When is large object heap collected

Of course, compaction involves overhead — both CPU cycles and additional memory for copying objects. Because the benefits of compaction outweigh the costs for small objects, compaction is performed automatically on the small object heap.

However, the cost of compaction is too high for large objects greater than 85KB in size. Copying and moving large objects not only would involve significant overhead for the garbage collector — the GC would need twice as much memory for garbage collection — but moving large objects would be very time-consuming as well.

Therefore, unlike the small object heap, the large object heap is not compacted during garbage collection. So, how is memory in the large object heap reclaimed?

Well, the GC never moves large objects — all it does is remove them when they are no longer needed. In doing so, memory holes are created in the large object heap. This is what is known as memory fragmentation. So, if you have adjacent free blocks in the large object heap, the GC combines them to create a larger free block and adds it to the free list as an optimization strategy.

Also keep in mind that the GC collects unused objects from the large object heap only during generation 2 collections.

In other words, the GC tries to reclaim memory residing in the small object heap before it tries to reclaim memory from the large object heap. Because this cost is substantial and can degrade performance, large objects are stored in a separate heap that is not compacted.

A recommended strategy is to identify the large objects in your application and then split them into smaller objects — perhaps using some wrapper class. You can also redesign your application to ensure that you avoid using large objects. Another approach is to recycle the application pool periodically.

The following code snippet illustrates how this can be achieved. Note also that there have been a number of significant improvements in the way the large object heap is managed in.

Connect and share knowledge within a single location that is structured and easy to search. I have read about Generations and Large object heap. But I still fail to understand what is the significance or benefit of having Large object heap? What could have went wrong in terms of performance or memory if CLR would have just relied on Generation 2 Considering that threshold for Gen0 and Gen1 is small to handle Large objects for storing large objects?

A garbage collection doesn't just get rid of unreferenced objects, it also compacts the heap. That's a very important optimization. It doesn't just make memory usage more efficient no unused holes , it makes the CPU cache much more efficient.

The cache is a really big deal on modern processors, they are an easy order of magnitude faster than the memory bus. Compacting is done simply by copying bytes. That however takes time. The larger the object, the more likely that the cost of copying it outweighs the possible CPU cache usage improvements. So they ran a bunch of benchmarks to determine the break-even point. And arrived at 85, bytes as the cutoff point where copying no longer improves perf.

With a special exception for arrays of double, they are considered 'large' when the array has more than elements. That's another optimization for bit code, the large object heap allocator has the special property that it allocates memory at addresses that are aligned to 8, unlike the regular generational allocator that only allocates aligned to 4. That alignment is a big deal for double, reading or writing a mis-aligned double is very expensive.

Oddly the sparse Microsoft info never mention arrays of long, not sure what's up with that. Fwiw, there's lots of programmer angst about the large object heap not getting compacted. This invariably gets triggered when they write programs that consume more than half of the entire available address space.

Followed by using a tool like a memory profiler to find out why the program bombed even though there was still lots of unused virtual memory available.

Such a tool shows the holes in the LOH, unused chunks of memory where previously a large object lived but got garbage collected. Such is the inevitable price of the LOH, the hole can only be re-used by an allocation for an object that's equal or smaller in size. The real problem is assuming that a program should be allowed to consume all virtual memory at any time. A problem that otherwise disappears completely by just running the code on a bit operating system. A bit process has 8 terabytes of virtual memory address space available, 3 orders of magnitude more than a bit process.

You just can't run out of holes. Long story short, the LOH makes code run more efficient. At the cost of using available virtual memory address space less efficient. NET 4. LargeObjectHeapCompactionMode property. Beware the consequences please. If the object's size is greater than some pinned value bytes in. This optimises:. Also note that obviously some of the links mentioned in this article may no longer exist. NET Garbage Collector divides objects up into small and large objects.

When an object is large some attributes associated with it become more significant than if the object is small. For instance, compacting it, meaning copying the memory elsewhere on the heap, is expensive.

In this article we are going to look at the large object heap in depth. We will talk about what qualifies an object as a large object, how these large objects are collected and what kind of performance implications large objects impose. NET 1. This number was determined by performance tuning.

What does this mean exactly? To understand this, it may be beneficial to explain some fundamentals about the. NET GC. As many of you are aware, the. NET Garbage Collect is a generational collector.

It has 3 generations — generation 0, generation 1 and generation 2. The reason behind having 3 generations is that we expect for a well tuned app, most objects die in Gen0.

For example, in a server app, the allocations associated each request should die after the request is finished. And the in flight allocation requests will make into Gen1 and die there.

Essentially Gen1 acts as a buffer between young object areas and long lived object areas. From the generation point of view, large objects belong to generation 2 because they are collected only when we do a generation 2 collection.

When a generation gets collected, all its younger generation s also get collected. So for example, when a generation 1 GC happens, both generation 1 and 0 get collected. And when a generation 2 GC happens, the whole heap gets collected.

For this reason a generation 2 GC is also called a full GC. So generations are the logical view of the GC heap. Physically, objects live on the managed heap segments.

A managed heap segment is a chunk of memory that the GC reserves from the OS via calling VirtualAlloc on behalf of managed code. The allocation requests are then satisfied by putting managed objects on this managed heap segment. Segments are committed in smaller chunks as more and more objects are allocated onto them.

For SOH, objects that survive a GC get promoted to the next generation; so objects that survive a generation 0 collection will now be considered generation 1 objects, and so on. Objects that survive the oldest generation, however, will still be considered in the oldest generation. In other words, survivors from generation 2 will be generation 2 objects; and survivors from LOH will be LOH objects collected with gen2.

The user code can only allocate in generation 0 small objects or LOH large objects. When a GC is triggered, we trace through the live objects and compact them. For LOH though, because compaction is expensive we choose to sweep them, meaning making a free list out of dead objects that can be reused later to satisfy large object allocation requests. Adjacent dead objects are made into one free object. Note that the figures below are only for illustration purposes — I use very few objects to show what happens on the heap.

In reality there are much more objects there. The space after the last object Obj3 till end of the segment can also be used to satisfy allocation requests.

If that fails then we will trigger a generation 2 GC in hope of freeing up some space. During a generation 2 GC we take the opportunity to release segments that have no live objects on them back to the OS by calling VirtualFree.