# How to accelerate matrix operations(multiplication, add, inverse) in a for loop?

No, it doesn’t. What is `I` and `CCore(X’*b)`? is it a function or multiplication? And put the code between two three back-tics like this:

```
code here
```

I apologize for the misunderstanding.

``````function test(b)
X=randn(16,12);
l=100;
local Weights
for i=1:500000
CCore=X.'*X+eye(12,12)*l;
Weights=CCore\(X.'*b);
end
return Weights
end
``````

OK., if you use `StaticArrays` you can get at least 2X speedup for free:

``````julia> using StaticArrays
julia> function test(b)
X = @SMatrix randn(16,12)
l = 100
local Weights
for i = 1:500000
CCore = X'*X + I*l
Weights = CCore \ (X'*b)
end
return Weights
end
test (generic function with 1 method)

julia> @btime test(@SMatrix randn(16,1))
593.777 ms (0 allocations: 0 bytes)
``````

Note that `eye()` was removed, just use `I`, also `X.'` was removed now use `transpose(X)` and for real matrices, `X'` is the same. When JuliaPro is out, you can gain more speedups from MKL as @Elrod pointed out.

EDIT:

As suggested by @DNF, you can also enable multi-threading by running `set JULIA_NUM_THREADS=8` in a cmd terminal in the same folder as `julia.exe` and run the following:

``````julia> function test(b)
X = @SMatrix randn(16,12)
l = 100
Weights = SArray{Tuple{12,1},Float64,2,12}
CCore = transpose(X)*X + I*l
Weights = CCore \ (transpose(X)*b)
end
return Weights
end
test (generic function with 1 method)

julia> @btime test(@SMatrix randn(16,1));
178.113 ms (477874 allocations: 50.99 MiB)
``````

This is near 8X faster than your original version.

Matlab uses multi-threading internally. I’m not sure if that happens here, but you can make sure Julia uses it.

First, make sure that Julia has access to several threads (on mac/linux you invoke `export JULIA_NUM_THREADS=4` if you have 4 cores from the command line – not inside Julia itself.) Then you add a decorator in front of the loop:

``````Threads.@threads for i = 1:500000
``````

On my computer, using StaticArrays and multi-threading with 4 threads, I get a speedup of 22x relative to Matlab R2018a.

BTW: Your loop is weird. Why do you repeat the same operation 500_000 times? Is it just to get averaged timings? In that case, don’t. Julia’s BenchmarkTools.jl and Matlab’s `timeit` command does the averaging for you, and then we don’t have to wait forever for the benchmarking to run.

1 Like

BTW, I think you actually should use `randn(16)`, not `randn(16, 1)`. It’s easy to stuck on the Matlabism here, but a vector has size `(16,)` not `(16, 1)`.

The OP was just comparing two loops doing matrix operations, and yes MATLAB is multi-threaded by default, so enabling `@threads` in not unfair but may be in the future, Julia will use multi-threaded matrix operations by default (I’m not sure if it already does that currently).

OK, let me explain you the big picture to make sense of the loop. What I am doing is translating a toy image de-noising Matlab code to Julia code to see how Julia performs in generic image processing task. It only means to learn and test Julia in a fast way in the context of a typical image processing task. The performance of the toy code is just trivial.

This de-noising algorithm is using the linear combination of non-local similar patches to predict each patch values. You can also view it as self-regression. Specifically, it executes the following steps:

1. Decompose an NXN image into (N-n+1)*(N-n+1) overlapped patches, each patch has nXn pixels (using im2col).
2. For each patch, search for its top K most similar patches within a window of size (2W+1)*(2W+1)
3. Using the patch and all its K most similar patches to find the weights that approximate itself(I use RIDGE regression to estimate the weights) and get an updated patch. (This serves as a de-noising step.)
4. Use all the updated patches to synthesize a de-noised image.

Let me post both the Julia and Matlab code to accelerate the discussion of benchmarking. My Julia coding is majorly from the answers of my recent questions since last week. I wish to thank all the people for answering. ([What is Julia's maxk(Matlab) that returns the indice of top K largest values?] and [What is Julia's im2col?])

Currently, the Matlab code (in Matlab 2018a) is roughly 1.1X the speed of the Julia code(in JuliaPro 0.6.4), which is NOT what I want. I really wish that Julia is doing a much faster job than Matlab in image processing regime.

