server : optimize get_token_probabilities (#24796)

Use std::partial_sort to order only the requested top-n tokens instead
of the full vocabulary

    logprobs sort: vocab=128000 n_top=0 iters=100
    full    sort:   8555.6 us/op
    partial sort:    704.3 us/op

Signed-off-by: Adrien Gallouët <angt@huggingface.co>

tools/server/server-common.cpp

@@ -12,6 +12,7 @@
 #include <random>
 #include <sstream>
 #include <fstream>
+#include <limits>
 
 json format_error_response(const std::string & message, const enum error_type type) {
     std::string type_str;
@@ -1238,7 +1239,7 @@ json format_response_rerank(
 // other utils
 //
 
-std::vector<llama_token_data> get_token_probabilities(llama_context * ctx, int idx) {
+std::vector<llama_token_data> get_token_probabilities(llama_context * ctx, int idx, size_t n_top) {
     std::vector<llama_token_data> cur;
 
     const auto * logits = llama_get_logits_ith(ctx, idx);
@@ -1257,21 +1258,34 @@ std::vector<llama_token_data> get_token_probabilities(llama_context * ctx, int i
         }
     }
 
-    // sort tokens by logits
+    // sort tokens by logits (partial: only the leading `n_top` need ordering)
-    std::sort(cur.begin(), cur.end(), [](const llama_token_data & a, const llama_token_data & b) {
+    if (n_top > cur.size()) {
+        n_top = cur.size();
+    }
+    if (n_top > 0) {
+        std::partial_sort(cur.begin(), cur.begin() + n_top, cur.end(),
+            [](const llama_token_data & a, const llama_token_data & b) {
                 return a.logit > b.logit;
             });
+    }
 
     // apply softmax
-    float max_l = cur[0].logit;
+    float max_l = -std::numeric_limits<float>::infinity();
+    if (n_top > 0) {
+        max_l = cur[0].logit; // partial_sort guarantees the absolute maximum is at index 0
+    } else {
+        for (const auto & t : cur) {
+            max_l = std::max(max_l, t.logit);
+        }
+    }
     float cum_sum = 0.0f;
-    for (size_t i = 0; i < cur.size(); ++i) {
+    for (auto & t : cur) {
-        float p = expf(cur[i].logit - max_l);
+        float p = expf(t.logit - max_l);
-        cur[i].p = p;
+        t.p = p;
         cum_sum += p;
     }
-    for (size_t i = 0; i < cur.size(); ++i) {
+    for (auto & t : cur) {
-        cur[i].p /= cum_sum;
+        t.p /= cum_sum;
     }
 
     return cur;

tools/server/server-common.h

@@ -326,7 +326,7 @@ json format_response_rerank(
 // other utils
 //
 
-std::vector<llama_token_data> get_token_probabilities(llama_context * ctx, int idx);
+std::vector<llama_token_data> get_token_probabilities(llama_context * ctx, int idx, size_t n_top);
 
 std::string safe_json_to_str(const json & data);
 

tools/server/server-context.cpp

@@ -1824,8 +1824,7 @@ private:
                 });
             }
         } else {
-            // TODO: optimize this with min-p optimization
+            std::vector<llama_token_data> cur = get_token_probabilities(ctx_tgt, idx, n_probs_request);
-            std::vector<llama_token_data> cur = get_token_probabilities(ctx_tgt, idx);
             const size_t max_probs = cur.size();
             const size_t n_probs = std::min(max_probs, n_probs_request);