# Blas dot doing weird things depending on size
**URL:** https://discourse.julialang.org/t/blas-dot-doing-weird-things-depending-on-size/30430
**Category:** Performance
**Tags:** question
**Created:** [October 29, 2019, 6:57am UTC](https://discourse.julialang.org/t/blas-dot-doing-weird-things-depending-on-size/30430 "2019-10-29T06:57:28Z")
**Posts on this page:** 4
**Page:** 1
### Author:  [@Oscar\_Smith](https://discourse.julialang.org/u/Oscar_Smith)
#### Post date: [October 29, 2019, 6:57am UTC](https://discourse.julialang.org/t/blas-dot-doing-weird-things-depending-on-size/30430/1 "2019-10-29T06:57:28Z")
I’m working on some by hand matrix multiplication.
```
function slowMult!(A::AbstractMatrix{T},B::AbstractMatrix{T}) where T<:Number
C = Matrix{T}(undef,size(A,1),size(B,2))
for i in 1:size(A,1)
for j in 1:size(B,2)
@inbounds @views C[i,j] = dot(A[i,:], B[:,j])
end
end
return C
end
```
This is obviously not going to be the fastest approach, but what’s really weird is that calling this function on 2 `512` by `512` matrices is about 2x slower than on `514` by `514` matrices. The timing code is
```
T,N=Int,512
A = rand(T,N,N)
B = rand(T,N,N)
slowMult!(rand(T,1,1),rand(T,1,1)
C= @btime slowMult!($A,$B)
```
---
### Author:  [@Tamas\_Papp](https://discourse.julialang.org/u/Tamas_Papp)
#### Post date: [October 29, 2019, 8:07am UTC](https://discourse.julialang.org/t/blas-dot-doing-weird-things-depending-on-size/30430/2 "2019-10-29T08:07:50Z")
Could be some alignment issue.
Incidentally, the first thing I would fix is memory access order:
[https://docs.julialang.org/en/v1/manual/performance-tips/#Access-arrays-in-memory-order,-along-columns-1](https://docs.julialang.org/en/v1/manual/performance-tips/#Access-arrays-in-memory-order,-along-columns-1)
---
### Author:  [@kristoffer.carlsson](https://discourse.julialang.org/u/kristoffer.carlsson)
#### Post date: [October 29, 2019, 9:45am UTC](https://discourse.julialang.org/t/blas-dot-doing-weird-things-depending-on-size/30430/3 "2019-10-29T09:45:13Z")
Not sure this is the case here but looping over multiple arrays that has lengths that are a power of two can have bad effects on the cache:
[https://stackoverflow.com/questions/7905760/matrix-multiplication-small-difference-in-matrix-size-large-difference-in-timi](https://stackoverflow.com/questions/7905760/matrix-multiplication-small-difference-in-matrix-size-large-difference-in-timi)
[http://scribblethink.org/Computer/cachekiller.html](http://scribblethink.org/Computer/cachekiller.html)
[https://stackoverflow.com/questions/11868087/avoiding-powers-of-2-for-cache-friendliness](https://stackoverflow.com/questions/11868087/avoiding-powers-of-2-for-cache-friendliness)
[https://stackoverflow.com/questions/8547778/why-are-elementwise-additions-much-faster-in-separate-loops-than-in-a-combined-l](https://stackoverflow.com/questions/8547778/why-are-elementwise-additions-much-faster-in-separate-loops-than-in-a-combined-l)
---
### Author:  [@Oscar\_Smith](https://discourse.julialang.org/u/Oscar_Smith)
#### Post date: [October 29, 2019, 12:56pm UTC](https://discourse.julialang.org/t/blas-dot-doing-weird-things-depending-on-size/30430/4 "2019-10-29T12:56:34Z")
Thanks so much! Now that I know about it, it should be easy enough to fix.