diff --git a/extension/llm/custom_ops/op_sdpa_impl.h b/extension/llm/custom_ops/op_sdpa_impl.h
index 8b923673a08..cf825bc5a0b 100644
--- a/extension/llm/custom_ops/op_sdpa_impl.h
+++ b/extension/llm/custom_ops/op_sdpa_impl.h
@@ -1098,7 +1098,7 @@ void cpu_flash_attention(
     }
   };
   torch::executor::parallel_for(
-      0, batchSize * num_head * qSlice, 1, compute_lambda);
+      0, batchSize * num_head * qSlice, 1, compute_lambda, num_thread);
 }
 } // namespace sdpa::impl
 } // namespace native
diff --git a/extension/threadpool/thread_parallel.cpp b/extension/threadpool/thread_parallel.cpp
index 0fd95019753..516dad4cb99 100644
--- a/extension/threadpool/thread_parallel.cpp
+++ b/extension/threadpool/thread_parallel.cpp
@@ -36,14 +36,21 @@ void set_thread_num(int64_t thread_num) {
   thread_num_ = thread_num;
 }
 
-inline std::tuple<int64_t, int64_t>
-calc_num_tasks_and_chunk_size(int64_t begin, int64_t end, int64_t grain_size) {
+inline std::tuple<int64_t, int64_t> calc_num_tasks_and_chunk_size(
+    int64_t begin,
+    int64_t end,
+    int64_t grain_size,
+    int64_t num_threads) {
   if ((end - begin) < grain_size) {
     return std::make_tuple(1, std::max((int64_t)0, end - begin));
   }
-  // Choose number of tasks based on grain size and number of threads.
-  int64_t chunk_size =
-      divup((end - begin), get_threadpool()->get_thread_count());
+  // Choose number of tasks based on grain size and number of threads. A
+  // caller-supplied num_threads pins this to the same count it sized its
+  // per-thread scratch with; <= 0 means use the threadpool's current count.
+  if (num_threads <= 0) {
+    num_threads = get_threadpool()->get_thread_count();
+  }
+  int64_t chunk_size = divup((end - begin), num_threads);
   // Make sure each task is at least grain_size size.
   chunk_size = std::max(grain_size, chunk_size);
   int64_t num_tasks = divup((end - begin), chunk_size);
@@ -54,7 +61,8 @@ bool parallel_for(
     const int64_t begin,
     const int64_t end,
     const int64_t grain_size,
-    runtime::FunctionRef<void(int64_t, int64_t)> f) {
+    runtime::FunctionRef<void(int64_t, int64_t)> f,
+    const int64_t num_threads) {
   ET_CHECK_OR_RETURN_FALSE(
       begin >= 0 && end >= 0 && end >= begin,
       "begin = %" PRId64 ", end = %" PRId64,
@@ -63,7 +71,7 @@ bool parallel_for(
   ET_CHECK_OR_RETURN_FALSE(grain_size > 0, "grain_size = %" PRId64, grain_size);
   int64_t num_tasks = 0, chunk_size = 0;
   std::tie(num_tasks, chunk_size) =
-      calc_num_tasks_and_chunk_size(begin, end, grain_size);
+      calc_num_tasks_and_chunk_size(begin, end, grain_size, num_threads);
 
   auto task = [&f, begin, end, chunk_size](size_t task_id) {
     set_thread_num(task_id);
diff --git a/runtime/kernel/thread_parallel_interface.h b/runtime/kernel/thread_parallel_interface.h
index 8cce610dcb4..eacc4fb09b3 100644
--- a/runtime/kernel/thread_parallel_interface.h
+++ b/runtime/kernel/thread_parallel_interface.h
@@ -60,6 +60,11 @@ constexpr int64_t GRAIN_SIZE = 32768;
  * described below
  * f: user function applied in parallel to the chunks, signature:
  *   void f(int64_t begin, int64_t end)
+ * num_threads: number of threads to partition the work across. When <= 0
+ *   (the default), the threadpool's current thread count is used. Callers that
+ *   pre-size per-thread scratch indexed by get_thread_num() should pass the
+ *   same count they sized with, so the number of chunks (and thus the maximum
+ *   get_thread_num()) cannot exceed that count.
  * Returns true if all work items are processed successfully, false otherwise
  *
  * Warning: parallel_for does NOT copy thread local states from the current
@@ -70,7 +75,8 @@ bool parallel_for(
     const int64_t begin,
     const int64_t end,
     const int64_t grain_size,
-    runtime::FunctionRef<void(int64_t, int64_t)> f);
+    runtime::FunctionRef<void(int64_t, int64_t)> f,
+    const int64_t num_threads = -1);
 
 int64_t get_thread_num();
 
@@ -81,7 +87,9 @@ bool parallel_for(
     const int64_t begin,
     const int64_t end,
     const int64_t grain_size,
-    const Func& func) {
+    const Func& func,
+    const int64_t num_threads = -1) {
+  (void)num_threads;
   return internal::parallel_for_no_threadpool(begin, end, grain_size, func);
 }