Sunday, June 7, 2009

SQL Server as a Runtime Host Part 1

If you are a SQL Server developer or database administrator, you might just be inclined to use the new Common Language Runtime (CLR) hosting feature to write stored procedures in C# or VB.NET without knowing how it works. But you should care. SQL Server is an enterprise application, perhaps one of the most important in your organization. When the CLR was added to SQL Server, there were three goals in the implementation, considered in this order:

1. Security
2. Reliability
3. Performance

The reasons for this order are obvious. Without a secure system, you have a system that runs reliably run code, including code introduced by hackers, very quickly. It’s not what you’d want for an enterprise application. Reliability comes next. Critical applications, like a database management system, are expected to be available 99.99% of the time.

You don’t want to wait in a long line at the airport or the bank while the database restarts itself. Reliability is therefore considered over performance when the two clash; a decision might be whether to allow stack overflows to potentially bring down the main application, or slow down processing to make sure they don’t. Since applications that perform transactional processing use SQL Server, SQL Server must ensure data integrity and its transactional correctness, which is another facet of reliability.

Performance is extremely important in an enterprise application as well. Database management systems can be judged on benchmarks, such as the TPC-C (Transaction Processing Performance Council benchmark C) benchmark, as well as programmer-friendly features. So although having stored procedures and user-defined types written in high-level languages is a nice feature, it has to be implemented in such a way as to maximize performance.

Since SQL Server 2005 is going to introduce fundamental changes such as loading .NET runtime engines and XML parsers, we’ll first consider how SQL Server 2005 works as a .NET runtime host, how it compares with other .NET runtime hosts, and what special features of the runtime are used to ensure security, reliability, and performance.

You may already know that an updated version of the .NET runtime, .NET 2.0, will be required for use with SQL Server. In this article, I will explain why. A runtime host is defined as any process that loads the .NET runtime and runs code in a managed environment. The most common scenario is that a runtime host is simply a bootstrap program that executes from the Windows shell, loads the runtime into memory, and then loads one or more managed assemblies.

An assembly is the unit of deployment in .NET roughly analogous to an executable program or DLL in prior versions of Windows. A runtime host loads the runtime by using the ICorRuntimeHost or CorBindToRuntimeEx, prior to Whidbey. These APIs call a shim DLL, MSCOREE.DLL, whose only job is to load the runtime.

Only a single copy of the runtime (also known as the CLR) engine can ever be loaded into a process during the process’s lifetime; it is not possible to run multiple versions of the CLR within the same host. In pre-Whidbey versions of .NET, a host could specify only a limited number of parameters to ICorRuntime Host or CorBindToRuntimeEx, namely the following:

• Server or workstation behavior
• Version of the CLR (for example, version 1.0.3705.0)
• Garbage collection behavior
• Whether or not to share Just-In-Time compiled code across
AppDomains (an AppDomain is a subdivision of the CLR runtime space)

Two examples of specialized runtime hosts are the ASP.NET worker process and Internet Explorer. The ASP.NET worker process differs in codelocation and how the executable code, threads, and AppDomains are organized. The ASP.NET worker process divides code into separate “applications,” application being a term that is borrowed from Internet Information Server to denote code running in a virtual directory. Code is located in virtual directories, which are mapped to physical directories in the IIS metabase.

Internet Explorer is another runtime host with behaviors that differ from the ASP.NET worker or SQL Server 2005. IE loads code when it encounters a specific type of <object> tag in a Web page.
The location of the code is obtained from an HTML attribute of the tag. SQL Server 2005 is an example of a specialized runtime host that goes far beyond ASP.NET in specialization and control of CLR semantics.

SQL Server’s special requirements of utmost security, reliability, and performance, in addition to the way that SQL Server works internally, have necessitated an overhaul in how the managed hosting APIs work as well as in how the CLR works internally. Although early versions of SQL Server 2005 did run on .NET version 1.0, the changes in the CLR are important in ensuring enterprise quality.

SQL Server is a specialized host like ASP.NET and IE, rather than a simple bootstrap echanism. The runtime is lazy loaded; if you never use a managed stored procedure or user-defined type, the runtime is never loaded. This is useful because loading the runtime takes a one-time memory allocation of approximately 10–15MB in addition to SQL Server’s buffers and unmanaged executable code, although this certainly will not be the exact number in SQL Server 2005.

How SQL Server manages its resources and locates the code to load is unique as well. To accommodate hosts that want to have hooks into the CLR’s resource allocation and management, .NET 2.0 hosts can use ICLRRuntimeHost instead of ICorRuntimeHost. The host can then call ICLRRuntimeHost::SetHostControl, which takes a pointer to an interface (IHostControl) that contains a method that the CLR can call (GetHostManager) to delegate things like thread management to the host.

SQL Server uses this interface to take control of some functions that the CLR usually calls down to the operating system for directly. SQL Server manages its own thread scheduling, synchronization and locking, and memory allocation. In .NET runtime hosts, these are usually managed by the CLR itself.
In SQL Server 2005 this conflict is resolved by layering the CLR’s mechanisms on top of SQL Server’s mechanisms.

