From a78b1ab1df9ca44acc5638e8f9d0ae2e62bd65ed Mon Sep 17 00:00:00 2001
From: Kwok Cheung Yeung <kcy@codesourcery.com>
Date: Thu, 29 Aug 2019 10:16:42 -0700
Subject: [PATCH] amdgcn: Tune default OpenMP/OpenACC GPU utilization
libgomp/
* plugin/plugin-gcn.c (parse_target_attributes): Automatically set
the number of teams and threads if necessary.
(gcn_exec): Automatically set the number of gangs and workers if
necessary.
Co-Authored-By: Andrew Stubbs <ams@codesourcery.com>
---
libgomp/plugin/plugin-gcn.c | 82 ++++++++++++++++++++++++++++++-------
1 file changed, 67 insertions(+), 15 deletions(-)
@@ -1219,24 +1219,55 @@ parse_target_attributes (void **input,
if (gcn_dims_found)
{
+ bool gfx900_workaround_p = false;
+
if (agent->device_isa == EF_AMDGPU_MACH_AMDGCN_GFX900
&& gcn_threads == 0 && override_z_dim == 0)
{
- gcn_threads = 4;
+ gfx900_workaround_p = true;
GCN_WARNING ("VEGA BUG WORKAROUND: reducing default number of "
- "threads to 4 per team.\n");
+ "threads to at most 4 per team.\n");
GCN_WARNING (" - If this is not a Vega 10 device, please use "
"GCN_NUM_THREADS=16\n");
}
+ /* Ideally, when a dimension isn't explicitly specified, we should
+ tune it to run 40 (or 32?) threads per CU with no threads getting queued.
+ In practice, we tune for peak performance on BabelStream, which
+ for OpenACC is currently 32 threads per CU. */
def->ndim = 3;
- /* Fiji has 64 CUs, but Vega20 has 60. */
- def->gdims[0] = (gcn_teams > 0) ? gcn_teams : get_cu_count (agent);
- /* Each thread is 64 work items wide. */
- def->gdims[1] = 64;
- /* A work group can have 16 wavefronts. */
- def->gdims[2] = (gcn_threads > 0) ? gcn_threads : 16;
- def->wdims[0] = 1; /* Single team per work-group. */
+ if (gcn_teams <= 0 && gcn_threads <= 0)
+ {
+ /* Set up a reasonable number of teams and threads. */
+ gcn_threads = gfx900_workaround_p ? 4 : 16; // 8;
+ def->gdims[0] = get_cu_count (agent); // * (40 / gcn_threads);
+ def->gdims[2] = gcn_threads;
+ }
+ else if (gcn_teams <= 0 && gcn_threads > 0)
+ {
+ /* Auto-scale the number of teams with the number of threads. */
+ def->gdims[0] = get_cu_count (agent); // * (40 / gcn_threads);
+ def->gdims[2] = gcn_threads;
+ }
+ else if (gcn_teams > 0 && gcn_threads <= 0)
+ {
+ int max_threads = gfx900_workaround_p ? 4 : 16;
+
+ /* Auto-scale the number of threads with the number of teams. */
+ def->gdims[0] = gcn_teams;
+ def->gdims[2] = 16; // get_cu_count (agent) * 40 / gcn_teams;
+ if (def->gdims[2] == 0)
+ def->gdims[2] = 1;
+ else if (def->gdims[2] > max_threads)
+ def->gdims[2] = max_threads;
+ }
+ else
+ {
+ def->gdims[0] = gcn_teams;
+ def->gdims[2] = gcn_threads;
+ }
+ def->gdims[1] = 64; /* Each thread is 64 work items wide. */
+ def->wdims[0] = 1; /* Single team per work-group. */
def->wdims[1] = 64;
def->wdims[2] = 16;
*result = def;
@@ -3031,13 +3062,34 @@ gcn_exec (struct kernel_info *kernel, size_t mapnum, void **hostaddrs,
if (hsa_kernel_desc->oacc_dims[2] > 0)
dims[2] = hsa_kernel_desc->oacc_dims[2];
- /* If any of the OpenACC dimensions remain 0 then we get to pick a number.
- There isn't really a correct answer for this without a clue about the
- problem size, so let's do a reasonable number of single-worker gangs.
- 64 gangs matches a typical Fiji device. */
+ /* Ideally, when a dimension isn't explicitly specified, we should
+ tune it to run 40 (or 32?) threads per CU with no threads getting queued.
+ In practice, we tune for peak performance on BabelStream, which
+ for OpenACC is currently 32 threads per CU. */
+ if (dims[0] == 0 && dims[1] == 0)
+ {
+ /* If any of the OpenACC dimensions remain 0 then we get to pick a
+ number. There isn't really a correct answer for this without a clue
+ about the problem size, so let's do a reasonable number of workers
+ and gangs. */
- if (dims[0] == 0) dims[0] = get_cu_count (kernel->agent); /* Gangs. */
- if (dims[1] == 0) dims[1] = 16; /* Workers. */
+ dims[0] = get_cu_count (kernel->agent) * 4; /* Gangs. */
+ dims[1] = 8; /* Workers. */
+ }
+ else if (dims[0] == 0 && dims[1] > 0)
+ {
+ /* Auto-scale the number of gangs with the requested number of workers. */
+ dims[0] = get_cu_count (kernel->agent) * (32 / dims[1]);
+ }
+ else if (dims[0] > 0 && dims[1] == 0)
+ {
+ /* Auto-scale the number of workers with the requested number of gangs. */
+ dims[1] = get_cu_count (kernel->agent) * 32 / dims[0];
+ if (dims[1] == 0)
+ dims[1] = 1;
+ if (dims[1] > 16)
+ dims[1] = 16;
+ }
/* The incoming dimensions are expressed in terms of gangs, workers, and
vectors. The HSA dimensions are expressed in terms of "work-items",
--
2.34.1