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Garbage Collector Basics and Performance Hints

 

Rico Mariani
Microsoft Corporation

April 2003

Summary: The .NET garbage collector provides a high-speed allocation service with good use of memory and no long-term fragmentation problems. This article explains how garbage collectors work, then goes on to discuss some of the performance problems that might be encountered in a garbage-collected environment. (10 printed pages)

Applies to:
   Microsoft® .NET Framework

Contents

Introduction
Simplified Model
Collecting the Garbage
Performance
Finalization
Conclusion

Introduction

In order to understand how to make good use of the garbage collector and what performance problems you might run into when running in a garbage-collected environment, it's important to understand the basics of how garbage collectors work and how those inner workings affect running programs.

This article is broken down into two parts: First I will discuss the nature of the common language runtime (CLR)'s garbage collector in general terms using a simplified model, and then I will discuss some performance implications of that structure.

Simplified Model

For explanatory purposes, consider the following simplified model of the managed heap. Note that this is not what is actually implemented.

Figure 1. Simplified model of the managed heap

The rules for this simplified model are as follows:

  • All garbage-collectable objects are allocated from one contiguous range of address space.
  • The heap is divided into generations (more on this later) so that it is possible to eliminate most of the garbage by looking at only a small fraction of the heap.
  • Objects within a generation are all roughly the same age.
  • Higher-numbered generations indicate areas of the heap with older objects—those objects are much more likely to be stable.
  • The oldest objects are at the lowest addresses, while new objects are created at increasing addresses. (Addresses are increasing going down in Figure 1 above.)
  • The allocation pointer for new objects marks the boundary between the used (allocated) and unused (free) areas of memory.
  • Periodically the heap is compacted by removing dead objects and sliding the live objects up toward the low-address end of the heap. This expands the unused area at the bottom of the diagram in which new objects are created.
  • The order of objects in memory remains the order in which they were created, for good locality.
  • There are never any gaps between objects in the heap.
  • Only some of the free space is committed. When necessary, more memory is acquired from the operating system in the reserved address range.

Collecting the Garbage

The easiest kind of collection to understand is the fully compacting garbage collection, so I'll begin by discussing that.

Full Collections

In a full collection we must stop the program execution and find all of the roots into the GC heap. These roots come in a variety of forms, but are most notably stack and global variables that point into the heap. Starting from the roots, we visit every object and follow every object pointer contained in every visited object marking the objects as we go along. In this way the collector will have found every reachable or live object. The other objects, the unreachable ones, are now condemned.

Figure 2. Roots into the GC heap

Once the unreachable objects have been identified we want to reclaim that space for later use; the goal of the collector at this point is to slide the live objects up and eliminate the wasted space. With execution stopped, it's safe for the collector to move all those objects, and to fix all the pointers so that everything is properly linked in its new location. The surviving objects are promoted to the next generation number (which is to say the boundaries for the generations are updated) and execution can resume.

Partial Collections

Unfortunately, the full garbage collection is simply too expensive to do every time, so now it's appropriate to discuss how having generations in the collection helps us out.

First let's consider an imaginary case where we are extraordinarily lucky. Let's suppose that there was a recent full collection and the heap is nicely compacted. Program execution resumes and some allocations happen. In fact, lots and lots of allocations happen and after enough allocations the memory management system decides it's time to collect.

Now here's where we get lucky. Suppose that in all of the time we were running since the last collection we didn't write on any of the older objects at all, only newly allocated, generation zero (gen0), objects have been written to. If this were to happen we would be in a great situation because we can simplify the garbage collection process massively.

Instead of our usual full collect we can just assume that all of the older objects (gen1, gen2) are still live—or at least enough of them are alive that it isn't worth looking at those objects. Furthermore, since none of them were written (remember how lucky we are?) there are no pointers from the older objects to the newer objects. So what we can do is look at all the roots like usual, and if any roots point to old objects just ignore those ones. For other roots (those pointing into gen0) we proceed as usual, following all the pointers. Whenever we find an internal pointer that goes back into the older objects, we ignore it.

When that process is done we will have visited every live object in gen0 without having visited any objects from the older generations. The gen0 objects can then be condemned as usual and we slide up just that region of memory, leaving the older objects undisturbed.

