Windows Phone 8: Evolution of the Runtime and Application Compatibility

Long time back at the wake of the release of Windows Phone 7 (WP7) I posted about the Windows Phone 7 series programming model. I also published how .NET Compact framework powered the applications on WP7.

Further simplifying the block diagram, we can think of the entire WP7 application system as follows

As with most block diagrams, this is gross simplifications. However, I hope it helps to easily picture the entire system.

Essentially the application can be purely managed (written in say C# or VB.net). The application can only utilize services exposed by the developer platform and core services provided by .NET Compact Framework. The application can in no way directly use native code or talk to the OS (say call an Win32 API). It has to always go through the runtime infrastructure and is in a security sandbox.

The application manager is the loose term I am using to encompass everything that is used to managed the application including the host.

Windows Phone 8 (WP8) is a huge huge change from Windows Phone 7.x (WP7). From the perspective of a WP7 application running on a WP8 device the system looks as follows

Everything in Green in this diagram got outright replaced with entire new codebase and the rest of the system other than the application was heavily modified to work with the new OS and the new managed runtime.

Shared Windows Core

The OS moved away from Windows Compact Embedded (WinCE) OS core that was used in WP7 to a new OS which shares it’s core with the desktop Windows 8. This means that a bunch of things in the WP8 OS is shared with the desktop implementation, this includes things like kernel, networking, driver framework and others. The shared core obviously brings great value as innovations and features will more easily flow across the two form factors (device and desktop) and also reduce engineering redundancy on Microsoft side. Some of the benefits are readily visible today like great multi-core support, WinRT interop and others are more subtle.

CoreCLR

.NET Compact Framework (NETCF) that was used in WP7 has a very different design philosophy and hence a completely different implementation from the desktop .NET. I will have a follow up post on this but for now it suffices to note that .NETCF is a very portable runtime that is designed to be very versatile and cross platform. Desktop CLR on the other hand is more closely tied with Windows and the processor architecture. It closely works with the OS and the underlying HW to give the maximum performance benefit to managed code running on it.

With the new Windows RT which works on ARM, desktop CLR was anyway updated to work on the ARM processor. So when the phone chose to move to shared core it was an obvious choice to move the CLR as well. This gave the same benefits of shared innovation and reduced engineering redundancy.

The full desktop CLR is more heavy and provides functionality that is not really required by the phone scenarios and hence a lighter variant of it (which is built from the same source) called CoreCLR was chosen for WP8. CoreCLR is the evolution of the lightweight runtime that powered Silverlight. With the move to CoreCLR developers get a much faster runtime with extended feature set that includes interop via WinRT.

Backward Compat

One of the simple statements made during all of these WP8 launch presentations was that applications in the store built for WP7 will work as is for WP8. This is a small statement but is a huge achievement and effort from the runtime implementation perspective. Making apps work as is when the entire runtime, OS and chipset has changed is non-trivial to say the least. Our team worked very hard to make this possible.

One of the biggest things that played out to our benefit was that the WP7 apps were fully sandboxed and couldn’t use any native code. This means that they didn’t have any means of taking behavioral dependence on the OS APIs or underlying HW. The OS APIs were used via the CLR and it could always add quirks to expose consistent behavior to the applications.

API Compatibility
This required us to ensure that CoreCLR exposes the same API set as NETCF. We ran various automated tools to managed API surface area changes and retain meaningful API compat between WP7 and WP8. With a closed application store it was possible for us to gather complete metrics on API usage and correctly prioritize engineering resources to ensure that majority of applications continued to see the same API set in signature and semantics.

We also needed to ensure that the same APIs behave as closely as possible with that provided by NETCF. We tested a lot of applications in the app store to get as close as we can and believe that we are at a place that should allow most WP7 application’s API usage to transparently fall over to the new runtime.

Runtime behavior changes
When a runtime changes there are behavioral changes that can expose pre-existing issues with applications. This includes things like timing differences. Even though these runtime behaviors are not documented, or in some cases especially called out that user code should not take dependence on them, some apps still did do that.