Julia Main Function:

``````using MAT
using QuantEcon: meshgrid
using Compat
include("PSNR.jl")
include("Image_Crop.jl")
include("Demo_Julia.jl")
include("Image_Extend.jl")
include("im2col.jl")
include("xx_yy_min_max.jl")
include("maxk2!.jl")
include("ind2sub_local.jl")
file = matopen("y1.mat")
close(file)
n=6
W=10
K=12
lambda=300
y_noisy=y1+randn(512,512)*3;
tic()
(y_denoised)=Demo_Julia(y_noisy,n,W,K,lambda)
toc()
println("PSNR Before Denoising:")
println(PSNR(y_noisy,y1))
println("PSNR After Denoising:")
println(PSNR(y_denoised,y1))
``````

Julia Subroutines:

``````function Demo_Julia(yNoisy,n,W,K,lambda)
yNoisy=Image_Extend(yNoisy,n)-128
(M,N)=size(yNoisy)
MAX_L=(2*W+1)^2
Coord_NL_Patches      =zeros(UInt32,K,(N-n+1)*(M-n+1))
Weight_NL_Patches     =zeros(Float32,K,(N-n+1)*(M-n+1))
NoisyPatch            =convert(Array{Float32},im2col(yNoisy,[n n]));
NoisyPatchNorm        =convert(Array{Float32},sqrt.(sum(NoisyPatch.^2,1))).';
x_half=convert(UInt32,M/2-n/2+1);
y_half=convert(UInt32,N/2-n/2+1);
for i=1:x_half*y_half
(xx_hLR,yy_wLR)=ind2sub_local(i,x_half)
yy=convert(UInt32,yy_wLR*2-1)
xx=convert(UInt32,xx_hLR*2-1)
(x_min,x_max,y_min,y_max)=xx_yy_min_max(xx,yy,M,N,W)
Search_Coord=zeros(UInt32,MAX_L,1)
(yyy,xxx)=meshgrid(y_min:y_max,x_min:x_max)
Search_Coord[1:(2*W+1)^2]=(yyy[:]-1)*(M-n+1)+xxx[:];
b=NoisyPatch[:,(yy-1)*(M-n+1)+xx]
s=Search_Coord[find(Search_Coord)]
PATCHES=NoisyPatch[:,s]
corr=PATCHES'*b./NoisyPatchNorm[s]
ix=collect(1:length(s))
ind=maxk2!(ix,corr,K,true);
Coord_NL_Patches[:,i]=s[ind]
X=NoisyPatch[:,s[ind]]
Core=transpose(X)*X+lambda*eye(K,K)
Weight_NL_Patches[:,i]=Core\(transpose(X)*b)
end
NoisyPatchHat=zeros(Float32,n^2,(N/2-n/2+1)*(M/2-n/2+1))
for i=1:x_half*y_half
(xx_hLR,yy_wLR)=ind2sub_local(i,x_half)
yy=convert(UInt32,yy_wLR*2-1)
xx=convert(UInt32,xx_hLR*2-1)
s=Coord_NL_Patches[:,i]
X=NoisyPatch[:,s]
omega=Weight_NL_Patches[:,i]
Y_Patch=X*omega
yPatch_Hat=reshape(Y_Patch,n,n)
yPatch_Hat[find(yPatch_Hat.>+127)]=+127
yPatch_Hat[find(yPatch_Hat.<-128)]=-128
NoisyPatchHat[:,i]=yPatch_Hat[:]
end