Now this is really a great situation for us because we know that most of the dead space is likely to be in younger objects where there is a great deal of churn. Many classes create temporary objects for their return values, temporary strings, and assorted other utility classes like enumerators and whatnot. Looking at just gen0 gives us an easy way to get back most of the dead space by looking at only very few of the objects.

Unfortunately, we're never lucky enough to use this approach, because at least some older objects are bound to change so that they point to new objects. If that happens it's not sufficient to just ignore them.

Making Generations Work with Write Barriers

To make the algorithm above actually work, we must know which older objects have been modified. To remember the location of the dirty objects, we use a data structure called the card table, and to maintain this data structure the managed code compiler generates so-called write barriers. These two notions are central the success of generation-based garbage collecting.

The card table can be implemented in a variety of ways, but the easiest way to think of it is as an array of bits. Each bit in the card table represents a range of memory on the heap—let's say 128 bytes. Every time a program writes an object into some address, the write barrier code must compute which 128-byte chunk was written and then set the corresponding bit in the card table.

With this mechanism in place, we can now revisit the collection algorithm. If we are doing a gen0 garbage collection, we can use the algorithm as discussed above, ignoring any pointers to older generations, but once we have done that we must then also find every object pointer in every object that lies on a chunk that was marked as modified in the card table. We must treat those just like roots. If we consider those pointers as well, then we will correctly collect just the gen0 objects.

This approach wouldn't help at all if the card table was always full, but in practice comparatively few of the pointers from the older generations actually get modified, so there is a substantial savings from this approach.

Performance

Now that we have a basic model for how things are working, let's consider some things that could go wrong that would make it slow. That will give us a good idea what sorts of things we should try to avoid to get the best performance out of the collector.

Too Many Allocations

This is really the most basic thing that can go wrong. Allocating new memory with the garbage collector is really quite fast. As you can see in Figure 2 above is all that needs to happen typically is for the allocation pointer to get moved to create space for your new object on the "allocated" side—it doesn't get much faster than that. However, sooner or later a garbage collect has to happen and, all things being equal, it's better for that to happen later than sooner. So you want to make sure when you're creating new objects that it's really necessary and appropriate to do so, even though creating just one is fast.

This may sound like obvious advice, but actually it's remarkably easy to forget that one little line of code you write could trigger a lot of allocations. For example, suppose you're writing a comparison function of some kind, and suppose that your objects have a keywords field and that you want your comparison to be case insensitive on the keywords in the order given. Now in this case you can't just compare the entire keywords string, because the first keyword might be very short. It would be tempting to use String.Split to break the keyword string into pieces and then compare each piece in order using the normal case-insensitive compare. Sounds great right?

Well, as it turns out doing it like that isn't such a good idea. You see, String.Split is going to create an array of strings, which means one new string object for every keyword originally in your keywords string plus one more object for the array. Yikes! If we're doing this in the context of a sort, that's a lot of comparisons and your two-line comparison function is now creating a very large number of temporary objects. Suddenly the garbage collector is going to be working very hard on your behalf, and even with the cleverest collection scheme there is just a lot of trash to clean up. Better to write a comparison function that doesn't require the allocations at all.

Too-Large Allocations

When working with a traditional allocator, such as malloc(), programmers often write code that makes as few calls to malloc() as possible because they know the cost of allocation is comparatively high. This translates into the practice of allocating in chunks, often speculatively allocating objects we might need, so that we can do fewer total allocations. The pre-allocated objects are then manually managed from some kind of pool, effectively creating a sort of high-speed custom allocator.

In the managed world this practice is much less compelling for several reasons:

First, the cost of doing an allocation is extremely low—there's no searching for free blocks as with traditional allocators; all that needs to happen is the boundary between the free and allocated areas needs to move. The low cost of allocation means that the most compelling reason to pool simply isn't present.

Second, if you do choose to pre-allocate you will of course be making more allocations than are required for your immediate needs, which could in turn force additional garbage collections that might otherwise have been unnecessary.

Finally, the garbage collector will be unable to reclaim space for objects that you are manually recycling, because from the global perspective all of those objects, including the ones that are not currently in use, are still live. You might find that a great deal of memory is wasted keeping ready-to-use but not in-use objects on hand.

This isn't to say that pre-allocating is always a bad idea. You might wish to do it to force certain objects to be initially allocated together, for instance, but you will likely find it is less compelling as a general strategy than it would be in unmanaged code.