Couple of the examples we saw

1. Taking dependence on finalization order
   Even though CLI specification clearly calls out that finalization order in .NET is not deterministic, code still took subtle dependency on them. In one particular case object F1 used a file and it’s finalizer released it. Later another object F2 opens the same file. With change in the runtime, the timing of GC changed so that at the time F2 tried to open the file, F1 which has been already collected hasn’t yet had its finalizer run. Hence the application crashed. We got in touch with the app developer and got the code moved to use the right dispose pattern/
2. GC Timing and changes in number of generations
   Application finalizers contrary to .NET guidelines modified other managed objects. Now changed GC timings and generation resulted in those objects to have already been collected and hence resulted in finalizer crashes
3. Threading issues
   This was one of the painful ones. With the change in OS and hence thread scheduler and addition of multiple cores in the ARM CPU, a lot of subtle races and deadlocks in the applications got exposed. These were pre-existing issues where synchronization primitives were not used correctly or some code relied on the quanta based thread scheduling WinCE did. With move to WP8, threads run in parallel on different cores and scheduled in different order, this lead to various deadlocks and race driven crashes. Again we had to get in touch with app developer to address these issues
4. There were other cases where exact precision of floating point math was relied on. This resulted in a board game where pucks flew off from its surface
5. Whether or not functions got inlined
6. Order of static constructor initialization (especially in conjunction of function in lining)

We addressed some of these issues where it was realistic to fix them in the runtime. For some of the others we got in touch with the application developers. Obviously all applications in the store and all of their features were not tested. So you should try to test your applications when you have access to WP8.

At one point we were all playing games on our phones telling our managers that I am compat testing Need For Speed and I need to test till level 10 :)

My first Windows Phone App

I have been involved with Windows Phone 7 from the very beginning and worked on delivering the CLR for the GA version and all the way through Nodo and Mango.

All this while I was busy working on one of the lower layers and seeing bits and bytes flying by. For some time I wanted to try out writing an app for the phone to experience how it feels to be our developer customer. Unfortunately my skills fall more on system programming than designing good looking UI. I had a gazillion ideas floating in my head but most were beyond my graphic and UI design skills. I was also waiting for some sort of inspiration.

Over the last school break our local King Country Library System which earned the best library system of the year award run an interesting effort. Students received a sheet which they filled up as they read books. On hitting 500 minutes and 1000 minutes of book reading they got prizes. One of the prize was a Kaleidoscope. Not the one with colored glass pieces like I had as a child, but one through which you could see everything around. This was the inspiration I was waiting for and I wrote an WP7 Mango app that does the same. Interestingly the app got a lot of good reviews and is rated at 5 stars (something I didn’t expect). It’s free, go check it out.

Get the app here, documentation is at http://bonggeek.com/Kaleidoscope/

Windows Phone Mango: Under the hood of Fast Application Switch

Fast Application Switch of FAS is kind of tricky for application developers to handle. There are a ton of documentation around how the developers need to handle the various FAS related events. I really liked the video http://channel9.msdn.com/Events/DevDays/DevDays-2011-Netherlands/Devdays059 which walks through the entire FAS experience (jump to around 8:30).

In this post I want to talk about how the CLR (Common Language Runtime or .NET runtime) handles FAS and what that means to your application. Especially the Active –> Dormant –> Active flow. Most of the documentation/presentation quickly skips over this with the vague “The application is made dormant”. This is equivalent to the “witches use brooms to fly”. What is the navigation mechanism or how the broom is propelled are the more important questions which no one seems to answer (given the time of year, I just couldn’t resist :P) . Do note that most developers can just follow the coding guidelines for FAS and never need to care about this. However, a few developers, especially the ones developing multi-threaded apps and using threading primitives may need to care about this. And hence this post

Design Principle

The entire Multi-threading design was made to ensure the following

Principle 1: Pre-existing WP7 apps shouldn’t break on Mango.
Principle 2: When an application is sent to the background it shouldn’t consume any resources
Principle 3: Application should be resumed fast (hence the name FAS)

As you’d see that these played a vital role in the design being discussed below.

States

The states an application goes through is documented in http://msdn.microsoft.com/en-us/library/ff817008(VS.92).aspx 

CLR Design

The diagram below captures the various phase that are used to rundown the application to make it dormant and later re-activated.  It gives the flow of an application as it goes through the Active –> Dormant –> Active state (e.g. the application was running and the user launches another application and then uses the back button to go back to the first application).