y_Hat=zeros(M,N)
Times=zeros(M,N)
for i=1:x_half*y_half
(row,col)=ind2sub_local(i,x_half)
r=convert(UInt32,row*2-1)
c=convert(UInt32,col*2-1)
patch=reshape(NoisyPatchHat[:,i],n,n)
y_Hat[r:r+n-1,c:c+n-1]= y_Hat[r:r+n-1,c:c+n-1]+patch
Times[r:r+n-1,c:c+n-1]=Times[r:r+n-1,c:c+n-1]+1
end
y_Hat=y_Hat./Times
y_DeNoise=Image_Crop(y_Hat,n)+128
return y_DeNoise
end
``````
``````function Image_Extend(im,L)
# extend the image by L.
(M,N)=size(im)
im_Extend=zeros(M+2*L,N+2*L)
im_Extend[L+1:M+L,L+1:N+L]=im[1:M,1:N]
im_Extend[1:M+2*L,    1:L    ]=flipdim(im_Extend[1:M+2*L,L+1:2*L], 2)
im_Extend[1:M+2*L,N+L+1:N+L*2]=flipdim(im_Extend[1:M+2*L,N+1:N+L], 2)
im_Extend[    1:L    ,1:N+2*L]=flipdim(im_Extend[L+1:2*L,1:N+2*L], 1)
im_Extend[M+L+1:M+L*2,1:N+2*L]=flipdim(im_Extend[M+1:M+L,1:N+2*L], 1)
return im_Extend
end
``````
``````function Image_Crop(im,L)
# crop the image by L.
(M,N)=size(im)
im_Crop=im[L+1:M-L,L+1:N-L]
return im_Crop
end
``````
``````function     im2col(a,block)
(ma,na) = size(a)
m = block
n = block
# Create Hankel-like indexing sub matrix.
mc = block;
nc = ma-m+1;
nn = na-n+1;
cidx = 0:1:mc-1;
ridx = (1:1:nc).';
t = repmat(cidx,1,nc)+repmat(ridx,mc,1);    # Hankel Subscripts
tt = zeros(mc*n,nc);
rows = 1:mc;
for i=0:n-1
tt[i*mc+rows,:] = t+ma*i;
end
ttt = zeros(UInt32,mc*n,nc*nn);
cols = 1:nc;
for j=0:nn-1
ttt[:,j*nc+cols] = tt+ma*j;
end
b = a[ttt];
return b
end
``````
``````function ind2sub_local(t,N)
y=mod(t,N);
x=(t-y)/N+1;
if (y==0)
y=N;
x=x-1;
end
return y,x
end
``````
``````function maxk2!(ix, a, k, initialized=false)
partialsortperm!(ix, a, 1:k, rev=true, initialized=initialized)
return ix[1:k]
end
``````
``````function xx_yy_min_max(xx,yy,M,N,W)
x_min=xx-W
x_max=xx+W
y_min=yy-W
y_max=yy+W
if  xx-W<=0
x_min=1
x_max=1+2*W
end
if  xx+W>M-n+1
x_max=M-n+1
x_min=M-n+1-2*W
end
if  yy-W<=0
y_min=1
y_max=1+2*W
end
if  yy+W>N-n+1
y_max=N-n+1
y_min=N-n+1-2*W;
end
return x_min,x_max,y_min,y_max
end
``````
``````function PSNR(x, y)
err = x - y
err = err[:]
PSNRdb = 20 * log10(255/sqrt(mean(err.^2)))
return PSNRdb
end
``````