Too Many Pointers

If you create a data structure that is a large mesh of pointers you'll have two problems. First, there will be a lot of object writes (see Figure 3 below) and, secondly, when it comes time to collect that data structure, you will make the garbage collector follow all those pointers and if necessary change them all as things move around. If your data structure is long-lived and won't change much, then the collector will only need to visit all those pointers when full collections happen (at the gen2 level). But if you create such a structure on a transitory basis, say as part of processing transactions, then you will pay the cost much more often.

Figure 3. Data structure heavy in pointers

Data structures that are heavy in pointers can have other problems as well, not related to garbage collection time. Again, as we discussed earlier, when objects are created they are allocated contiguously in the order of allocation. This is great if you are creating a large, possibly complex, data structure by, for instance, restoring information from a file. Even though you have disparate data types, all your objects will be close together in memory, which in turn will help the processor to have fast access to those objects. However, as time passes and your data structure is modified, new objects will likely need to be attached to the old objects. Those new objects will have been created much later and so will not be near the original objects in memory. Even when the garbage collector does compact your memory your objects will not be shuffled around in memory, they merely "slide" together to remove the wasted space. The resulting disorder might get so bad over time that you may be inclined to make a fresh copy of your whole data structure, all nicely packed, and let the old disorderly one be condemned by the collector in due course.

Too Many Roots

The garbage collector must of course give roots special treatment at collection time—they always have to be enumerated and duly considered in turn. The gen0 collection can be fast only to the extent that you don't give it a flood of roots to consider. If you were to create a deeply recursive function that has many object pointers among its local variables, the result can actually be quite costly. This cost is incurred not only in having to consider all those roots, but also in the extra-large number of gen0 objects that those roots might be keeping alive for not very long (discussed below).

Too Many Object Writes

Once again referring to our earlier discussion, remember that every time a managed program modified an object pointer the write barrier code is also triggered. This can be bad for two reasons:

First, the cost of the write barrier might be comparable to the cost of what you were trying to do in the first place. If you are, for instance, doing simple operations in some kind of enumerator class, you might find that you need to move some of your key pointers from the main collection into the enumerator at every step. This is actually something you might want to avoid, because you effectively double the cost of copying those pointers around due to the write barrier and you might have to do it one or more times per loop on the enumerator.

Second, triggering write barriers is doubly bad if you are in fact writing on older objects. As you modify your older objects you effectively create additional roots to check (discussed above) when the next garbage collection happens. If you modified enough of your old objects you would effectively negate the usual speed improvements associated with collecting only the youngest generation.

These two reasons are of course complemented by the usual reasons for not doing too many writes in any kind of program. All things being equal, it's better to touch less of your memory (read or write, in fact) so as to make more economical use of the processor's cache.

Too Many Almost-Long-Life Objects

Finally, perhaps the biggest pitfall of the generational garbage collector is the creation of many objects, which are neither exactly temporary nor are they exactly long-lived. These objects can cause a lot of trouble, because they will not be cleaned up by a gen0 collection (the cheapest), as they will still be necessary, and they might even survive a gen1 collection because they are still in use, but they soon die after that.

The trouble is, once an object has arrived at the gen2 level, only a full collection will get rid of it, and full collections are sufficiently costly that the garbage collector delays them as long as is reasonably possible. So the result of having many "almost-long-lived" objects is that your gen2 will tend to grow, potentially at an alarming rate; it might not get cleaned up nearly as fast as you would like, and when it does get cleaned up it will certainly be a lot more costly to do so than you might have wished.

To avoid these kinds of objects, your best lines of defense go like this:

  1. Allocate as few objects as possible, with due attention to the amount of temporary space you are using.
  2. Keep the longer-lived object sizes to a minimum.
  3. Keep as few object pointers on your stack as possible (those are roots).

If you do these things, your gen0 collections are more likely to be highly effective, and gen1 will not grow very fast. As a result, gen1 collections can be done less frequently and, when it becomes prudent to do a gen1 collection, your medium lifetime objects will already be dead and can be recovered, cheaply, at that time.

If things are going great then during steady-state operations your gen2 size will not be increasing at all!

