PLINQ Support

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PLINQ Support

LinqConnect Documentation
PLINQ Support
[email protected]

The .NET Framework 4.0 release included PLINQ, an engine for parallel execution of LINQ queries. This technology provides considerable speed-up of LINQ to Objects query execution on multi-core machines, as it implicitly partitions the query data source into segments each being processed by a separate processor. To prepare a data source for parallel querying, one only needs to call the AsParallel extension method. For example, in the following code we get discontinued products (see the CRM Demo sample database) from the database, and process the resulting list in a parallel fashion:

C#csharp	Copy Code
public static double XMasDiscountedPrice(double? price) { return price.GetValueOrDefault() / 2; } static void Main(string[] args) { //... var query = from product in context.Products where product.Discontinued.GetValueOrDefault() == true select product; var discontinuedProducts = query.ToList(); var productsForSale = from product in discontinuedProducts.AsParallel() // This is the place where PLINQ is enabled. select new { Name = product.ProductName, Price = XMasDiscountedPrice(product.Price) }; }

C#csharp

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public static double XMasDiscountedPrice(double? price) {
    return price.GetValueOrDefault() / 2;
}
 
static void Main(string[] args)
{
    //...
    var query = from product in context.Products 
                where product.Discontinued.GetValueOrDefault() == true 
                select product;
    var discontinuedProducts = query.ToList();
    var productsForSale = 
        from product in
            discontinuedProducts.AsParallel()  // This is the place where PLINQ is enabled.
        select new
        {
            Name = product.ProductName,
            Price = XMasDiscountedPrice(product.Price)
        };
}

Visual Basic	Copy Code
Public Shared Function XMasDiscountedPrice(price As System.Nullable(Of Double)) As Double Return price.GetValueOrDefault() / 2 End Function Private Shared Sub Main(args As String()) '... Dim query = From product In context.Products Where product.Discontinued.GetValueOrDefault() = True Select product Dim discontinuedProducts = query.ToList() ' PLINQ is enabled at the .AsParallel() call Dim productsForSale = _ From product In discontinuedProducts.AsParallel() _ Select New With { _ Key .Name = product.ProductName, _ Key .Price = XMasDiscountedPrice(product.Price) _ } End Sub

Visual Basic

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Public Shared Function XMasDiscountedPrice(price As System.Nullable(Of Double)) As Double
    Return price.GetValueOrDefault() / 2
End Function
 
Private Shared Sub Main(args As String())
    '...
    Dim query = From product In context.Products 
                Where product.Discontinued.GetValueOrDefault() = True
                Select product
    Dim discontinuedProducts = query.ToList()
    ' PLINQ is enabled at the .AsParallel() call
    Dim productsForSale = _
        From product In discontinuedProducts.AsParallel() _
        Select New With { _
            Key .Name = product.ProductName, _
            Key .Price = XMasDiscountedPrice(product.Price) _
        }
End Sub

Notes on this sample:

PLINQ is enabled for a local collection here, though the collection itself is retrieved via LinqConnect. We mention it to note that the PLINQ-related part of this sample is not LinqConnect-specific, and you could use any other data source as well.
XMasDiscountedPrice is a lightweight method, while the PLINQ advantages are best shown on more expensive functions (or large data sources, which is also not the case). The point is that distributing tasks between threads and processors is not free, and overhead caused by it can negate the positive effects of parallelizing. However, we use a simple method here to make the sample more clear.

