Fix floating point issues in block_selection.py (#89)

README.md

@@ -60,7 +60,7 @@ A stable version of the code and a public swarm open to everyone will be release
 
 ### 📋 Terms of use
 
-Before using Petals to run a language model, please make sure that you are familiar with its terms of use, risks, and limitations. For BLOOM, they are described in its [model card](https://huggingface.co/bigscience/bloom) and [license](https://huggingface.co/spaces/bigscience/license).
+Before using Petals to run a language model, please make sure that you are familiar with its terms of use, risks, and limitations. In case of BLOOM, they are described in its [model card](https://huggingface.co/bigscience/bloom) and [license](https://huggingface.co/spaces/bigscience/license).
 
 ### 🔒 Privacy and security
 
@@ -101,7 +101,7 @@ For macOS, you can *probably* run everything normally if you manage to install d
 
 ## 🚀 Getting Started
 
-This is a toy example running on a local machine without GPU and with a tiny model. 
+This is a toy example running on a local machine without GPU and with a tiny model.
 For a detailed instruction with larger models, see ["Launch your own swarm"](https://github.com/bigscience-workshop/petals/wiki/Launch-your-own-swarm).
 
 First, run a couple of servers, each in a separate shell. To launch your first server, run:
@@ -133,7 +133,7 @@ You can assign `--initial_peers` to one or multiple addresses of other servers,
 The only requirement is that at least one of them is running at the time.
 
 Before you proceed, __please run 3 servers__ for a total of 24 blocks (3x8). If you are running a different model,
-make sure your servers have enough total `--num_blocks` to cover that model. 
+make sure your servers have enough total `--num_blocks` to cover that model.
 
 Once your have enough servers, you can use them to train and/or inference the model:
 ```python
@@ -162,8 +162,8 @@ print("Gradients (norm):", model.transformer.word_embeddings.weight.grad.norm())
 ```
 
 Of course, this is a simplified code snippet. For actual training, see the example notebooks with "deep" prompt-tuning:
-- Simple text semantic classification: [examples/prompt-tuning-sst2.ipynb](./examples/prompt-tuning-sst2.ipynb).
-- A personified chatbot: [examples/prompt-tuning-personachat.ipynb](./examples/prompt-tuning-personachat.ipynb).
+- Simple text semantic classification: [examples/prompt-tuning-sst2.ipynb](./examples/prompt-tuning-sst2.ipynb)
+- A personified chatbot: [examples/prompt-tuning-personachat.ipynb](./examples/prompt-tuning-personachat.ipynb)
 
 Here's a [more advanced tutorial](https://github.com/bigscience-workshop/petals/wiki/Launch-your-own-swarm) that covers 8-bit quantization and best practices for running Petals.
 

src/server/block_selection.py

@@ -32,7 +32,10 @@ def _compute_spans(module_infos: List[Optional[RemoteModuleInfo]]) -> Tuple[Dict
         if module is None:
             continue
 
-        for peer_id, server in module.servers.items():
+        # We sort servers here to ensure that we get exactly the same throughputs for a given set of servers.
+        # If the order were not defined, we would get slightly different values due to floating point errors,
+        # which may cause excess block replacements.
+        for peer_id, server in sorted(module.servers.items()):
             if server.state == ServerState.OFFLINE:
                 continue
 
@@ -47,17 +50,14 @@ def _compute_spans(module_infos: List[Optional[RemoteModuleInfo]]) -> Tuple[Dict
     return spans, throughputs
 
 
-def _choose_best_start(throughputs: np.ndarray, num_blocks: int, cur_start: Optional[int]) -> int:
-    options = (
-        (sorted(throughputs[i : i + num_blocks]), i != cur_start, i)
-        for i in range(0, len(throughputs) - num_blocks + 1)
-    )
+def _choose_best_start(throughputs: np.ndarray, num_blocks: int) -> int:
+    options = ((sorted(throughputs[i : i + num_blocks]), i) for i in range(0, len(throughputs) - num_blocks + 1))
     return min(options)[-1]
 
 
 def choose_best_blocks(num_blocks: int, module_infos: List[Optional[RemoteModuleInfo]]) -> List[int]:
     _, throughputs = _compute_spans(module_infos)
-    start = _choose_best_start(throughputs, num_blocks, None)
+    start = _choose_best_start(throughputs, num_blocks)
     return list(range(start, start + num_blocks))
 
 
@@ -69,16 +69,22 @@ def should_choose_other_blocks(
 
     spans, throughputs = _compute_spans(module_infos)
     initial_throughput = throughputs.min()
+    eps = 1e-3
 
     assert local_peer_id in spans, "Span served by this server is not present in the DHT"
     local_span = spans[local_peer_id]
-    throughputs[local_span.start : local_span.end] -= local_span.throughput
+    throughputs[local_span.start : local_span.end] -= local_span.throughput * (1 + eps)
+    # Without (1 + eps) here, we would sometimes subtract a value slightly less than local_span.throughput
+    # due to the floating point error, which would cause excess block replacements.
+    # Also, subtracting local_span.throughput * (1 + eps) makes _choose_best_start() prefer
+    # the previous server position in case of other things being almost equal.
 
-    new_start = _choose_best_start(throughputs, local_span.length, local_span.start)
+    new_start = _choose_best_start(throughputs, local_span.length)
     if local_span.start == new_start:
         return False  # This server is on its best place already
-    local_span.move_to(new_start)
 
+    throughputs[local_span.start : local_span.end] += local_span.throughput * eps
+    local_span.move_to(new_start)
     throughputs[local_span.start : local_span.end] += local_span.throughput
 
     moved = True
@@ -89,18 +95,18 @@ def should_choose_other_blocks(
         moved = False
         for peer_id in servers:
             span = spans[peer_id]
-            throughputs[span.start : span.end] -= span.throughput
+            throughputs[span.start : span.end] -= span.throughput * (1 + eps)
 
-            new_start = _choose_best_start(throughputs, span.length, span.start)
+            new_start = _choose_best_start(throughputs, span.length)
+
+            throughputs[span.start : span.end] += span.throughput * eps
             if span.start != new_start:
                 span.move_to(new_start)
                 moved = True
-
             throughputs[span.start : span.end] += span.throughput
 
     new_throughput = throughputs.min()
     actual_quality = initial_throughput / new_throughput
     logger.info(f"Swarm balance quality: {actual_quality * 100:.1f}%")
 
-    eps = 1e-6
     return actual_quality < balance_quality - eps