Deactivated

The Deactivated event is sent to the application to notify it that the user is navigating away from the application. After this there is 3 possible outcomes. It will either remain dormant, gets tombstoned or finally gets killed as well. Since there is no way to know which would happen, the application should store its transient state into the PhoneApplicationPage.State and it’s persistent state into some persistent store like the IsolatedStorage or even in the cloud. However, do note that the application has 10 seconds to handle the Deactivated event. In the 3 possible situations this is how the stored data will be used back

1. Deactivate –> Dormant –> Active
   In this situation the entire process was intact in memory and just it’s execution was stopped (more about it below). In the Activated event the application can just check the IsApplicationInstancePreserved property. If this is true then the application is coming back from Dormant state and can just use the in-memory state. Nothing needs to be re-read in.
2. Deactivate –> Dormant –> Tombstoned –> Active
   In this case the application’s in-memory state is gone. However, the PhoneApplicationPage.State is serialized back. So the application should read persistent user data from IsolatedStorage or other permanent sources in the Activated event. At the same time it case use the PhoneApplicationPage.State in the OnNavigatedTo event.
3. Deactivate –> Dormant –> Terminated
   This case is no different from the application being re-launched. So in the Launching event the user data needs to be re-created from the permanent store. PhoneApplicationPage.State  is empty in this case

The above supports the #1 principle of not breaking pre-existing WP7 apps. A WP7 app would’ve been designed without considering the dormant stage. Hence it would’ve just skipped the #1 option. So the only issue will be that a WP7 app will result in re-creating the application state each time and not get the benefit of the Dormant stage (it will get the performance of Tombstoning but not break in Mango).

Post this event the main thread never transitions to user code (e.g. no events are triggered). The requirement on the application for deactivate is that

1. It shouldn’t run any user code post this point. This means it should voluntarily stop background threads, cancel timers it started and so on
2. This event needs to be handled in 10 seconds

If app continues to run code, e.g. in another thread and modifies any application state then that state cannot be persisted (as there will be no subsequent Deactivated type event)

Paused

This event is an internal event that is not visible to the application. If the application adhered to the above guideline it shouldn’t care about it anyway.

The CLR does some interesting stuff on this event. Adhering to the “no resource consumption” principle is very important. Consider that the application had used ManualResetEvent.WaitOne(timeout). Now this timeout can expire in the time when the application was dormant. If that happened it would result in some code running when the application is dormant. This is not acceptable because the phone maybe behind locked screen and this context switch can get the phone out of a low-power state. To handle this the runtime detaches Waits, Thread.Sleep at Paused. Also it cancels all Timers so that no Timer callbacks happen post this Pause event.

Since Pause event is not visible to the application, it should consider that some time post Deactivated this detach will happen. This is completely transparent to user code. As far as the user code is considered, it just that these handles do not timeout or sleeps do not return during the time the application is dormant. The same WaitHandle objects or Thread.Sleeps start working as is after the application is activated (more about timeout adjustment below).

This is also the place where other parts of the tear-down happens. E.g. things like asynchronous network calls cancelled, media is stopped.

Note that the background user threads can continue to execute. Obviously that is a problem because the user code is supposed to voluntarily stop them at Deactivated.

Freeze

Besides user code there are a lot of other managed code running in the system. These include but not limited to Silverlight managed code, XNA managed code. Sometime after Paused all managed code is required to stop. This is called the CLRFreeze. At this point the CLR freezes or blocks all managed execution including user background threads. To do that it uses the same mechanism as used for foreground GC. In a later post I’d cover the different mechanics NETCF and desktop CLR uses to stop managed execution.

Around freeze the application enters the Dormant stage where it’s in 0 CPU utilization mode.

Thaw

Managed threads stopped at Freeze are re-started at this point.

Resuming

At Resuming the WaitHandle, Thread.Sleep detached in Paused is re-attached. Also timeout adjustments are made during this time. Consider that the user had two handles on which the user code started Waits with 5 seconds and 10 seconds timeouts. After 3 seconds of starting the Waits the application is made dormant. When the application is re-activated, the Waits are restarted with the amount of timeout remaining at the point of the application getting deactivated. So essentially in the case below the first Wait is restarted with 2 seconds and the later with 7. This ensures that relative gap between Sleeps, Waits are maintained.

Note timers are still not restarted.

Activated

This is the event that the application gets and it is required to re-build it’s state when the activation is from Tombstone or just re-use the state in memory when the activation is from Dormant stage.

