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128 lines
5.6 KiB
128 lines
5.6 KiB
from math import pow
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import numpy as np
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def get_submesh_choices(num_hosts, num_devices_per_host, mode="new"):
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submesh_choices = []
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i = 1
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p = -1
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while i <= num_devices_per_host:
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i *= 2
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p += 1
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assert pow(2, p) == num_devices_per_host, ("Only supports the cases where num_devices_per_host is power of two, "
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f"while now num_devices_per_host = {num_devices_per_host}")
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if mode == "alpa":
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for i in range(p + 1):
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submesh_choices.append((1, pow(2, i)))
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for i in range(2, num_hosts + 1):
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submesh_choices.append((i, num_devices_per_host))
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elif mode == "new":
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for i in range(p // 2 + 1):
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for j in range(i, p - i + 1):
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submesh_choices.append((pow(2, i), pow(2, j)))
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return submesh_choices
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def alpa_dp_impl(num_layers, num_devices, num_microbatches, submesh_choices, compute_cost, max_stage_cost,
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best_configs):
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"""Implementation of Alpa DP for pipeline strategy
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Paper reference: https://www.usenix.org/system/files/osdi22-zheng-lianmin.pdf
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Arguments:
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num_layers: K
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num_devices: N*M
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num_microbatches: B
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submesh_choices: List[(n_i,m_i)]
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compute_cost: t_intra
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"""
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# For f, layer ID start from 0
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# f[#pipeline stages, layer id that is currently being considered, number of devices used]
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f = np.full((num_layers + 1, num_layers + 1, num_devices + 1), np.inf, dtype=np.float32)
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f_stage_max = np.full((num_layers + 1, num_layers + 1, num_devices + 1), 0.0, dtype=np.float32)
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f_argmin = np.full((num_layers + 1, num_layers + 1, num_devices + 1, 3), -1, dtype=np.int32)
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f[0, num_layers, 0] = 0
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for s in range(1, num_layers + 1):
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for k in range(num_layers - 1, -1, -1):
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for d in range(1, num_devices + 1):
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for m, submesh in enumerate(submesh_choices):
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n_submesh_devices = np.prod(np.array(submesh))
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if n_submesh_devices <= d:
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# TODO: [luzgh]: Why alpa needs max_n_succ_stages? Delete.
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# if s - 1 <= max_n_succ_stages[i, k - 1, m, n_config]:
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# ...
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for i in range(num_layers, k, -1):
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stage_cost = compute_cost[k, i, m]
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new_cost = f[s - 1, k, d - n_submesh_devices] + stage_cost
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if (stage_cost <= max_stage_cost and new_cost < f[s, k, d]):
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f[s, k, d] = new_cost
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f_stage_max[s, k, d] = max(stage_cost, f_stage_max[s - 1, i, d - n_submesh_devices])
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f_argmin[s, k, d] = (i, m, best_configs[k, i, m])
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best_s = -1
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best_total_cost = np.inf
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for s in range(1, num_layers + 1):
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if f[s, 0, num_devices] < best_total_cost:
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best_s = s
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best_total_cost = f[s, 0, num_devices]
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if np.isinf(best_total_cost):
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return np.inf, None
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total_cost = f[best_s, 0, num_devices] + (num_microbatches - 1) * f_stage_max[best_s, 0, num_devices]
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current_s = best_s
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current_layer = 0
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current_devices = num_devices
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res = []
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while current_s > 0 and current_layer < num_layers and current_devices > 0:
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next_start_layer, submesh_choice, autosharding_choice = (f_argmin[current_s, current_layer, current_devices])
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assert next_start_layer != -1 and current_devices != -1
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res.append(((current_layer, next_start_layer), submesh_choice, autosharding_choice))
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current_s -= 1
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current_layer = next_start_layer
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current_devices -= np.prod(np.array(submesh_choices[submesh_choice]))
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assert (current_s == 0 and current_layer == num_layers and current_devices == 0)
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return total_cost, res
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def alpa_dp(num_layers,
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num_devices,
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num_microbatches,
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submesh_choices,
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num_autosharding_configs,
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compute_cost,
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gap=1e-6):
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"""Alpa auto stage dynamic programming.
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Code reference: https://github.com/alpa-projects/alpa/blob/main/alpa/pipeline_parallel/stage_construction.py
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Arguments:
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submesh_choices: List[(int,int)]
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num_autosharding_configs: Max number of t_intra(start_layer, end_layer, LogicalMesh)
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compute_cost: np.array(num_layers,num_layers,num_submesh_choices,num_autosharding_configs)
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"""
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assert np.shape(compute_cost) == (num_layers, num_layers, len(submesh_choices),
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num_autosharding_configs), "Cost shape wrong."
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all_possible_stage_costs = np.sort(np.unique(compute_cost))
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best_cost = np.inf
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best_solution = None
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last_max_stage_cost = 0.0
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# TODO: [luzgh]: Why alpa needs the num_autosharding_configs dimension in compute_cost?
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# In dp_impl it seems the argmin n_config will be chosen. Just amin here.
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best_configs = np.argmin(compute_cost, axis=3)
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best_compute_cost = np.amin(compute_cost, axis=3)
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assert len(all_possible_stage_costs), "no solution in auto stage construction."
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for max_stage_cost in all_possible_stage_costs:
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if max_stage_cost * num_microbatches >= best_cost:
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break
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if max_stage_cost - last_max_stage_cost < gap:
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continue
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cost, solution = alpa_dp_impl(num_layers, num_devices, num_microbatches, submesh_choices, best_compute_cost,
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max_stage_cost, best_configs)
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if cost < best_cost:
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best_cost = cost
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best_solution = solution
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last_max_stage_cost = max_stage_cost
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return best_cost, best_solution
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