... | ... | @@ -30,15 +30,13 @@ In other words, `int **firstneigh` is a pointer to an array of pointers to array |
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`firstneigh[i]` contains an array of the atoms `j` that are neighbors of atom `i`.
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In this protein input, there are 32.000 `i` atoms.
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For example, `firstneigh[i][0]` would be the first neighbor of atom `i`.
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Both `i` and are atoms `firstneigh[i][0]`, and are represented using a 32 bit `int`.
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Both `i` and `j` are atom identifiers, and are represented using a 32 bit `int`.
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To retrieve the properties of an atom, this `int` value needs to be used as an index for the arrays found in file `atom_vec.h` or `atom.h` such as `double **x` (position), `**f` (force) or `*q` (charge).
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`x` and `f` are pointer to pointer since the second index in needed to split the magnitudes in XYZ components.
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Function `PairLJCharmmCoulLong::compute` has an inner and an outer for loop.
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The outer loop (i-loop) iterates through all 32.000 atoms in the protein simulation, while the inner loop (j-loop) iterates through each atom `j` that is a neighbor of `i`.
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For each pair of atoms `i, j`, the algorithm first computes the distance between the two atoms.
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Then, the distance is compared to different values which act as a threshold.
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... | ... | @@ -49,6 +47,7 @@ The alternative form with `X Y Z` parameters ([see](https://docs.lammps.org/pair |
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These values always follow:
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- `cut_lj_innersq < cut_ljsq`
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- `tabinnersq < cut_coulsq`
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- `cut_ljsq = cut_coulsq` (only in optimized function)
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- `cut_bothsq = MIN(cut_ljsq, cut_coulsq)`
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In the code, `rsq` represents the distance between atoms `i,j`. It is saved in squared form to avoid computing an expensive `sqrt`.
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... | ... | @@ -56,11 +55,22 @@ In the code, `rsq` represents the distance between atoms `i,j`. It is saved in s |
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- If `rsq` is smaller than `tabinnersq`, then `forcecoul` is computed using a *fast* table method. If not, it is computed using a *slow* method with calls to `sqrt` and `exp` functions.
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- If `rsq` is bigger than `cut_lj_innersq`, then `forcelj` needs a few additional computations.
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The following flowchart shows the different cases based on `rsq`.
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The condition on `forcelj`is not shown in order to focus on the `tabinnersq` condition.
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![flowchart](uploads/2c10a18b3cda9496b9eaed9e54e001db/flowchart.png)
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This flowcharts shows the different cases based on `rsq`.
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The condition on `forcelj`is not shown to focus on the `tabinnersq` condition.
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At the end, the code uses the computed `forcelj` and `forcecoul` values to update the `f` (force) values for both atoms `i` and `j`.
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# Implementation
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In this section we discuss the implementation of the optimized `PairLJCharmmCoulLong::compute` function, with a focus on the issues that presented when adapting the code for the RISC-V 0.7 scalable vector extension and how have been overcame.
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### Specialization
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The end of `PairLJCharmmCoulLong::compute` contains function calls to `ev_tally` or `virial_fdotr_compute`, which are run depending on the value or `evflag` and `vflag_fdotr` variables.
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An analysis using GDB breakpoints showed that, for the protein input, these funcions are only called on the first and last timesteps of the execution.
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This paraver trace
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atom_vec.h -> contains `**x` and `**f` (3D)
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neigh_list.h -> contains `**firstneigh` (for each i, store array of neighbors j)
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