Resumed

This is the final stage or FAS. This is where the CLR restarts the Timers. The idea behind the late start of timers is that they are essentially asynchronous callbacks. So the callbacks are not sent until the application is activated (built its state) and ready to consume those callbacks.

Conclusion

1. Ideally application developer needs to take care of the FAS by properly supporting the various events like Deactivated, Activated
2. Background threads continue to run post the Deactivated event. This might lead to issues by corrupting application state and losing state changes. Handle this by terminating the background threads at Deactivated
3. While making application dormant Waits, Sleeps and Timers are deactivated. They are later activated with timeout adjustments. This happens transparently to user code
4. Not all waiting primitives are time adjusted. E.g. Thread.Join(timeout) is not adjusted.

WP7 Mango: The new Generational GC

In my previous post “Mark-Sweep collection and how does a Generational GC help” I discussed how a generational Garbage Collector (GC) works and how it helps in reducing collection latencies which show up as long load times (startup as well as other load situations like game level load) and gameplay or animation jitter/glitches. In this post I want to discuss how those general principles apply to the WP7 Generational GC (GenGC) specifically.

Generations and Collection Types

We use 2 generations on the WP7 referred to as Gen0 and Gen1. A collection could be any of the following 4 types

1. An ephemeral or Gen0 collection that runs frequently and only collects Gen0 objects. Object surviving the Gen0 collection is promoted to Gen1
2. Full mark-sweep collection that collects all managed objects (both Gen1 and Gen0)
3. Full mark-sweep-compact collection that collects all managed objects (both Gen1 and Gen0)
4. Full-GC with code-pitch. This is run under severe low memory and can even throw away JITed code (something that desktop CLR doesn’t support)

The list above is in the order of increasing latency (or time they take to run)

Collection triggers

GC triggers are the same and as outlined in my previous post WP7: When does the GC run. The distinction between #2 and #3 above is that at the end of all full-GC the collector considers the memory fragmentation and can potentially run the memory compactor as well.

1. After significant allocation
   After significant amount of managed allocation the GC is started. The amount today is 1MB (called GC quanta) but is open to change. This GC can be ephemeral or full-GC. In general it’s an ephemeral collection. However, it might be a full collection under the following cases
   1. After significant promotion of objects from Gen0 to Gen1 the collections become full collections. Today 5MB of promotion triggers a full GC (again this number is subject to change).
   2. Application’s total memory usage is close to the maximum memory cap that apps have (very little free memory left). This indicates that the application will get terminated if the memory utilization is not cut-back.
   3. Piling up of native resources. We use different heuristics like native to managed memory ratio and finalizer queue heuristics to detect if GC needs to turn to full collection to release native resources being held-up due to Gen0 only collections
2. Resource allocation failure
   All resource allocation failure means that the system is under memory pressure and hence such collections are always full collection. This can lead to code pitch as well
3. User code triggered GC
   User code can start collections via the System.GC.Collect() managed API. This results in a full collection as documented by that API. We have not added the method overload System.GC.Collect(generation). Hence there is no way for the developer to start a ephemeral or Gen0 only collection
4. Sharing server initiated
   Sharing server can detect phone wide memory issue and start GC in all managed processes running. These are full-GC and can potentially pitch code as well.

 

So from all of the above, the 3 key takeaways are

1. Low memory or memory cap related collections are always full-collections. These could also turn out to be the more costly compacting collection and/or pitch JITed code
2. Collections are in general ephemeral and become full-collection after significant object promotion
3. No fundamental changes to the GC trigger policies. So an app written for WP7 will not see any major changes to the number of GC’s that happen. Some GC will be ephemeral and others will be full-GCs.

 

Write Barriers/Card-table

As explained in my previous post, to keep track of Gen1 to Gen0 reference we use write-barrier/card-table.

Card-table can be visualized as a memory bitmap. Each bit in the card-table covers n bytes of the net address space. Each such bit is called a Card. For managed reference updates like  A.b = C in addition to JITing the real assignment, calls are added to Write-barrier functions. This  write barrier locates the Card corresponding to the address of write and sets it. Later during collection the collector checks all Gen-1 objects covered by a set card-bit and marks Gen-0 references in those objects.

This essentially brings in two additional cost to the system.

