Why is printf in F# so slow?

Question

I've just been really surprised by how slow printf from F# is. I have a number of C# programs that process large data files and write out a number of CSV files. I originally started by using fprintf writer "%s,%d,%f,%f,%f,%s" thinking that that would be simple and reasonably efficient.

However after a while I was getting a bit fed up with waiting for the files to process. (I've got 4gb XML files to go through and write out entries from them.).

When I ran my applications through a profiler, I was amazed to see printf as being one of the really slow methods.

I changed the code to not use printf and now performance is so much better. Printf performance was killing my overall application performance.

To give an example, my original code is:

fprintf sectorWriter "\"%s\",%f,%f,%d,%d,\"%s\",\"%s\",\"%s\",%d,%d,%d,%d,\"%s\",%d,%d,%d,%d,%s,%d"
    sector.Label sector.Longitude sector.Latitude sector.RNCId sector.CellId
    siteName sector.Switch sector.Technology (int sector.Azimuth) sector.PrimaryScramblingCode
    (int sector.FrequencyBand) (int sector.Height) sector.PatternName (int sector.Beamwidth) 
    (int sector.ElectricalTilt) (int sector.MechanicalTilt) (int (sector.ElectricalTilt + sector.MechanicalTilt))
    sector.SectorType (int sector.Radius)

And I've changed it to be the following

seq {
    yield sector.Label; yield string sector.Longitude; yield string sector.Latitude; yield string sector.RNCId; yield string sector.CellId; 
    yield siteName; yield sector.Switch; yield sector.Technology; yield string (int sector.Azimuth); yield string sector.PrimaryScramblingCode;
    yield string (int sector.FrequencyBand); yield string (int sector.Height); yield sector.PatternName; yield string (int sector.Beamwidth); 
    yield string (int sector.ElectricalTilt); yield string (int sector.MechanicalTilt); 
    yield string (int (sector.ElectricalTilt + sector.MechanicalTilt));
    yield sector.SectorType; yield string (int sector.Radius)
}
|> writeCSV sectorWriter

Helper functions

let writeDelimited delimiter (writer:TextWriter) (values:seq<string>) =
    values
    |> Seq.fold (fun (s:string) v -> if s.Length = 0 then v else s + delimiter + v) ""
    |> writer.WriteLine

let writeCSV (writer:TextWriter) (values:seq<string>) = writeDelimited "," writer values

I'm writing out files with about 30,000 rows. Nothing special.

Answer 1

I am not sure how much it matters, but...

Inspecting the code for printf:

https://github.com/fsharp/fsharp/blob/master/src/fsharp/FSharp.Core/printf.fs

I see

// The general technique used this file is to interpret
// a format string and use reflection to construct a function value that matches
// the specification of the format string.  

and I think the word 'reflection' probably answers the question.

printf is great for writing simple type-safe output, but if you want good perf in an inner loop, you might want to use a lower-level .NET API to write output. I haven't done my own benchmarking to see.