Jonathan Blow of “The Witness” fame likes to talk about just typing the obvious code first. Usually it will turn out to be fast enough. If it doesn’t, you can go back and optimize it later. His thoughts come in the context of working on games in C/C++. I think these languages, with modern incarnations of their compilers, are compatible with this philosophy. Not only are the compilers very mature but they are low level enough that you are forced to do things by hand, and think about what the machine is doing most of the time, especially if you stick to C or a ‘mostly C’ subset of C++. However in most higher level languages, there tend to be performance traps where the obvious, or idiomatic solution is particularly bad.

What counts as obvious or idiomatic, is of course often a matter of opinion. The language itself may encourage certain choices by making them easier to type, or highlighting them in documentation and teaching materials. The community that grows up around a language may just come to prefer certain constructs and encourage others to use them. It is very common to see programmers encouraged to use high level constructs over lower level ones, in the interest of readability and simplicity. This is a worthy ideal, but often people aren’t aware of what the cost really is. Some of these constructs have a much higher cost than people realize.

In this article I will explore a number of languages, with a toy map and reduce example. Within each language, I will explore a number of approaches, ranging from high level to hand coded imperative loops and SIMD operations. Some of the performance pitfalls I will show may be specific to this toy example. With a different toy example, the languages that excel and those that do poorly could be totally different. This is meant merely to explore, and get people thinking about the performance cost of abstractions. For each case I will show code examples so you can consider the differences in complexity.

The Task

We wish to take an array of 32 million 64bit floating point values, and compute the sum of their squares. This will let us explore some fundamental abilities of various languages. Their ability to iterate over arrays efficiently, whether they can vectorize basic loops, and whether higher order functions like map and reduce compile to efficient code. When applicable, I will show runtimes of both map and reduce, so we get insight into whether the language can stream higher order functions together, and also the runtime with a single reduce or fold operation.

The Results

C - 17 milliseconds

    double sum = 0.0;    
    for (int i = 0; i < COUNT; i++) 
        double v = values[i] * values[i]; 
        sum += v;

ANSI C is a bare bones language, no higher order functions or loop abstractions exist to even think about, so this imperative loop is what most programmers wil turn to to complete this task. If I thought that this would be a performance critical piece of code, I might use SIMD intrinsics, which requires this nasty mess:

C - SIMD Explicit - 17 milliseconds

    __m256d vsum = _mm256_setzero_pd();
    for(int i = 0; i < COUNT/4; i=i+1) {
        __m256d v = values[i];
        vsum = _mm256_add_pd(vsum,_mm256_mul_pd(v,v));
    double *tsum = &vsum;
    double sum = tsum[0]+tsum[1]+tsum[2]+tsum[3];

However, notice that the runtime is the same for the obvious and SIMD versions! It turns out that the obvious code was automatically turned into SIMD enhanced machine instructions. A process called “Auto vectorization”. Visual C++ is not known for being the most clever of C++ compilers but it still gets this right:

double sum = 0.0;    
	for (int i = 0; i < COUNT; i++) {
00007FF7085C1120  vmovupd     ymm0,ymmword ptr [rcx]  
00007FF7085C1124  lea         rcx,[rcx+40h]  
		double v = values[i] * values[i];  //square em
00007FF7085C1128  vmulpd      ymm2,ymm0,ymm0  
00007FF7085C112C  vmovupd     ymm0,ymmword ptr [rcx-20h]  
00007FF7085C1131  vaddpd      ymm4,ymm2,ymm4  
00007FF7085C1135  vmulpd      ymm2,ymm0,ymm0  
00007FF7085C1139  vaddpd      ymm3,ymm2,ymm5  
00007FF7085C113D  vmovupd     ymm5,ymm3  
00007FF7085C1141  sub         rdx,1  
00007FF7085C1145  jne         imperative+80h (07FF7085C1120h)  
		sum += v;

To get the SIMD instructions used here, which can operate on 4 doubles at a time, you have to specify to the compiler that you want ‘fast floating point’ and specify that you want to target AVX2 instructions as well. Results will be different when vectorized, though they will actually be more accurate, not less. (in this case, maybe all?)

C# Linq Select Sum - 260 milliseconds

    var sum = values.Sum(x => x * x);

C# Linq Aggregate - 280 milliseconds

    var sum = values.Aggregate(0.0,(acc, x) => acc + x * x);

C# for loop - 34 milliseconds

    double sum = 0.0;
    foreach (var v in values)
        double square = v * v;
        sum += square;       

Stepping up a level to C#, we have a couple of idiomatic solutions. Many C# programmers today might use Linq which as you can see is much slower. It also creates a lot of garbage, putting more pressure on the garbage collector. Oddly, the Aggregate function, which is equivalent to fold or reduce in most other languages, is slower despite being a single step instead of two. The foreach loop in the second example is also commonly used. While this pattern has big performance pitfalls when used on collections like List<T>, with arrays it compiles to efficient code. This is nice as it saves you some typing without runtime penalty. The runtime here is still twice as slow as the C code, but that is entirely due to not being automatically vectorized.
With the .NET JIT, it is not considered a worthwhile tradeoff to do this particular optimization.

