speculative : update default params (ggml-org#11954)

* speculative : update default params

* speculative : do not discard the last drafted token

Author: ggerganov

common/common.h

@@ -178,10 +178,10 @@ struct common_params_speculative {
 
     int32_t n_ctx        =     0; // draft context size
     int32_t n_max        =    16; // maximum number of tokens to draft during speculative decoding
-    int32_t n_min        =     5; // minimum number of draft tokens to use for speculative decoding
+    int32_t n_min        =     0; // minimum number of draft tokens to use for speculative decoding
     int32_t n_gpu_layers =    -1; // number of layers to store in VRAM for the draft model (-1 - use default)
     float   p_split      =  0.1f; // speculative decoding split probability
-    float   p_min        =  0.9f; // minimum speculative decoding probability (greedy)
+    float   p_min        = 0.75f; // minimum speculative decoding probability (greedy)
 
     struct cpu_params cpuparams;
     struct cpu_params cpuparams_batch;

common/speculative.cpp

@@ -252,11 +252,6 @@ llama_tokens common_speculative_gen_draft(
         // add drafted token for each sequence
         const llama_token id = cur_p->data[0].id;
 
-        // only collect very high-confidence draft tokens
-        if (cur_p->data[0].p < params.p_min) {
-            break;
-        }
-
         common_sampler_accept(smpl, id, true);
 
         result.push_back(id);
@@ -265,6 +260,11 @@ llama_tokens common_speculative_gen_draft(
             break;
         }
 
+        // only collect very high-confidence draft tokens
+        if (cur_p->data[0].p < params.p_min) {
+            break;
+        }
+
         common_batch_add(batch, id, n_past + i + 1, { 0 }, true);
 
         // evaluate the drafted tokens on the draft model

common/speculative.h

@@ -9,7 +9,7 @@ struct common_speculative_params {
     int n_draft = 16;  // max drafted tokens
     int n_reuse = 256;
 
-    float p_min = 0.9f; // min probability required to accept a token in the draft
+    float p_min = 0.75f; // min probability required to accept a token in the draft
 };
 
 struct common_speculative * common_speculative_init(struct llama_context * ctx_dft);

examples/server/server.cpp

@@ -274,7 +274,7 @@ struct server_task {
         params.speculative.p_min = json_value(data, "speculative.p_min", defaults.speculative.p_min);
 
         params.speculative.n_min = std::min(params.speculative.n_max, params.speculative.n_min);
-        params.speculative.n_min = std::max(params.speculative.n_min, 2);
+        params.speculative.n_min = std::max(params.speculative.n_min, 0);
         params.speculative.n_max = std::max(params.speculative.n_max, 0);
 
         // Use OpenAI API logprobs only if n_probs wasn't provided