Need for Speed Boost Ruby with FFI @johnlinvc 

Me • John Lin ( ྛ⇉ᠳʣ • @johnlinvc • Head of Algorithm @ Spoonrocket 

What’s SpoonRocket? 

The Problem 

heroku/router: at=error code=H12 
 desc="Request timeout" 

Real World® Problem 

We Need Speed 

Change Language • Switch to Go / Scala / Java / C++ 

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Go Native • C extension • FFI 

C Extension • Write the critical part using Ruby API in C • Need compiler on production environment • Not portable between Platforms 

Foreign Function Interface • Mounting C function as Ruby method • Call functions using C Library libffi • Doesn’t need compiler • Portable between platforms 

C-ext vs FFI 

How Libffi works in C 

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C Library Structure(ELF) 9/8/2015 https://upload.wikimedia.org/wikipedia/commons/7/77/Elf-layout--en.svg ... .data .rodata .text Program header table ELF header Section header table { { https://en.wikipedia.org/wiki/Executable_and_Linkable_Format 

Load Library void* dlopen(const char* path, int mode); • load a shared library file at path & relocate the address 

Function Address λ ~/ nm /usr/lib/libruby.dylib 0000000000027a75 T _InitVM_Enumerator 00000000000088de T _Init_Array 000000000012a65e T _Init_BareVM 00000000000164e2 T _Init_Bignum 0000000000037829 T _Init_Binding 000000000001b08e T _Init_Comparable 000000000001be5c T _Init_Complex 000000000013c869 T _Init_Cont 0000000000020721 T _Init_Dir 00000000000032ec T _Init_Encoding 00000000000239a6 T _Init_Enumerable 000000000002906a T _Init_Enumerator 000000000002caa7 T _Init_Exception 

Find Function Address void* dlsym(void* handle, const char* symbol); • get address of a symbol from a library 

Function call with cdecl 

Call Function With Address In C typedef int func(void); func * f = (func*) 0x12345566; int res = f(); 

Define Function Signature ffi_status ffi_prep_cif( ffi_cif *cif, ffi_abi abi, unsigned int nargs, ffi_type *rtype, ffi_type **atypes); Define the signature of a function. cif stands for Call InterFace 

Invoke Function void ffi_call(ffi_cif *cif, void (*fn)(void), void *rvalue, void **avalue); • ffi_call(): call the actual function by address, with return address and input parameters 

dlopen() dlsym() ffi_prep_cif() ffi_call() 

Ruby-FFI • Wrapper of C Library libffi • First appeared in Rubinius • Maintained by JRuby team • Available in MRI, JRuby & Rubinius(same interface) 

Installing • gem install ffi 

Hello FFI #!/usr/bin/env ruby require 'ffi' module LIBC extend FFI::Library ffi_lib FFI::Library::LIBC attach_function :printf, [:string], :int end LIBC.printf("hello FFI\n") 

Loading Library • Loading Multiple Libs
 ffi_lib "foo", "bar" • Loading all possible names of a Lib
 ffi_lib [“foo", “foo-1.2”] 

Attach Function • Function rename
 attach_function :c_print, :printf, [:string], :int • Function with variable length
 attach_function :printf, [:string, :varargs], :int
 

Supported C Types • Integer : singed/unsigned of various length • Floating Point: float / double • String: NULL terminated char array. • Pointer: pointer 

Define C Structs class Timeval < FFI::Struct layout :tv_sec => :ulong, :tv_usec => :ulong end struct timeval { unsinged long tv_sec; /* seconds since 1/1/1970 */ unsinged long tv_usec;/* and microseconds */ }; C Ruby 

Accessing C Struct module LibC extend FFI::Library ffi_lib FFI::Library::LIBC attach_function :gettimeofday, [ :pointer, :pointer ], :int end t = Timeval.new LibC.gettimeofday(t.pointer, nil) puts "t.tv_sec=#{t[:tv_sec]} t.tv_usec=#{t[:tv_usec]}" int gettimeofday(struct timeval * tp, void * tzp); C signature Ruby 

Define Callback module LibC extend FFI::Library ffi_lib FFI::Library::LIBC callback :qsort_cmp, [ :pointer, :pointer ], :int attach_function :qsort, [ :pointer, :ulong, :ulong, :qsort_cmp ], :void end void qsort(void *base, size_t nel, size_t width, int (*compar)(const void *, const void *)); C signature Ruby 

Using Callback pointer = FFI::MemoryPointer.new(:int, 2) ary = [ 2, 1 ] pointer.write_array_of_int32(ary) size_of_int = 4 LibC.qsort(pointer, ary.count, size_of_int) do |p1, p2| i1 = p1.read_int32 i2 = p2.read_int32 i1 < i2 ? -1 : i1 > i2 ? 1 : 0 end p pointer.get_array_of_int32(0, ary.count) 

Features • Load C library • Invoke C function • Auto convert between Ruby types & C types • Define/Access C Struct • Using block/Proc as callback 

Benchmark • Find Eigenvalue & Eigenvector of a matrix • Benchmark the runtime of • Ruby Matrix in Stdlib • C Extension with LAPACK • FFI with LAPACK 

Performance N=1000 N=10000 N=100000 Ruby 0.3 2.87 30.3 FFI 0.02 0.19 2.1 C-Ext 0.01 0.1 0.99 

Take aways • Don’t give up ruby if you need speed. • How FFI works under the hood • FFI core features 

Q&A 

References • Linker & Loaders • Libffi • ruby-ffi Gem • Race car Image • x86 call convention