1. Memory cost of adding those calls to the WB in the JITed code
2. Cost of executing the write barrier while modifying reference

Both of the above are optimized to ensure they have minimum execution impact. We only JIT calls to WB when absolutely required and even then we have an overhead of a single instruction to make the call. The WB are hand-tuned assembly code to ensure they take minimum cycles. In effect the net hit on process memory due to write barriers is way less than 0.1%. The execution hit in real-world applications scenarios is also not in general measureable (other than real targeted testing).

Differences from desktop

In principle both the desktop GC and the WP7 GC are similar in that they use mark-sweep generational GC. However, there are differences based on the fact that the WP7 GC targets a more constrained device.

1. 2 generations as opposed to 3 on the desktop
2. No background or incremental collection supported on the phone
3. WP7 GC has additional logic to track and handle application policies like application memory caps and total memory utilization
4. The phone CLR uses a very different memory layout which is pooled and not linear. So no concept of Large Object Heap. So lifetime of large objects is no different
5. No support for particular generation collection from user code

WP7 Mango: Mark-Sweep collection and how does a Generational GC help

About a month back we announced that in the next release of Windows Phone 7 (codenamed Mango) we will ship a new garbage collector in the CLR. This garbage collector (GC) is a generational garbage collector.

This post is a “back to basics” post where I’ll try to examine how a mark-sweep GC works and how adding generational collection helps in boosting it’s performance. We will take a simplified look at how mark-sweep-compact GC works and how generational GC can enhance it’s performance. In later posts I’ll try to elaborate on the specifics of the WP7 generational GC and how to ensure you get the best performance out of it.

Object Reference Graph

Objects in the memory can be considered to be a graph. Where every object is a node and the references from one object to another are edges. Something like

To use an object the code should be able to “reach” it via some reference. These are called reachable objects (in blue). Objects like a method’s local variable, method parameters, global variables, objects held onto by the runtime (e.g. GCHandles), etc. are directly reachable. They are the starting points of reference chains and are called the roots (in black).

Other objects are reachable if there are some references to them from roots or from other objects that can be reached from the roots. So Object4 is reachable due to the Object2->Object4 reference. Object5 is reachable because of Object1->Object3->Object5 reference chain. All reachable objects are valid objects and needs to be retained in the system.

On the other hand Object6 is not reachable and is hence garbage, something that the GC should remove from the system.

Mark-Sweep-Compact GC

A garbage collector can locate garbage like Object6 in various ways. Some common ways are reference-counting, copying-collection and Mark-Sweep. In this section lets take a more pictorial view of how mark-sweep works.

Consider the following object graph

At first the GC pauses the entire application so that the object graph is not being mutated (as in no new objects or references are being created). Then it goes into the mark phase. In mark phase the GC traverses the graph starting at the roots and following the references from object to object. Each time it reaches an object through a reference it flips a bit in the object header indicating that this object is marked or in other words reachable (and hence not garbage). At the end everything looks as follows

So the 2 roots and the objects A, C, D are reachable.

Next it goes into the sweep phase. In this phase it starts from the very first object and examines the header. If the header’s mark bit is set it means that it’s a reachable object and the sweep resets that bit. If the header’s bit is not set, it’s not reachable and is flagged as garbage.

So B and E gets flagged as garbage. Hence these areas are free to be used for other objects or can be released back to the system

This is where the GC is done and it may resume the execution of the application. However, if there are too many of those holes (released objects) created in the system, then the memory gets fragmented. To reduce memory fragmentation. The GC may compact the memory by moving objects around. Do note that compaction doesn’t happen for every GC, it is run based on some fragmentation heuristics.

Both C and D is moved here to squeeze out the hole for B. At the same time all references to these objects in the system is also update to point to the correct location.

One important thing to note here is that unlike native objects, managed objects can move around in memory due to compaction and hence taking direct references to it (a.k.a memory pointers) is not possible. In case this is ever required, e.g. a managed buffer is passed to say the microphone driver native code to copy recorded audio into, the GC has to be notified that the corresponding managed object cannot be moved during compaction. If the GC runs a compaction and object moves during that microphone PCM data copy, then memory corruption will happen because the object being copied into would’ve moved. To stop that, GCHandle has to be created to that object with GCHandleType.Pinned to notify the GC that the corresponding object should never move.

