The following multithreaded algorithm computes transpose of a matrix in parallel:

$p\ Trans(X,Y,N)$

if $N=1$

then $Y[1,1]\leftarrow X[1,1]$

else partition $X$ into four $(N/2)\times(N/2)$ submatrices $X_{11},X_{12},X_{21},X_{22}$

partition $Y$ into four $(N/2)\times(N/2)$ submatrices $Y_{11},Y_{12},Y_{21},Y_{22}$

spawn $p\ Trans(X_{11},Y_{11},N/2)$

spawn $p\ Trans(X_{12},Y_{12},N/2)$

spawn $p\ Trans(X_{21},Y_{21},N/2)$

spawn $p\ Trans(X_{22},Y_{22},N/2)$

What is the asymptotic parallelism of the algorithm?