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fix(scheduler): reserve paged KV at allocation granularity #367

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fix(scheduler): reserve paged KV at allocation granularity #367

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22 changes: 18 additions & 4 deletions python/freetoken/scheduler/prefill.py
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Original file line number Diff line number Diff line change
Expand Up @@ -48,6 +48,13 @@ class PrefillAdder:
# across the admission loop -- without this, successive admits all see the full pool.
reserved_swa: int = 0

def _kv_reservation_size(self, total_len: int, cached_len: int) -> int:
"""Return the token-equivalent cost of the additional KV pages for a request."""
page_size = self.cache_manager.page_size
return (
div_ceil(total_len, page_size) - div_ceil(cached_len, page_size)
) * page_size

def _try_allocate_one(self, req: PendingReq):
if self.table_manager.available_size == 0:
return None
Expand All @@ -58,12 +65,14 @@ def _try_allocate_one(self, req: PendingReq):
cached_len = handle.cached_len
# TODO: better estimate policy
extend_len = req.input_len - cached_len
estimated_len = extend_len + req.output_len
estimated_size = self._kv_reservation_size(
req.input_len + req.output_len, cached_len
)

if estimated_len + self.reserved_size > self.cache_manager.available_size:
if estimated_size + self.reserved_size > self.cache_manager.available_size:
return None
self.cache_manager.lock(handle)
if estimated_len + self.reserved_size > self.cache_manager.available_size:
if estimated_size + self.reserved_size > self.cache_manager.available_size:
return self.cache_manager.unlock(handle)

# Second currency (hybrid GDN): reserve 1 live + 2 ping-pong state slots; evict tree
Expand Down Expand Up @@ -166,7 +175,12 @@ def _add_one_req(
is_chunked = chunk_size < remain_len
CLS = ChunkedReq if is_chunked else Req
self.token_budget -= chunk_size
self.reserved_size += remain_len + pending_req.output_len
# CacheManager allocates each request independently in whole pages. Reserve the same
# page span here; charging raw tokens can admit several short requests against one page
# even though allocate_paged() needs a separate page for each request.
self.reserved_size += self._kv_reservation_size(
pending_req.input_len + pending_req.output_len, cached_len
)
# NOTE: update the tokens ids only; new pages will be allocated in the scheduler
_slice = slice(cached_len, cached_len + chunk_size)
device_ids = self.table_manager.token_pool[table_idx, _slice]
Expand Down