On the WP7 the interfaces to these peripherals and sensors are wrapped by managed interfaces and hence the WP7 developer doesn’t really have to do these things, they are taken care offm under the hood by those managed interfaces.

The performance issue

As mentioned before during the entire GC the execution of the application is stopped. So as long as the GC is running the application is frozen. This isn’t a problem in general because the GC runs pretty fast and infrequently. So small latencies of the order of 10-20ms is not really noticeable.

However, with WP7 the capability of the device in terms of CPU and memory drastically increased. Games and large Silverlight applications started coming up which used close to 100mb of memory. As memory increases the number of references those many objects can have also increases exponentially. In the scheme explained above the GC has to traverse each and every object and their reference to mark them and later remove them via sweep. So the GC time also increases drastically and becomes a function of the net workingset of the application. This results in very large pauses in case of large XNA games and SL applications which finally manifests as long startup times (as GC runs during startup) or glitches during the game play/animation.

Generational Approach

If we take a look at a simplified allocation pattern of a typical game (actually other apps are also similar), it looks somewhat like below

The game has a large steady state memory which contains much of it’s steady state data (which are not released) and then per-frame it goes on allocating/de-allocating some more data, e.g. for physics, projectiles, frame-data. To collect this memory layout the traditional GC has to walk or traverse the entire 50+ mb of data to locate the garbage in it. However, most of the data it traverses will almost never be garbage and will remain in use.

This application behavior is used for the Generational GC premise

1. Most objects die young
2. If an object survives collection (that is doesn’t die young) it will survive for a very long time

Using these premise the generational GC tries to segregate the managed heap into older and younger generations objects. The younger generation called Gen-0 is collected in each GC (premise 1), this is called the Ephemeral or Gen0 Collection. The older generation is called Gen-1. The GC rarely collects the Gen-1 as the probability of finding garbage in it is low (premise 2).

So essentially the GC becomes free of the burden of the net working set of the application.

Most GC will be ephemeral GC and it will only traverse the recently allocated objects, hence the GC latency remains very low. Post collection the surviving objects are promoted to the higher generation. Once a lot of objects are promoted, the higher generation starts becoming full and then a full collection is run (which collects both gen-1 and gen-0). However, due to premise 1, the ephemeral collection finds a lot of garbage in their runs and hence promotes very few objects. This means the growth rate of the higher generation is low and hence full-collection will run very infrequently.

Ephemeral/Gen-0 collection

Even in ephemeral collection the GC needs to deterministically find all objects in Gen-0 which are not reachable. This means the following objects needs to survive a Gen-0 collection

1. Objects directly reachable from roots
2. Root –> Gen0 –> Gen-0 objects (indirectly reachable from roots)
3. Objects referenced from Gen1 to Gen0

Now #1 and #2 pose no issues as in the Ephemeral GC, we will anyway scan all roots and Gen-0 objects. However to find objects from Gen1 which are referencing objects in Gen-0, we would have to traverse and look into all Gen1 objects. This will break the very purpose of having segregating the memory into generation. To handle this write-barrier+card-table technique is used.

The runtime maintains a special table called card-table. Each time any references are taken in the managed code e.g. a.b = c; the code JITed for this assignment also updates an internal data-structure called CardTable to capture that write. Later during the ephemeral collection, the GC looks into that table to find all the ‘a’ which took new references. If that ‘a’ is a gen-1 object and ‘c’ a gen-0 object then it marks ‘c’ recursively (which means all objects reachable from ‘c’ is also marked). This technique ensures that without examining all the gen-1 objects we can still find all live objects in Gen-0. However, the cost paid is

1. Updating object reference is a little bit more costly now
2. Making old object taking references to new objects increases GC latency (more about these in later posts)

Putting it all together, the traditional GC would traverse all references shown in the diagram below. However, an ephemeral GC can work without traversing the huge number of Red references.

It scans all the Roots to Gen-0 references (green) directly. It traverses all the Gen1->Gen0 references (orange) via the CardTable data structure.

Conclusion

1. Generational GC reduces the GC latency by avoiding looking up all objects in the system
2. Most collections are gen-0 or ephemeral collection and are of low latency this ensures fast startup and low latency in game play
3. However, based on how many objects are being promoted full GC’s are run sometimes. When they do, they exhibit the same latency as a full GC on the previous WP7 GC

Given the above most applications will see startup and in-execution perf boost without any modification. E.g today if an application allocates 5 MB of data during startup and GC runs after every MB of allocation then it traverses 15mb (1 + 2 + 3 + 4 + 5). However, with GenGC it might get away with traversing as low as only 5mb.

