Why is spdiags() much slower now than in previous versions

Question

Mads 2021-5-6

3
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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/823610-why-is-spdiags-much-slower-now-than-in-previous-versions

编辑： Brandon Caasenbrood 2023-4-19

I have used spdiags() over many years.

Lately MathWorks have updated the script and the response is much slower.

I have compared spdiags() from R2019a with R2021a on a mac.

The older spdiags() runs in my present test 15 to 40 times faster than the latest spdiags().

I wonder what benefit I get from waiting that much longer, for something that involves plugging numbers into a sparse matrix.

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Matt J 2021-5-9

在 MATLAB Online 中打开

Mads' comment moved here:

When I run your code, I also see no problem. I confirm your results.

I can't send the entire code of mine or the data.

However:

P = 4096

Dm2_... are complex, but with zeros in some places.

Profiler showed quite some time spent in makeSparseGeneral(B, d, A)

    version
    disp('old')
    tic
    matrix = spdiags_old([Dm2_,Dm1_,Dp1_,Dp2_],[-2,-1,1,2],P*3,P*3);
    toc
    disp('new')
    tic
    matrix = spdiags([Dm2_,Dm1_,Dp1_,Dp2_],[-2,-1,1,2],P*3,P*3);
    toc

Two outputs from R2020b for reference:

ans =
    '9.9.0.1592791 (R2020b) Update 5'
old
Elapsed time is 0.007126 seconds.
new
Elapsed time is 0.261705 seconds.
ans =
    '9.9.0.1592791 (R2020b) Update 5'
old
Elapsed time is 0.006587 seconds.
new
Elapsed time is 0.270015 seconds.

MANY outsputs from R2021a, which by the way runs very unstable.

ans =
    '9.10.0.1602886 (R2021a)'
old
Elapsed time is 0.011837 seconds.
new
Elapsed time is 0.250930 seconds.
ans =
    '9.10.0.1602886 (R2021a)'
old
Elapsed time is 0.006751 seconds.
new
Elapsed time is 0.249982 seconds.
ans =
    '9.10.0.1602886 (R2021a)'
old
Elapsed time is 0.006735 seconds.
new
Elapsed time is 0.250596 seconds.
ans =
    '9.10.0.1602886 (R2021a)'
old
Elapsed time is 0.006552 seconds.
new
Elapsed time is 0.254115 seconds.
ans =
    '9.10.0.1602886 (R2021a)'
old
Elapsed time is 0.006326 seconds.
new
Elapsed time is 0.249205 seconds.
ans =
    '9.10.0.1602886 (R2021a)'
old
Elapsed time is 0.007143 seconds.
new
Elapsed time is 0.256610 seconds.
ans =
    '9.10.0.1602886 (R2021a)'
old
Elapsed time is 0.006193 seconds.
new
Elapsed time is 0.254189 seconds.
ans =
    '9.10.0.1602886 (R2021a)'
old
Elapsed time is 0.006665 seconds.
new
Elapsed time is 0.267214 seconds.

Bruno Luong 2023-4-18

在 MATLAB Online 中打开

@Brandon Caasenbrood "I can confirm the same. spdiags has poor performance compared to previous matlab version (e.g. 2021a outperforms 2023a signficantly)"

I just test the same code I posted above with R2023a, and it is faster than R2021a.

>> testspdiags
ans =
'9.14.0.2206163 (R2023a)'
Elapsed time is 0.153604 seconds. % REMINDER: Elapsed time is 0.188962 seconds. for R2021a

Brandon Caasenbrood 2023-4-19

编辑：Brandon Caasenbrood 2023-4-19

在 MATLAB Online 中打开

I might have shouted wolf too early. But I'm getting subpar performance with spdaig in comparison to speye when make diagonal matrices. I've recently moved from 2021a to 2023a, and only now noticed the performance dip; which made me believe the "6 May 2021" issue above was related. Consider the following code (a segment from the MMA solver by Krister Svanberg - https://gitlab.gbar.dtu.dk/s144426/s144426-TopOpt/-/blob/9e34bb5bb19b2e0f5aebbb4640f4d296627140d6/subsolv.m)

GG and GG_ are identical, but speye outperforms spdiags here. Nevertheless, running the below code, gives indeed similar performance. What am I missing here?

clc;
M = 3e4;
N = 1e3;
tic;
disp('test: spdiags');
a = rand(M,1);
for ii = 1:N
    A = spdiags(a,0,M,M) - spdiags(a/10,0,M,M);
end
toc
tic;
disp('test: speye');
a = rand(M,1);
for ii = 1:N
    A = a.*speye(M,M) - (a/10).*speye(M,M);
end
toc
% test: spdiags
% Elapsed time is 0.814878 seconds.
% test: speye
% Elapsed time is 0.847202 seconds.

The profiler says the bottleneck is in spdiags>makeSparseGeneral.Slightly odd, since I, J, and V are preallocated above, and I also never seen a profiler highlighting the iteration counter before.

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Why is spdiags() much slower now than in previous versions

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Why is spdiags() much slower now than in previous versions

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