server : fix context limit check to use slot.n_past

ggml-ci

Author: ggerganov

examples/server/server.cpp

@@ -1102,12 +1102,13 @@ struct server_context {
         }
 
         // if context shift is disabled, we stop when it reaches the context limit
-        if (slot.n_decoded >= slot.n_ctx) {
+        if (slot.n_past >= slot.n_ctx) {
             slot.truncated      = true;
             slot.stopped_limit  = true;
             slot.has_next_token = false;
 
-            SLT_DBG(slot, "stopped due to running out of context capacity, n_decoded = %d, n_ctx = %d\n", slot.n_decoded, slot.n_ctx);
+            SLT_DBG(slot, "stopped due to running out of context capacity, n_past = %d, n_prompt_tokens = %d, n_decoded = %d, n_ctx = %d\n",
+                    slot.n_decoded, slot.n_prompt_tokens, slot.n_past, slot.n_ctx);
         }
 
         if (llama_token_is_eog(model, result.tok)) {
@@ -1797,7 +1798,7 @@ struct server_context {
         // apply context-shift if needed
         // TODO: simplify and improve
         for (server_slot & slot : slots) {
-            if (slot.is_processing() && slot.n_past >= slot.n_ctx - 1) {
+            if (slot.is_processing() && slot.n_past + 1 >= slot.n_ctx) {
                 if (!params.ctx_shift) {
                     // this check is redundant (for good)
                     // we should never get here, because generation should already stopped in process_token()
@@ -1960,6 +1961,8 @@ struct server_context {
                         } else {
                             if (!params.ctx_shift) {
                                 // if context shift is disabled, we make sure prompt size is smaller than KV size
+                                // TODO: there should be a separate parameter that control prompt truncation
+                                //       context shift should be applied only during the generation phase
                                 if (slot.n_prompt_tokens >= slot.n_ctx) {
                                     slot.release();
                                     send_error(slot, "the request exceeds the available context size. try increasing the context size or enable context shift", ERROR_TYPE_INVALID_REQUEST);

examples/server/tests/features/ctx_shift.feature

@@ -13,6 +13,10 @@ Feature: llama.cpp server
     And   32 as batch size
     And   2 slots
 
+    # the prompt is 301 tokens
+    # the slot context is 256/2 = 128 tokens
+    # the prompt is truncated to keep the last 109 tokens
+    # 64 tokens are generated thanks to shifting the context when it gets full
   Scenario: Inference with context shift
     And   64 server max tokens to predict
     Then  the server is starting