In addition, especially game developers can optimize their in-gameplay allocations such that during the entire game play there is no full collection and hence only low-latency ephemeral collections happens.

How well the generational scheme works depend on a lot of parameters and has some nuances. In the subsequent posts I will dive into the details of our implementation and some of the design choices we made and what the developers needs to do to get the most value out of it.

Windows Phone 7 App Development: When does the GC run

Many moons ago I made a post on When does the .NET Compact Framework Garbage Collector run. Given that a lot of things have changed since then, it’s time to make another post about the same thing.

For the developers coming to Windows Phone 7 (WP7) from the Windows desktop let me first clarify that the runtime (CLR) that is running on the WP7 is not the same as the one running on the desktop. The WP7 runtime is known as .NET Compact Framework (NETCF) and it works differently than the “desktop CLR”. For 90% of cases this is irrelevant as the WP7 developer targets the XNA or Silverlight programming model and hence what is running underneath is really not important. E.g when you drive you really do not care about the engine. This post is for the other 5% where folks do run into issues (smoke coming out of the car).

Moreover do note that when the GC is run is really an implementation detail that is subject to change.

Now that we have all the disclaimers behind us lets get down to the list.

The Garbage Collector is run in the following situations

1. After some significant allocation:
   When an application tries to allocate managed memory the allocator first checks a counter that indicates the number of bytes of managed data allocated since the last GC. If this counter crosses a threshold (which is changeable and set to 1MB currently) then GC is fired (and the counter is obviously reset).
   The basic idea is that there has been significant allocation since the last GC and hence do it again.
   
2. Resource allocation failure
   If some internal native allocation fails, like loadlibrary fails or JIT buffer allocation fails due to out-of-memory condition then GC is started to free up some memory and the allocation is re-attempted (only 1 re-attempt)
   
3. User code can trigger GC
   Using the managed API System.GC.Collect(), user code can force a GC
   
4. Sharing server initiated
   One of the new features in the WP7 CLR is the sharing server (see my post http://blogs.msdn.com/b/abhinaba/archive/2010/04/28/we-believe-in-sharing.aspx for details). In WP7 there is a central server coordinating all the managed processes. If this sharing server is notified of low memory condition, it starts GC in all the managed processes in the system.

The GC is NOT run in the following cases (I am explicitly calling these out because in various conferences and interactions I’ve heard folks thinking it might be)

1. GC is not run on some timer. So if a process is not allocating any memory and there is no low-memory situation then GC will never be fired irrespective of how long the application is running
2. The phone is never woken up by the CLR to run GC. GC is always in response to an active request OR allocation failure OR low memory notification.
3. In the same lines there is no GC thread or background GC on WP7

 

For folks migrating from NETCF 3.5 the list below gives you the changes

1. WinForm Application going to background used to fire GC. On WP7 this is no longer true
2. Sharing server based changes are obviously new
3. The GC quantum cannot be changed by user

Inside the Windows Phone Emulator

Learn from the dev lead and PM of Windows Phone 7 Emulator on how it works and delivers the awesome performance.

Some key points

1. The emulator is essentially a x86 based Virtual Machine running full image of Windows Phone 7 OS
2. It emulates various peripherals (e.g. audio, networking), the list hopefully will grow in the future
3. Supports multi-touch if your host PC has a touch screen
4. It uses the GPU on the host PC to accelerate the graphics
5. Shameless plug: One of the reason it’s possible to emulate x86 and still have managed apps running as-is, is because the runtime (.NET Compact Framework) supports both x86 and ARM seamlessly (provides JITer for both architectures and runs the same managed code on both seamlessly)

We Believe in Sharing

In Building NETCF for Windows Phone 7 series we put in couple of features to enhance startup performance and ensure that the working set remains small. One of these features added is code/data sharing.

Native applications have inherent sharing where multiple processes can share the same executable code. However, in case of managed code running on NETCF 3.5 even when multiple applications use the same assembly, the managed code in them are JITed in context of each process separately. This results in suboptimal resource utilization because of the following

1. Repeated JITing of the same code
2. Memory overhead of having identical copies of the same JITed code for every process running them. The execution engine also maintains records of the various types loaded and even though two (or more) processes may be loading the same types from the same assemblies; per process copies are maintained for them.