With C# you also have to take some care with array access in loops, or bounds checking overhead can be introduced. In this case the JIT gets it right, and there is no bounds checking overhead.

C# SIMD Explicit - 17 milliseconds

    Vector<double> vsum = new Vector<double>(0.0);
    for (int i = 0; i < COUNT; i += Vector<double>.Count)
        var value = new Vector<double>(values, i);
        vsum = vsum + (value * value);
    double sum = 0;
    for(int i = 0; i < Vector<double>.Count;i++)
        sum += vsum[i];

While the .NET JIT won’t do SIMD automatically, we can explicitly use some SIMD instructions, and achieve performance nearly identical to C. An advantage here for C# is that the SIMD code is a bit less nasty than using intrinsics, and that particular instructions whether they be AVX2, SSE2, NEON, or whatever the hardware supports, can be decided upon at runtime. Whereas the C code above would require separate compilation for each architecture. A disadvantage for C# is that not all SIMD instructions are exposed by the Vector library, so something like SIMD enhanced noise functions can’t be done with nearly the same performance. As well, the machine code produced by the Vector library is not always as efficient when you step out of toy examples.

F# - 127 milliseconds

    let sum =
        |> squares
        |> Array.sum

The obvious F# code is beautiful, I like typing this, and I like working with it. But performance is terrible. Just as with C# you get no auto vectorization, as they use the same JIT. Additionally the array is iterated over twice, once to map them to squares, and once to sum them. Finally, since immutability is the default, each operation returns a new array, incurring allocation costs and GC pressure. So the total performance impact on an application is likely to be worse than this micro benchmark would suggest.

F# Streams - 98 milliseconds

    let sum = 
        |> square
        |> Stream.sum

F# is a functional first language, rather than a pure functional language like Haskell. If you do happen to use pure functions, you can stream your map and sum operations together, and avoid iterating over the array twice. The Nessos Streams library provides this, with a nice performance improvement as a result.

F# Fold - 75 milliseconds

    let sum = 
        |> Array.fold (fun acc x -> acc + x*x) 0.0

When we use a single fold operation, we no longer iterate over the collection twice and allocate extra memory, and runtime improves even more. Since there is no overhead for streaming together multiple higher order functions as there is in the Streams library, it does slightly better.

F# Imperative - 38 milliseconds

    let mutable sum = 0.0
    for i = 0 to values.Length-1 do
            let x = values.[i]
            sum <- sum + x*x            

One of the nice things about F#, is that while it is a functional leaning language, very few barriers are put in your way if you want to go imperative for the sake of speed. Write a normal for loop, and you get the same performance as SSE vectorized C.

F# SIMD - 18ms

    let sum =
        |> Array.SIMD.fold (fun acc v -> acc +v*v) (+) 0.0    

Now to get serious. First we use fold, so that we can combine the summing and squaring into a single pass. Then we use the SIMDArray extensions that I have been working on which let you take full advantage of SIMD with more idiomatic F#. Performance here is great, nearly as fast as C, but it took a lot of work to get here. At the moment there is no way to combine the lazy stream optimization with the SIMD ones. If you want to filter->map->reduce you will still be doing a lot of extra work. This should be possible in principle though. Please submit a PR!

Rust - 34ms

    let sum = values.iter().
                map(|x| x*x).        

Rust achieves impressive numbers with the most obvious approach. This is super cool. I feel that this behavior should be the goal for any language offering these kinds of higher order functions as part of the language or core library. Using a traditional for loop or a ‘for x in y’ style loop is also just as fast. It is also possible to use rust intrinsics to get the same speed as the AVX2 vectorized C code here, but to use those you have to write out the loop explicitly:

Rust SIMD - 17ms

    let mut sum = 0.0;
    unsafe {
        for v in values {
            let x : f64 = std::intrinsics::fmul_fast(*v,*v);
            sum = std::intrinsics::fadd_fast(sum,x); 

It would be nice if the rustc compiler had an option to just apply this globally, so you could use the higher order functions. Also, these features are marked as unstable, and likely to remain unstable forever. This might make it problematic to use this feature for any important production project. It would also be nice if the unsafe block was not required. Hopefully the Rust maintainers have a plan to make this better.

Javascript map reduce (node.js) 10,000ms

var sum = => x*x).
                 reduce( (total,num,index,array) => total+num,0.0);

Javascript reduce (node.js) 800 and then 300 milliseconds

var sum = values.reduce( (total,num,index,array) => total+num*num,0.0)

It is common to see these higher order javascript functions suggested as the most elegant way to do this, but it is incredibly slow. Simplifying the combined map and reduce improves runtime by an order of magnitude to 800ms, though after 3 or 4 iterations the JIT does some magic and runtime drops to 300ms thereafter. This represents the first time I have seen any substantive JIT optimization happen during runtime in the wild!

