Refactor the allocation such that we utilise just enough memory pressure
to invoke the shrinker, and just enough processes to spread across the
CPUs and contend on the shrinker.

Signed-off-by: Chris Wilson <ch...@chris-wilson.co.uk>
---
 tests/i915/gem_shrink.c | 11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

diff --git a/tests/i915/gem_shrink.c b/tests/i915/gem_shrink.c
index dba62c8fa..e8a814fe6 100644
--- a/tests/i915/gem_shrink.c
+++ b/tests/i915/gem_shrink.c
@@ -426,6 +426,7 @@ igt_main
        int num_processes = 0;
 
        igt_fixture {
+               const int ncpus = sysconf(_SC_NPROCESSORS_ONLN);
                uint64_t mem_size = intel_get_total_ram_mb();
                int fd;
 
@@ -434,16 +435,16 @@ igt_main
 
                /*
                 * Spawn enough processes to use all memory, but each only
-                * uses half the available mappable aperture ~128MiB.
+                * uses half of the available per-cpu memory.
                 * Individually the processes would be ok, but en masse
                 * we expect the shrinker to start purging objects,
                 * and possibly fail.
                 */
-               alloc_size = gem_mappable_aperture_size() / 2;
-               num_processes = 1 + (mem_size / (alloc_size >> 20));
+               alloc_size = (mem_size + ncpus - 1) / ncpus / 2;
+               num_processes = ncpus + (mem_size / alloc_size);
 
-               igt_info("Using %d processes and %'lluMiB per process\n",
-                        num_processes, (long long)(alloc_size >> 20));
+               igt_info("Using %d processes and %'"PRIu64"MiB per process\n",
+                        num_processes, alloc_size);
 
                intel_require_memory(num_processes, alloc_size,
                                     CHECK_SWAP | CHECK_RAM);
-- 
2.29.2

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