Together the overhead can be significant.

In the latest version of the runtime shipping with Windows Phone 7 (.NET Compact Framework 3.7) we added a feature to share both the JITed code and these type information across multiple managed processes.

The runtime essentially uses a client server architecture. We added a new kernel mode server
which is responsible of maintaining a system wide shared heap. The server on receiving requests
from the various client processes (each managed app is a client) JITs code and type information
into this shared heap. The shared heap is accessed Read-only from all the client apps and
Read/write from the server.

When a process requests for the same type to be loaded or code to be JITed it just re-uses the already loaded type info or JITed code from the shared heap.
What is shared

1. Various Execution engine data-structures like loader type-information
2. JITed code from a predefined list of platform assemblies (user code is not shared).

Do note that user code is not shared and neither is all platform code. Sharing arbitrarily would actually increase cost if they ultimately didn’t get reused in a lot of applications. The list of assemblies/data shared was carefully chosen and tuned to ensure only those are shared that provided the maximum performance benefit.

Sharing provides the following advantages

1. Warm startup time is significantly reduced. When a new application is coming up a large
   percentage of the initial code and type-info is already found on the shared heap and hence
   there is significant perf boost. The exact number depends on the applications but it’s common
   to get around 30% gains
2. Significant reduction in net working set as a lot of commonly used platform assembly JITed code is re-used
3. Obviously there are other goodness like less JITing means less processing on the device

Like user code the system is also capable of pitching the shared code when there is a low memory situation on the device.

Silverlight on Nokia S60 platform

Today at MIX 2010 we announced Silverlight beta for Nokia S60 phones. Go download the beta from http://silverlight.net/getstarted/devices/symbian/. 

Currently the supported platform is S60 5th Edition phones and in particular the 5800 XpressMusic, N97 and N97 mini. It works in-browser and only in the Nokia default web-kit based browser.

Silverlight on S60 uses .NET Compact Framework (NETCF) underneath it. I work in the dev team for the Execution Engine of NETCF and I was involved at the very beginning with getting NETCF to work on the S60 platform.

NETCF is extremely portable (more about that in later posts) and hence we didn’t have a lot of trouble getting it onto S60. We abstract away the platform by coding against a generic platform abstraction layer (PAL), but even then we had some challenges. Getting .NET platform on a non-Windows OS is challenging because so much of our code relies on Windows like functionality (semantics) which might not be available directly on other platforms.

As I was digging through emails I found the following screenshot that I had sent out to the team when I just got NETCF to work on the S60 Emulator…

As you can see the first code to run was a console application and it was not Hello World :). We have come a long way since then as evident from the touch based casual game running on the N97 shown below

MIX 2010 Announcements

Today is a very big day for me and my team. The stuff we have been working on is finally went live on stage MIX 2010.

Windows Phone 7 is easily the most dramatic re-entry/reset Microsoft has undertaken in any field it’s in. We announced the phone in MWC and the reception it got was phenomenal. However, in the new connected devices world, the application platform+market story is as or more important than the device itself. Today at MIX we just disclosed details of that.

1. The application model supported on Windows Phone 7 series will be managed only and will leverage Silverlight, XNA and the .NET Compact Framework. So basically you can reuse your C#/.NET and Silverlight skills to build amazing experience on the phone. I will get into details of this in the coming posts
2. Microsoft just announced free Windows Phone Developer Tools (preview bits available for download head over to http://developer.windowsphone.com/ )
   1. This builds on the amazing development experience brought in by the Visual Studio 2010 shell.
   2. You can either get a standalone Visual Studio 2010 Express for Windows Phone Or an addin for your previously installed VS 2010
   3. Windows Phone 7 series emulator (yes you do not even need a device to develop for Windows Phone)
3. A version of Expression Blend is also demonstrated for the Windows Phone (a free version will be available for download in the coming months)
4. A new Windows Phone Marketplace is being put into place
   1. Developers can earn 70% revenue from the applications they sell of the marketplace
   2. The registration to the market place is not free but it’ll be free for the DreamSpark program for students

Our team was involved with building the IDE, the emulator and the core .NET runtime that powers the applications. So watch out and in the coming posts I’ll deep dive into these areas.