Finalization

Now that we've covered a few topics with the simplified allocation model, I'd like to complicate things a little bit so that we can discuss one more important phenomenon, and that is the cost of finalizers and finalization. Briefly, a finalizer can be present in any class—it's an optional member that the garbage collector promises to call on otherwise dead objects before it reclaims the memory for that object. In C# you use the ~Class syntax to specify the finalizer.

How Finalization Affects Collection

When the garbage collector first encounters an object that is otherwise dead but still needs to be finalized it must abandon its attempt to reclaim the space for that object at that time. The object is instead added to a list of objects needing finalization and, furthermore, the collector must then ensure that all of the pointers within the object remain valid until finalization is complete. This is basically the same thing as saying that every object in need of finalization is like a temporary root object from the collector's perspective.

Once the collection is complete, the aptly named finalization thread will go through the list of objects needing finalization and invoke the finalizers. When this is done the objects once again become dead and will be naturally collected in the normal way.

Finalization and Performance

With this basic understanding of finalization we can already deduce some very important things:

First, objects that need finalization live longer than objects that do not. In fact, they can live a lot longer. For instance, suppose an object that is in gen2 needs to be finalized. Finalization will be scheduled but the object is still in gen2, so it will not be re-collected until the next gen2 collection happens. That could be a very long time indeed, and, in fact, if things are going well it will be a long time, because gen2 collections are costly and thus we want them to happen very infrequently. Older objects needing finalization might have to wait for dozens if not hundreds of gen0 collections before their space is reclaimed.

Second, objects that need finalization cause collateral damage. Since the internal object pointers must remain valid, not only will the objects directly needing finalization linger in memory but everything the object refers to, directly and indirectly, will also remain in memory. If a huge tree of objects was anchored by a single object that required finalization, then the entire tree would linger, potentially for a long time as we just discussed. It is therefore important to use finalizers sparingly and place them on objects that have as few internal object pointers as possible. In the tree example I just gave, you can easily avoid the problem by moving the resources in need of finalization to a separate object and keeping a reference to that object in the root of the tree. With that modest change only the one object (hopefully a nice small object) would linger and the finalization cost is minimized.

Finally, objects needing finalization create work for the finalizer thread. If your finalization process is a complex one, the one and only finalizer thread will be spending a lot of time performing those steps, which can cause a backlog of work and therefore cause more objects to linger waiting for finalization. Therefore, it is vitally important that finalizers do as little work as possible. Remember also that although all object pointers remain valid during finalization, it might be the case that those pointers lead to objects that have already been finalized and might therefore be less than useful. It is generally safest to avoid following object pointers in finalization code even though the pointers are valid. A safe, short finalization code path is the best.

IDisposable and Dispose

In many cases it is possible for objects that would otherwise always need to be finalized to avoid that cost by implementing the IDisposable interface. This interface provides an alternative method for reclaiming resources whose lifetime is well known to the programmer, and that actually happens quite a bit. Of course it's better still if your objects simply use only memory and therefore require no finalization or disposing at all; but if finalization is necessary and there are many cases where explicit management of your objects is easy and practical, then implementing the IDisposable interface is a great way to avoid, or at least reduce, finalization costs.

In C# parlance, this pattern can be quite a useful one:

class X:  IDisposable
{
   public X(…)
   {
   … initialize resources … 
   }

   ~X()
   {
   … release resources … 
   }

   public void Dispose()
   {
// this is the same as calling ~X()
        Finalize(); 

// no need to finalize later
System.GC.SuppressFinalize(this); 
   }
};

Where a manual call to Dispose obviates the need for the collector to keep the object alive and call the finalizer.

Conclusion

The .NET garbage collector provides a high-speed allocation service with good use of memory and no long-term fragmentation problems, however it is possible to do things that will give you much less than optimal performance.

To get the best out of the allocator you should consider practices such as the following:

  • Allocate all of the memory (or as much as possible) to be used with a given data structure at the same time.
  • Remove temporary allocations that can be avoided with little penalty in complexity.
  • Minimize the number of times object pointers get written, especially those writes made to older objects.
  • Reduce the density of pointers in your data structures.
  • Make limited use of finalizers, and then only on "leaf" objects, as much as possible. Break objects if necessary to help with this.

A regular practice of reviewing your key data structures and conducting memory usage profiles with tools like Allocation Profiler will go a long way to keeping your memory usage effective and having the garbage collector working its best for you.

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