Matlab Main Function:

``````close all;clear;clc;
[M,N]=size(y1);
n=6;
W=10;
K=12;
lambda=300;
y_noisy=y1+randn(512)*3;
tic;
y_denoised=Demo_Matlab(y_noisy,n,W,K,lambda);
toc;
disp(["PSNR before denoising:", num2str(PSNR(y_noisy,y1))]);
disp(["PSNR after denoising:", num2str(PSNR(y_denoised,y1))]);

``````

Matlab Subroutines:

``````function y_denoised=Demo_Matlab(yPre,n,W,K,lambda)
yPre=Image_Extend(yPre,n)-128;
[M,N]=size(yPre);
MAX_L=ceil((2*W+1)^2/4);
Coord_NL_Patches  =zeros(K,(N/2-n/2+1)*(M/2-n/2+1),'uint32');
Weight_NL_Patches =zeros(K,(N/2-n/2+1)*(M/2-n/2+1),'single');
NoisyPatch           =single(im2col(yPre,[n n],'sliding'));
NoisyPatchNorm       =single(vecnorm(NoisyPatch).');
for i=1:(M/2-n/2+1)*(N/2-n/2+1)
[xx_LR,yy_LR]=ind2sub([M/2-n/2+1 N/2-n/2+1],i);
xx=xx_LR*2-1;
yy=yy_LR*2-1;
x_min=xx-W;% W is always even, x_min is always odd.
x_max=xx+W;
y_min=yy-W;% W is always even, y_min is always odd.
y_max=yy+W;
if  xx-W<=0
x_min=1;
x_max=1+2*W;
end
if  xx+W>M-n+1
x_min=M-n+1-2*W;
x_max=M-n+1;
end
if  yy-W<=0
y_min=1;
y_max=1+2*W;
end
if  yy+W>N-n+1
y_max=N-n+1;
y_min=N-n+1-2*W;
end
Search_Coord=zeros(MAX_L,1,'uint32');
[yyy,xxx]=meshgrid(y_min:y_max,x_min:x_max);
Search_Coord(1:(2*W+1)^2)=(yyy-1)*(M-n+1)+xxx;
b=NoisyPatch(:,(yy-1)*(M-n+1)+xx);
pos=Search_Coord>0;
s=Search_Coord(pos);
PATCHES=NoisyPatch(:,s);
corr=PATCHES'*b./NoisyPatchNorm(s);
[~,indd]=maxk(corr,K);
Coord_NL_Patches(:,i)=s(indd);
X=NoisyPatch(:,s(indd));
Core=X'*X+lambda*eye(K);
Weight_NL_Patches(:,i)=Core\(X'*b);
end
NoisyPatch_Hat=zeros(n^2,(N/2-n/2+1)*(M/2-n/2+1),'single');
for i=1:(M/2-n/2+1)*(N/2-n/2+1)
s=Coord_NL_Patches(:,i);
X=NoisyPatch(:,s);
omega=Weight_NL_Patches(:,i);
Y_Patch=X*omega;
Y_Patch(Y_Patch>+127)=+127;
Y_Patch(Y_Patch<-128)=-128;
NoisyPatch_Hat(:,i)=Y_Patch(:);
end
% ======
y_Hat=zeros(M,N);
Times=zeros(M,N);
for i=1:(M/2-n/2+1)*(N/2-n/2+1)
[row,col]=ind2sub([M/2-n/2+1 N/2-n/2+1],i);
patch=reshape(NoisyPatch_Hat(:,i),[n n]);
r=row*2-1;
c=col*2-1;
y_Hat(r:r+n-1,c:c+n-1)= y_Hat(r:r+n-1,c:c+n-1)+patch;
Times(r:r+n-1,c:c+n-1)=Times(r:r+n-1,c:c+n-1)+1;
end
y_Hat=y_Hat./Times;
y_denoised=Image_Crop(y_Hat,n)+128;
end
function im_Crop=Image_Crop(im,L)
% crop the image by L.
[M,N]=size(im);
im_Crop=im(L+1:M-L,L+1:N-L);
end
function im_Extend=Image_Extend(im,L)
% extend the image by L.
[M,N]=size(im);
im_Extend=zeros(M+2*L,N+2*L);
im_Extend(L+1:M+L,L+1:N+L)=im(1:M,1:N);

im_Extend(1:M+2*L,    1:L    )=fliplr(im_Extend(1:M+2*L,L+1:2*L));
im_Extend(1:M+2*L,N+L+1:N+L*2)=fliplr(im_Extend(1:M+2*L,N+1:N+L));

im_Extend(    1:L    ,1:N+2*L)=flipud(im_Extend(L+1:2*L,1:N+2*L));
im_Extend(M+L+1:M+L*2,1:N+2*L)=flipud(im_Extend(M+1:M+L,1:N+2*L));
end
``````

and

``````function PSNRdb = PSNR(x, y)

err = x - y;
err = err(:);
PSNRdb = 20 * log10(255/sqrt(mean(err .^2)));
end
``````

The y1.mat file that Julia is using is the .MAT file version of 512X512 Lena image. They can be both downloaded here:

There are some points need to be modified:
a). May could use the

``````M=5.;N=3.;
I=1:Int(fld(M,N));
I=1:Int(cld(M,N));
I=1:Int(round(M/N));
``````

if the type of I isn’t Int;
b). Could you explain each sub-modules of the Julia such as include(…).
Do they recognize the functions of images processing?

In addition, if M is an array, there is an effective solution:

``````A=rand(1000)
B = fill(1,1000)
for i=1:length(A)
B[i]=Int(round(A[i]/1.0));
end
``````

``````include("PSNR.jl") # compute the peak signal-to-noise ratio of the images.
include("Image_Crop.jl") # shrink the size of the image by cropping the pixels near the boundary
include("Demo_Julia.jl")
include("Image_Extend.jl") # extend the size of the image by copying the pixels near the boundary
include("im2col.jl")     # Decompose an NXN image into (N-n+1)*(N-n+1) overlapped patches, each patch has nXn pixels
include("xx_yy_min_max.jl") # compute the boundary of the search window
include("maxk2!.jl")  # compute the indices of the top largest  K values from a 1D array.
include("ind2sub_local.jl") # transfer index of a 2D array to the subscripts in row and in column

``````

When you say “recognize the functions of image processing”, what do you mean? I am sorry I am not quite sure.