SQL Server uses its own memory allocation scheme, managing real memory rather than using virtual memory. It attempts to optimize memory, balancing between data and index buffers, query caches, and other internal data structures. SQL Server can do a better job if it manages all of the memory in its process.

As an example, prior to SQL Server 2000, it was possible to specify that the TEMPDB database should be allocated in memory. In SQL Server 2000 that option was removed, based on the fact that SQL Server can manage this better than the programmer or DBA. SQL Server manages its memory directly by, in effect, controlling paging of memory to disk itself rather than letting the operating system do it.

Because SQL Server attempts to use as much memory as is allocated to the process, this has some repercussions in exceptional condition handling, which I will discuss next. SQL Server also uses its own thread management algorithm, putting threads “to sleep” until it wants them to run. This facility is known as UMS (user-mode scheduler).

Optionally, SQL Server can use fibers rather than threads through a configuration option, though this option is rarely used. The CLR also maintains thread pools and allows programmers to create new threads. The key point is that SQL Server uses cooperative thread scheduling; the CLR uses preemptive thread scheduling.

Cooperative thread scheduling means that a thread must voluntarily yield control of the rocessor; in preemptive thread scheduling the processor takes control back from the thread after its time slice has expired. SQL Server uses cooperative thread scheduling to minimize thread context switches.

With threading come considerations of thread synchronization and locking. SQL Server manages locking of its own resources, such as database rows and internal data structures. Allowing programmers to spin up a thread is not to be taken lightly. SQL Server 2005 CLR code executes under different permission levels with respect to CLR activities.

The hosting APIs in .NET 2.0 are enhanced to enable the runtime host to either control or “have a say in” resource allocation. The APIs manage units of work called Tasks, which can be assigned to a thread or a fiber. The SQL scheduler manages blocking points, and hooks PInvoke and interop calls out of the runtime to control switching the scheduling mode.

The new control points allow SQL Server to supply a host memory allocator, to be notified of low memory conditions at the OS level, and to fail memory allocations if desired. SQL Server can also use the hosting API to control I/O completion ports usually managed by the CLR.

Although this may slow things down a little in the case of an allocation callback, it is of great benefit in allowing SQL Server to manage all of its resources, as it does today. In .NET 1.0 certain exceptional conditions, such as an out-of-memory condition or a stack overflow, could bring down a running process (or App Domain). This cannot be allowed to happen in SQL Server.

Although transactional semantics might be preserved, reliability and performance would suffer dramatically.
In addition, unconditionally stopping a thread (using Thread.Abort or other API calls) can conceivably leave some system resources in an indeterminate state and, though using garbage collection minimizes memory leakage, leak memory. Different runtime hosts deal with these hard-to-handle conditions in different ways.

In the ASP.NET worker process, for example, recycling both the AppDomain and the process itself is considered acceptable since disconnected, short-running Web requests would hardly notice. With SQL Server, rolling back all the in-flight transactions might take a few minutes. Process recycling would ruin long-running batch jobs in progress.

Therefore, changes to the hosting APIs and the CLR exceptional condition handling needed to be made. Out-of-memory conditions are particularly difficult to handle correctly, even when you leave a safety buffer of memory to respond to them. In SQL Server the situation is exacerbated because SQL Server manages its own memory and attempts to use all memory available to it to maximize throughput.

This leaves us between a rock and a hard place. As we increase the size of the “safety net” to handle out-of-memory conditions, we also increase the occurrence of out-of-memory conditions. The Whidbey runtime handles these conditions more robustly; that is, it guarantees availability and reliability after out-of-memory conditions without requiring SQL Server to allocate a safety net, letting SQL Server tune memory usage to the amount of physical memory.

The CLR will notify SQL Server about the repercussions of failing each memory request. Low-memory conditions may be handled by permitting the garbage collector to run more frequently, waiting for other procedural code to finish before invoking additional procedures, or aborting running threads if needed.

There is also a failure escalation policy at the CLR level that will allow SQL Server to determine how to deal with exceptions. SQL Server can decide to abort the thread that causes an exception and, if necessary, unload the AppDomain. On resource failures, the CLR will unwind the entire managed stack of the session that takes the resource failure.

If that session has any locks, the entire AppDomain that session is in is unloaded. This is because having locks indicates there is some shared state to synchronize, and thus that shared state is not likely to be consistent if just the session was aborted. In certain cases this might mean that finally blocks in CLR code may not run.

In addition, finalizers, hooks that programmers can use to do necessary but not time-critical resource cleanup, might not get run. Except in UNSAFE mode (discussed later in the chapter), finalizers are not permitted in CLR code that runs in SQL Server.

Stack overflow conditions cannot be entirely prevented, and are usually handled by implementing exceptional condition handling in the program. If the program does not handle this condition, the CLR will catch these exceptions, unwind the stack, and abort the thread if needed. In exceptional circumstances, such as when memory allocation during a stack overflow causes an out-of-memory condition, recycling the App Domain may be necessary.

In all the cases just mentioned, SQL Server will maintain transactional semantics. In the case of AppDomain recycling, this is needed to assert that the principal concern is reliability, if needed, at the expense of performance. In addition, all the Framework class libraries (FX libraries) that SQL Server will load have gone through a careful review and testing to ensure that they clean up all memory and other esources after a thread abort or an AppDomain unload.

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