Working With SGen

SGen is a new and powerful garbage collector that we implemented for Mono. This new engine can be used for a wide range of applications from server workloads to desktop and mobile workloads and near-real time applications.

Different workloads will require different options in the garbage collector to be enabled. For example, server or high-performance computing applications can allocate as much memory from the operating system on startup in exchange for faster execution times. Meanwhile, mobile applications would rather only allocate as much memory as it is needed.

This document describes the options that you have available when it comes to tuning the garbage collector as well as a basic explanation of the inner workings of the garbage collector.

SGen Basics

SGen is a generational collector, which is a popular technique used by garbage collectors to improve the performance of the garbage collector.

SGen uses three heaps to allocate your objects: the nursery, the major heap and the large object space.


For users familiar with the terminology used in other corners of the industry: The Nursery is the “Young Generation” while our “Major Heap” corresponds to the “Old Generation”.

During the course of execution of your program, you will create some objects with long live spans and others that are short lived. Your objects are initially allocated in an area of memory called the nursery.


The size of the nursery is set to 4 megabytes. When you run of memory in the nursery, the garbage collector process starts a nursery collection.

It stops your threads and moves all of the live objects from the nursery into the major generation clearing the nursery in the process.


As the objects are evacuated, they are moved into the major heap. If there is not enough memory in the major heap, then a major collection is triggered. If the major collection fails to free up enough memory, then SGen might request more memory from the operating system (this is the default).

SGen considers any objects using more than than 8,000 bytes to be large objects (SGEN_MAX_SMALL_OBJ_SIZE). Large objects are not actually allocated on the nursery, instead they are tracked by the Large Object Space (LOS) manager.

Major Heap Collectors

SGen implements two algorithms for collecting the major heap. You can pick between the copying collector and the mark and sweep/copying collector. The mark and sweep/copying collector is the default.

The copying collector is the same technique used by the nursery where live objects are copied from their current location to a new place, tightly packing objects in the process. The problem with using the copying collector for the major heap is that most of the objects in the major heap are actually aged objects, so they tend to stay alive for a long period of time and much time is spent moving objects which might bring very small gains in memory usage at the cost of CPU usage.

The mark and sweep collector does not move the objects, but instead keeps the objects where they are. To avoid fragmentation, objects are distributed in buckets of different sizes and if these buckets reach a 66% of fragmentation then this automatically performs a copying evacuation. The mark and sweep collector basically provides the best of both worlds: fast execution and if the memory becomes fragmented, it automatically performs a copying collection.

The evacuation threshold for fragmented blocks is set to 66%, but you can change that by setting the evacuation-threshold option to a value between 0 and 100 (the default is 66).

You can also disable evacuation completely, even if the blocks become fragmented, by setting the value to zero.

Single CPU vs Multiple CPU Garbage Collection

When you use the mark and sweep collector, Mono can distribute the mark phase across multiple CPUs. The default is to only use a single CPU.

To enable the multi-processor garbage collection, set the major variable to marksweep-par or marksweep-par-fixed

When using the mark and sweep collector, you can also instruct SGen to perform a parallel sweep. When this is enabled a parallel thread runs side-by-side with your application performing sweeps in the background. To enable this feature set the concurrent-sweep flag. Fixed vs Variable Major Heaps

Unlike some other virtual machines, the major heap in Mono does not have to be fixed. Mono can grow its major heap on demand to satisfy the memory usage requirements of the application.

Having the major heap grow on demand comes with a minimal CPU cost. This cost might not be something that you want to incur for high-performance computing workloads or for server applications where memory usage is not a problem.

By default Mono will use variable size major heaps, to adjust the memory usage to the need of your application. But you can configure the collector use a fixed amount of memory. You must set the major collector to be marksweep-fixed or marksweep-par-fixed. If you do not specify the size of the heap, Mono will default to 512 megabytes. If you want a different value, set the major-heap-size parameter to the number of desired bytes for the heap.

For example:


The above uses a fixed heap size with 2 gigabytes in size. Other common suffixes include “k” for kilobyte, and “m” for megabyte in addition to “g” which stands for gigabytes.

Controlling Collections

Some interactive, and near-real time applications want to minimize pause times caused by the GC running in the middle of their work (for example, video games. Game users do not really appreciate pauses in their gaming experience). The best mechanism to avoid these pauses is to not allocate memory during time sensitive operations and instead move the majority of the object allocations out of the sensitive code paths or your main game loop for example.

But it is not possible to avoid all allocations, specially with complicated applications. In those cases, developers can benefit from running controlled and limited collections of the nursery which are very fast instead of waiting for the GC heuristics to collect and triggering not only a nursery collection, but also a fairly time consuming major collection.

You can manually trigger a nursery collection by calling the GC.Collect method with the generation parameter set to zero, like this:

GC.Collect (0);

This instructs the garbage collector to process the nursery, which is a relatively quick process due to the limited size of the nursery.

If you call GC.Collect without a parameter or with any value higher than zero, Mono will perform a global garbage collection. This means that the garbage collector will look not only at the objects that live in the nursery, but also the objects that live in the mature object space.

Nursery Size

The default size of the nursery is four megabytes.

Since being in control of the nursery collection is important to some interactive/near-real-time applications, you might want to customize the nursery size.

You might want to make it smaller, to make nursery collections take place more often, or for the nursery collections to be done in a smaller amount of time, or you might want to make it larger, to allow a player for example to go through an entire game level without ever hitting the garbage collector.

To change the nursery size, set the nursery-size flag to the size in bytes that you want for the nursery. The nursery size must be a power of two and specified in bytes. Just like the major heap size configuration, you can use the “k”, “m” and “g” suffixes as shortcuts for kilobyte, megabyte and gigabyte respectively.

Stack Scanning

On some platforms, SGen can scan your stacks precisely (Linux), in other platforms, it can only scan the stack conservatively.

Precise stack scanning removes false positives when it comes to determining whether an object is alive or not, but it requires more memory to be used at runtime for bookkeeping purposes.

Configuring SGen

To set any of the variables discussed so far, you must set the environment variable MONO_GC_PARAMS and pass the options discussed in this article separated by commas.

In addition to the various options to configure SGen, you can learn more about the behavior of your application by using some of the SGen debugging flags. This is done by setting the MONO_GC_DEBUG environment variable when using SGen.

You can learn more about the debugging options for SGen from Mono’s manual page: mono(1).


On systems that have DTrace (Solaris, macOS), Mono is able to report the SGen Garbage Collector events to Dtrace. See the SGen_DTrace page for details on how to use DTrace with Sgen to profile your application.