Javascript foreach (node.js) 800 and then 300 milliseconds

    var sum = 0.0;
    array.forEach( (element,index,array) => sum += element*element  )

Slightly less elegant but also a popular idiom in javascript, this is faster than map and reduce, but is still amazingly slow. Again, after 3 or 4 iterations the JIT does some magic and it speeds up from around 800 to 300 milliseconds.

Javascript imperative (node.js) 37 milliseconds

    var sum = 0.0;
    for (var i = 0; i < values.length;i++){
        var x = values[i];
        sum += x*x;

Finally, when we get down to a basic imperative for loop, javascript performs comparably to SEE vectorized C.

Java Streams Map Sum 138 milliseconds

    double sum =
                        map(x -> x*x).

Java Streams Reduce 34 milliseconds

    double sum =
                        reduce(0,(acc,x) -> acc+x*x);

Java 8 includes a very nice library called stream which provides higher order functions over collections in a lazy evaluated manner, similar to the F# Nessos streams library and Rust. Given that this is a lazy evaluated system, it is odd that there is such a performance difference between map then sum and a single reduction. The reduce function is compiling down to the equivalent of SSE vectorized C, but the map then sum is not even close. It turns out that the sum() method on DoubleStream:

may be implemented using compensated summation or other technique to reduce the error bound in the numerical sum compared to a simple summation of double values.

A nice feature, but not clearly communicated by the method name! If we tweak the java code to do normal summation the runtime remains as fast as SSE vectorized C, a nice accomplishment:

Java Streams Map Reduce 34 milliseconds

    double sum =
                        map(x -> x*x).
                        reduce(0,(acc,x) -> acc+x);

There does not appear to be a way to get SIMD out of Java, either explicitly or via automatic vectorization by the Hotspot JVM. There are 3rd party libraries available that do it by calling C++ code. I do see some literature stating that the JVM can and does auto-vectorize, but I’m not seeing evidence of that in this case, or when I use a for loop, either.

Go for Range 37 milliseconds

    sum := 0.0
    for _,v := range values[:] {
        sum = sum +  v*v

Go for loop 37 milliseconds

    sum := 0.0
    for i := 0; i < len(values); i++ {
        x := values[i]
        sum = sum +  x*x

Go has good performance with the both the usual imperative loop and their ‘range’ idiom which is like a ‘foreach’ in other languages.
Neither auto vectorization nor explicit SIMD support appears to be completely not on the Go radar. There are no map/reduce/fold higher order functions in the standard library, so we can’t compare them. Go does a good thing here by not providing a slow path at all.


I have shown some performance pitfalls in various languages here. One should not read too much into this as an argument for general performance of these languages. Every language has some pitfalls where the preferred or easiest approaches to solving a problem can lead to performance pitfalls. In Java, for instance, everything is objects. Objects all allocate on the heap (unless the JIT does some work at runtime to determine it doesn’t need to go on the heap, but that isn’t a freebie). Since Java is also a garbage collected language, this can lead to performance pitfalls when you type the obvious code. With experience, you can learn about these pitfalls and do work to avoid them, just like you can avoid pitfalls of Linq in C#, by not using it, or the pitfalls of F# by using Stream or SIMD libraries instead of the core ones. But even then, you have to take extra care, and type extra code, or take on more dependencies to do that. This is partially purpose defeating, since high level languages are supposed to let you type less, and get things working faster.

What I would like to see is more of an attitude change among high level language designers and their communities. None of the issues above need to exist. Java could (and will, soon) provide value types (as C# does) to make it less painful to avoid GC pressure if you use lots of small, short lived constructs. Go could provide more SIMD support, either via a SIMD library or better auto vectorization. F# could provide efficient Streams as part of the core library like Java does. .NET could auto vectorize in the JIT and/or provide more complete coverage of SIMD instructions in the Vector library. We, the community, can help by providing libraries and submitting PRs to make the obvious code faster. Time and energy will be saved, batteries will last longer, users will be happier.

Benchmark Details

All benchmarks run with what I believe to be the latest and greatest compilers available for Windows for each language. JIT warmup time is accounted for when applicable. If you identify cases where code or compiler/environment choices are sub optimal, email me please.


Host Process Environment Information:
OS=Microsoft Windows NT 6.2.9200.0
Processor=Intel(R) Core(TM) i7-4712HQ CPU 2.30GHz, ProcessorCount=8
Frequency=2240907 ticks, Resolution=446.2479 ns, Timer=TSC

F# / C# Runtime Details

CLR=MS.NET 4.0.30319.42000, Arch=64-bit RELEASE [RyuJIT]
GC=Concurrent Workstation
Type=SIMDBenchmark  Mode=Throughput  Platform=X64  
Jit=RyuJit  GarbageCollection=Concurrent Workstation  

C Details

Visual Studio 2015 Update 3, fast floating point, 64 bit, AVX2 instructions enabled, all speed optimizations on

Rust Details

v1.13 Nightly, –release -opt-level=3

Javascript/Node Details

v6.4.0 64bit NODE_ENV=production

Java Details

Oracle Java 64bit version 8 update 102

Go Details

Go 1.7