env: add knob to control SWA eviction interval (#22645)

python/sglang/srt/environ.py

@@ -263,6 +263,7 @@ class Envs:
     SGLANG_NCCL_ALL_GATHER_IN_OVERLAP_SCHEDULER_SYNC_BATCH = EnvBool(False)
     SGLANG_REQ_RUNNING_TIMEOUT = EnvFloat(-1)  # in seconds
     SGLANG_DISAGGREGATION_BOOTSTRAP_ENTRY_CLEANUP_INTERVAL = EnvInt(120)
+    SGLANG_SWA_EVICTION_INTERVAL_MULTIPLIER = EnvFloat(1.0)
 
     # Test: pd-disaggregation
     SGLANG_TEST_PD_DISAGG_BACKEND = EnvStr("mooncake")

python/sglang/srt/managers/schedule_batch.py

@@ -2482,12 +2482,22 @@ class ScheduleBatch(ScheduleBatchDisaggregationDecodeMixin):
             sliding_window_size = self.tree_cache.sliding_window_size
             server_args = get_global_server_args()
 
+            # Eviction_interval: trade-off between SWA token waste and eviction overhead
+            page_size = self.tree_cache.page_size
+            eviction_interval = max(
+                page_size,
+                int(
+                    sliding_window_size
+                    * envs.SGLANG_SWA_EVICTION_INTERVAL_MULTIPLIER.get()
+                ),
+            )
+            eviction_interval = (eviction_interval // page_size) * page_size
             for idx, req in enumerate(self.reqs):
                 if self.forward_mode.is_decode():
                     # We set evict_swa condition here with two reasons:
                     # 1. In overlap scheduler, we cannot evict swa when req.decode_batch_idx == 0 since the prev extend batch is still running.
-                    # 2. Evict swa every window_size tokens to reduce the overhead.
-                    if req.decode_batch_idx % sliding_window_size == 1:
+                    # 2. Evict swa every eviction_interval tokens to reduce the overhead.
+                    if req.decode_batch_idx % eviction_interval == 1:
                         self._evict_swa(req, req.seqlen - 1)
                 elif self.forward_mode.is_extend() and self.tree_cache.is_chunk_cache():
                     pre_len = self.prefix_lens[idx]