Constant hashjoin::measurement::hash_join_bench::GPU_MEM_SLACK_BYTES

const GPU_MEM_SLACK_BYTES: usize = 32 * 1024 * 1024; // 0x0000_0000_0200_0000usize

GPU memory to leave free when allocating a hybrid hash table

Getting the amount of free GPU memory and allocating the memory is not atomic. Sometimes the allocation spuriously fails.

Instead of allocating every last byte of GPU memory, leave some slack space.