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1. Summary
2. Preview
3. Tests
4. Review

Summary

Action	Implement 2D tiling (with the LFRA version of NEMO)
PI(S)	Daley Calvert, Andrew Coward
Digest	Implement 2D tiling to reduce traffic between main memory and L3 cache
Dependencies	DO loop macros (2020WP/KERNEL-02_Coward_DoLoopMacros_part1), extended haloes (Italo Epicoco, Seb Masson and Francesca Mele), extension of XIOS to accept 2D tiles of data (Yann Meurdesoif & Seb Masson)
Branch	source:/NEMO/branches/{YEAR}/dev_r{REV}_{ACTION_NAME}
Previewer(s)	Gurvan Madec
Reviewer(s)	Gurvan Madec
Ticket	#2365

Description

Implement loop tiling over horizontal dimensions (i and j).

Implementation

A trial implementation of tra_ldf_iso is described in this document.

The main code changes in the preferred approach (using public variables) are described below.

Summary of method

The full processor domain (1:jpi, 1:jpj) is split into one or more subdomains (tiles).

To work on a tile, the DO loop macros in do_loop_substitute are modified to use a new set of domain indices. A new subroutine DOM/domain/dom_tile sets the values of these indices and is also used to initialise the tile to the full domain in DOM/domain/dom_init.

A loop over tiles is implemented at the timestepping level in OCE/step/stp. The domain indices for the tile subdomain are set within this loop by dom_tile, then 'unset' (set back to the full domain) after exiting the loop. All DO loops within the tiling loop therefore work on the current tile, instead of the full processor domain.

The number of tiles is determined by the tile lengths, nn_tile_i and nn_tile_j defined in a new namelist namtile, with respect to the full domain.

Branch

​dev_r12745_HPC-02_Daley_Tiling_trial_public

New subroutines

• dom_tile - Set domain indices

Modified modules

NOTE: the number of affected modules is expected to be much larger in the final implementation

• cfgs/SHARED/namelist_ref - Add namelist namtile
• OCE/DOM/dom_oce - Declare namelist variables
• OCE/DOM/domain - Read namtile namelist and calculate tiling decomposition, add dom_tile, initialise domain indices
• OCE/TRA/traldf - Changes to account for domain indices
• OCE/TRA/traldf_iso - Changes to account for domain indices
• OCE/do_loop_substitute - Implement domain indices
• OCE/par_oce - Declare domain indices and tiling decomposition parameters
• OCE/step - Add tiling loop and set domain indices using dom_tile
• OCE/step_oce - Import dom_tile

Variables

• Global variables
  • ntsi, ntsj- start index of tile
  • ntei, ntej- end index of tile
  • ntsim1, ntsjm1- start index of tile, minus 1
  • nteip1, ntejp1- end index of tile, plus 1
  • ntile- tile number
• Parameters
  • jpnitile, jpnjtile, jpnijtile- number of tiles
• Loop indices
  • jtile- loop over tiles
• Namelist
  • ln_tile- Logical control on use of tiling
  • nn_tile_i, nn_tile_j- tile length
• Pre-processor macros
  • IND_2D- substitution for ALLOCATE or DIMENSION arguments
• Working variables
  • iitile, ijtile- tile number
• Dummy arguments
  • kntile (ntile)

Namelist

   !-----------------------------------------------------------------------
   &namtile        !   parameters of the tiling
   !-----------------------------------------------------------------------
      ln_tile = .false.     !  Use tiling (T) or not (F)
      nn_tile_i = 10        !  Length of tiles in i
      nn_tile_j = 10        !  Length of tiles in j
   /

Documentation updates

Error: Failed to load processor box

No macro or processor named 'box' found

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Preview

Error: Failed to load processor box

No macro or processor named 'box' found

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Tests

Error: Failed to load processor box

No macro or processor named 'box' found

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Review

Error: Failed to load processor box

No macro or processor named 'box' found

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