LinqConnect supports the PLINQ engine, meaning that you can execute the AsParallel method on LinqConnect queries, causing the resulting collections to be processed in parallel. E.g., the above sample can be rewritten as

C#csharp	Copy Code
public static double XMasDiscountedPrice(double? price) { return price.GetValueOrDefault() / 2; } static void Main(string[] args) { //... var query = from product in context.Products where product.Discontinued.GetValueOrDefault() == true select new { Name = product.ProductName, Price = XMasDiscountedPrice(product.Price) }; var productsForSale = query .AsParallel() // This is the place where PLINQ is enabled. .ToList(); }

C#csharp

Copy Code

public static double XMasDiscountedPrice(double? price) {
    return price.GetValueOrDefault() / 2;
}
 
static void Main(string[] args)
{
    //...
    var query = from product in context.Products
                where product.Discontinued.GetValueOrDefault() == true
                select new
                {
                    Name = product.ProductName,
                    Price = XMasDiscountedPrice(product.Price)
                };
    var productsForSale =
      query
        .AsParallel() // This is the place where PLINQ is enabled.
        .ToList();
}

Visual Basic	Copy Code
Public Shared Function XMasDiscountedPrice(price As System.Nullable(Of Double)) As Double Return price.GetValueOrDefault() / 2 End Function Private Shared Sub Main(args As String()) '... Dim query = _ From product In context.Products _ Where product.Discontinued.GetValueOrDefault() = True _ Select New With { _ Key .Name = product.ProductName, _ Key .Price = XMasDiscountedPrice(product.Price) _ } ' PLINQ is enabled at the .AsParallel() call Dim productsForSale = query _ .AsParallel() _ .ToList() End Sub

Visual Basic

Copy Code

Public Shared Function XMasDiscountedPrice(price As System.Nullable(Of Double)) As Double
    Return price.GetValueOrDefault() / 2
End Function
 
Private Shared Sub Main(args As String())
    '...
    Dim query = _
        From product In context.Products _
        Where product.Discontinued.GetValueOrDefault() = True _
        Select New With { _
            Key .Name = product.ProductName, _
            Key .Price = XMasDiscountedPrice(product.Price) _
        }
    ' PLINQ is enabled at the .AsParallel() call
    Dim productsForSale = query _
        .AsParallel() _
        .ToList()
End Sub

What should be kept in mind when using PLINQ for LinqConnect queries is:

Parallel execution is done at the client side only; it does not affect SQL commands performed at a server in any way.
As a corollary of the first point, it only makes sense to use PLINQ if you do process the query result in your application. If you just need to materialize a query result set, PLINQ can hardly give you any performance gain. The cases when it does make sense to use PLINQ are, e.g., when the processing function is expensive (see the above note on the XMasDiscountedPrice method) or when joining results from two heterogeneous data sources (like queries to different DataContext instances).

The AsParallel method should only be invoked on the 'final' queries, as after you call it, the rest of extension methods and LINQ expressions are considered to be of LINQ to SQL, not of LinqConnect. To explain what we mean saying this, let us apply AsParallel to the Products table in the previous sample:

C#csharp	Copy Code
var query = from product in context.Products.AsParallel() where product.Discontinued.GetValueOrDefault() == true select new { Name = product.ProductName, Price = XMasDiscountedPrice(product.Price) }; var productsForSale = query.ToList();

C#csharp

Copy Code

var query = from product in context.Products.AsParallel()
            where product.Discontinued.GetValueOrDefault() == true
            select new
            {
                Name = product.ProductName,
                Price = XMasDiscountedPrice(product.Price)
            };
var productsForSale = query.ToList();

Visual Basic	Copy Code
Dim query = From product In context.Products.AsParallel() _ Where product.Discontinued.GetValueOrDefault() = True _ Select New With { _ Key .Name = product.ProductName, _ Key .Price = XMasDiscountedPrice(product.Price) _ } Dim productsForSale = query.ToList()

Visual Basic

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Dim query =
    From product In context.Products.AsParallel() _
    Where product.Discontinued.GetValueOrDefault() = True _
    Select New With { _
        Key .Name = product.ProductName, _
        Key .Price = XMasDiscountedPrice(product.Price) _
    }
Dim productsForSale = query.ToList()

As you can see, the generated SQL statement has no WHERE clause, i.e., all products are fetched to the client, and only then the limitation 'Discontinued == 1' is checked. The cause of this is that the parallel execution is enabled at the Products table, making context.Products.AsParallel() a local collection subject to LINQ to Objects processing.

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