System.OutOfMemoryException for a tail recursive function

Question

I have isolated the problematic code to this function (that uses ASP.NET's Membership class):

let dbctx = DBSchema.GetDataContext()
let rec h1 (is2_ : int) (ie2_ : int) : unit =
    match is2_ >= ie2_ with
    | true ->
        let st2 = query {
            for row in dbctx.Tbl_Students do
            where (row.Id = is2_)
            head}
        let l2 =
            Membership.FindUsersByEmail (st2.Email_address)
            |> Seq.cast<_>
            |> Seq.length
        match l2 >= 1 with
        | true -> 
            ()
        | false ->
            Membership.CreateUser (st2.Email_address, password, st2.Email_address)
            |> ignore
        h1 (is2_ - 1) ie2_
    | false ->
        ()

I am getting a System.OutOfMemoryException after exactly 5626 iterations of h1. But my system's memory consumption is only at 20 percent. (I have a very powerful, 16 GB machine.)

Why should the above function overflow the stack? Is it not written tail recursively?

Thanks in advance for your help.

Answer 1

I don't think this is a tail-recursion issue -- if so, you'd be getting a StackOverflowException instead of an OutOfMemoryException. Note that even though you have 16GB of memory in your machine, the process your program is executing in may be limited to a smaller amount of memory. IIRC, it's 3GB for some combinations of .NET framework version and OS version -- this would explain why the process is crashing when you reach ~20% memory usage (20% of 16GB = 3.2GB).

I don't know how much it'll help, but you can simplify your code to avoid creating some unnecessary sequences:

let dbctx = DBSchema.GetDataContext()
let rec h1 (is2_ : int) (ie2_ : int) : unit =
    if is2_ >= ie2_ then
        let st2 = query {
            for row in dbctx.Tbl_Students do
            where (row.Id = is2_)
            head }
        let existingUsers = Membership.FindUsersByEmail st2.Email_address
        if existingUsers.Count < 1 then
            Membership.CreateUser (st2.Email_address, password, st2.Email_address)
            |> ignore

        h1 (is2_ - 1) ie2_

EDIT : Here's a link to a previous question with details about the CLR memory limitations for some versions of the .NET framework and OS versions: Is there a memory limit for a single .NET process

Answer 2

OutOfMemoryException usually doesn't have anything to do with the amount of RAM you have. You get it at ~3 GB most likely because your code runs as a 32-bit process. But switching it to 64 bit will only solve your issue if you actually need that much memory and the exception is not caused by some bug.