From 4b48a53b6cc60e051f35f2acbd06264a909bb255 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Adrien=20Gallou=C3=ABt?= <angt@huggingface.co>
Date: Fri, 19 Jun 2026 23:26:54 +0200
Subject: [PATCH] server : optimize get_token_probabilities (#24796)
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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  | 36 +++++++++++++++++++++++----------
 tools/server/server-common.h    |  2 +-
 tools/server/server-context.cpp |  3 +--
 3 files changed, 27 insertions(+), 14 deletions(-)

diff --git a/tools/server/server-common.cpp b/tools/server/server-common.cpp
index 75729e62dd..3dc686bb46 100644
--- a/tools/server/server-common.cpp
+++ b/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
-    std::sort(cur.begin(), cur.end(), [](const llama_token_data & a, const llama_token_data & b) {
-        return a.logit > b.logit;
-    });
+    // sort tokens by logits (partial: only the leading `n_top` need ordering)
+    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) {
-        float p = expf(cur[i].logit - max_l);
-        cur[i].p = p;
+    for (auto & t : cur) {
+        float p = expf(t.logit - max_l);
+        t.p = p;
         cum_sum += p;
     }
-    for (size_t i = 0; i < cur.size(); ++i) {
-        cur[i].p /= cum_sum;
+    for (auto & t : cur) {
+        t.p /= cum_sum;
     }
 
     return cur;
diff --git a/tools/server/server-common.h b/tools/server/server-common.h
index f286b3d156..efd31733b0 100644
--- a/tools/server/server-common.h
+++ b/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);
 
diff --git a/tools/server/server-context.cpp b/tools/server/server-context.cpp
index 791188b1e7..1f0e1bfd42 100644
--- a/tools/server/server-context.cpp
+++ b/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);
+            std::vector<llama_token_data> cur = get_token_probabilities(ctx_tgt, idx, n_probs_request);
             const size_t max_probs = cur.size();
             const size_t n_probs = std::min(max_probs, n_probs_request);