I could imagine that major GC could be run very soon after a minor GC, when objects are promoted to an almost-full old generation. Thus, the young generation would (presumably) not contain too many objects to collect.

However, if I'm remembering things correctly, the old generation is usually larger than the young generation, so not only does the GC have to analyze more space, it also has to go over permgen again, as well as the remaining objects in the young generation (again). So that would probably be why major GC is slower -- simply because there's more stuff to do. You might be able to make major GC faster than minor GC by changing the sizes of the generation spaces such that the young generation is larger than both the old generation and permgen, but I don't think that would be a common setting to use...