OK, this is a lot to digest, and very little comments. It will take a lot of work and time to get into it.

For your own sake and those who wish to help: Can you start by identifying bottlenecks? Look into profiling. In Matlab it’s easy (if you’re unsure how to start, just ask). In Julia it’s supposed to also be easy, at least in Juno, though I haven’t tried myself.

Once you identify the bottleneck(s), especially the ones where you feel that Julia should do better, please report back.

Which Julia version are you using?

Superficial impression:

• you have a lot of Matlab-isms, `repmat`, slicing and other unnecessary allocations. (And what is this: `ridx = (1:1:nc).';??)` You know, stuff like `y_Hat=y_Hat./Times` that totally unnecessarily creates a new array.
• `Image_Extend` looks like it could use some smart array type instead of all the copying etc.
• You use `UInt32` and `Float32` a lot (why?) with lots of conversions that seem unnecessary (this mainly makes the code messy, not sure if it hurts performance)

Edit: I see you are using `meshgrid`, that’s a bad sign, indicating wasted memory use. Look into broadcasting instead. I think even Matlab is abandoning `meshgrid`, `repmat` etc. in favor of broadcasting operations.

Oh, and any chance you can clean up the code? You know, put spaces around operators, and after commas, and stuff. My eyes hurt from reading squashed together code.

Note that,

Matlab version: `MAX_L=ceil((2*W+1)^2/4);`

Julia version: `MAX_L=(2*W+1)^2`

why?

Are you looking for JuliaImages?

You have lots of stuff like this:

``````x_half = convert(UInt32,M/2-n/2+1)
``````

This is slow, and makes no sense, as far as I can tell. Later you use this for `1:x_half*y_half`, which turns it back into `Int64`, and then you use floating point division: `i/x_half` that turns it into `Float64`, and then you `convert` it back to `Int32` again for indexing.

This is not your bottleneck, I’m pretty sure, but there is a lot of noise like this, which achieves nothing but create confusion (and at worst, type-instabilities). Just stick to normal `Int` and `Float64`. Then, at the end, when squeezing out the last nanoseconds, maybe consider using `Float32`, especially if on a GPU.

So use ordinary `Int` for indexing, and use `div` for integer division. You can get rid of pretty much all your `convert` calls:

``````x_half = div(M-n, 2) + 1 # or (M - n) ÷ 2 + 1   or (M - n) >> 1 + 1
``````

I’m just picking up random stuff here, but the above creates unnecessary arrays (and it will fail on Julia 1.0). Use `clamp` instead:

``````yPatch_Hat .= clamp.(yPatch_Hat, -128, 127)  # or clamp!(yPatch_Hat, -128, 127)
``````

A short advice because the code is too long, but you will get the point.

As others noticed, you are translating MATLAB code line-by-line to what you think is Julia equivalent, this is not true, it doesn’t work like this. We, like you, were deceived at the early versions of Julia that Julia was a better MATLAB, this now is very far from being true, Julia has deviated too much, as of version 0.6 onward, away from MATLAB that it is now a totally different language with its own idioms. So, simply my advice is to write your code from scratch in Julia’s own idioms and expressions avoiding this matrix-focused style. Julia is not good at working with slices and allocations of too many matrices and vectors, if you avoid these, you will gain much higher speedups, but as I said, your code will have no resemblance to what you wrote in MATLAB.

1 Like

I think you are basically right in your remarks, but we should avoid creating a misunderstanding here. Julia can do slices and vectorization etc. etc. just fine. But in Matlab you do it because you have to. In Julia you don’t, and you can do much better.

2 posts were split to a new topic: Julia vs Matlab side thread

Thank you so much!

Hi all, I cleaned up the code a little bit, it turns out the bottleneck is at this line:

`corr = (noisyPatch[px,py] ⋅ b) / noisyPatchNorm[px,py]`

any idea on how to optimize this?

1 Like