From 992c325323f925cb82c86778e5e91a63de199063 Mon Sep 17 00:00:00 2001
From: Georgi Gerganov <ggerganov@gmail.com>
Date: Wed, 29 Jul 2026 14:59:44 +0300
Subject: [PATCH] server : add trace logging for slot similarity checking
 (#26271)

Adds trace logging in server-context.cpp for slot similarity checking
during prompt cache slot selection, including skip reasons and similarity
calculation details.

Assisted-by: llama.cpp:Qwen3.6-27B
---
 tools/server/server-context.cpp | 19 ++++++++++++-------
 tools/server/server-task.cpp    | 14 +++++++-------
 2 files changed, 19 insertions(+), 14 deletions(-)

diff --git a/tools/server/server-context.cpp b/tools/server/server-context.cpp
index dba15d426f..749bd9aac1 100644
--- a/tools/server/server-context.cpp
+++ b/tools/server/server-context.cpp
@@ -1537,7 +1537,7 @@ private:
 
         // find the slot that has at least n% prompt similarity
         if (slot_prompt_similarity != 0.0f) {
-            float sim_best = 0;
+            float f_sim_best = 0;
 
             for (server_slot & slot : slots) {
                 if (task.id_slot != -1 && slot.id != task.id_slot) {
@@ -1546,6 +1546,7 @@ private:
 
                 // skip the slot if it is not available
                 if (slot.is_processing()) {
+                    SLT_TRC(slot, " - skipping, is_processing = %d\n", slot.is_processing());
                     continue;
                 }
 
@@ -1553,26 +1554,30 @@ private:
 
                 // skip the slot if it does not contains cached tokens
                 if (tokens.empty()) {
+                    SLT_TRC(slot, "%s", " - skipping, slot is empty\n");
                     continue;
                 }
 
                 // fraction of the Longest Common Prefix length with respect to the input prompt length
-                const float sim_cur = float(tokens.get_common_prefix(task.tokens)) / task.tokens.size();
+                const size_t lcp_len = tokens.get_common_prefix(task.tokens);
+                const float f_sim_cur = float(lcp_len) / task.tokens.size();
+
+                SLT_TRC(slot, " - checking sim = %.3f (%zu/%zu) > %.3f\n", f_sim_cur, lcp_len, task.tokens.size(), slot_prompt_similarity);
 
                 // select the current slot if the criteria match
-                if (sim_cur > sim_best && sim_cur > slot_prompt_similarity) {
-                    sim_best = sim_cur;
+                if (f_sim_cur > f_sim_best && f_sim_cur > slot_prompt_similarity) {
+                    f_sim_best = f_sim_cur;
 
                     ret = &slot;
                 }
             }
 
             if (ret != nullptr) {
-                const float f_keep = (sim_best*task.tokens.size()) / ret->prompt.tokens.size();
+                const float f_keep = (f_sim_best*task.tokens.size()) / ret->prompt.tokens.size();
 
                 if (task.id_slot == -1) {
-                    SLT_INF(*ret, "selected slot by LCP similarity, sim_best = %.3f (> %.3f thold), f_keep = %.3f\n",
-                            sim_best, slot_prompt_similarity, f_keep);
+                    SLT_INF(*ret, "selected slot by LCP similarity, f_sim_best = %.3f (> %.3f thold), f_keep = %.3f\n",
+                            f_sim_best, slot_prompt_similarity, f_keep);
                 }
 
                 // if we are about to lose a large portion of the existing context - save it in the prompt cache
diff --git a/tools/server/server-task.cpp b/tools/server/server-task.cpp
index 99e63b05f5..070f1ade24 100644
--- a/tools/server/server-task.cpp
+++ b/tools/server/server-task.cpp
@@ -1742,9 +1742,9 @@ bool server_prompt_cache::load(server_prompt & prompt, const server_tokens & tok
     const int lcp_best = prompt.tokens.get_common_prefix(tokens_new);
 
     float f_keep_best = prompt.tokens.size() > 0 ? float(lcp_best) / prompt.tokens.size() : -1.0f; // empty slot: any cache entry wins
-    float sim_best    = float(lcp_best) / tokens_new.size();
+    float f_sim_best  = float(lcp_best) / tokens_new.size();
 
-    SRV_TRC(" - looking for better prompt, base f_keep = %.3f, sim = %.3f\n", f_keep_best, sim_best);
+    SRV_TRC(" - looking for better prompt, base f_keep = %.3f, f_sim = %.3f\n", f_keep_best, f_sim_best);
 
     auto it_best = states.end();
 
@@ -1753,25 +1753,25 @@ bool server_prompt_cache::load(server_prompt & prompt, const server_tokens & tok
         const int lcp_cur = it->prompt.tokens.get_common_prefix(tokens_new);
 
         const float f_keep_cur = float(lcp_cur) / it->prompt.tokens.size();
-        const float sim_cur    = float(lcp_cur) / tokens_new.size();
+        const float f_sim_cur  = float(lcp_cur) / tokens_new.size();
 
-        SRV_TRC("   - prompt with length %7zu, lcp = %7d, f_keep = %.3f, sim = %.3f\n", it->prompt.tokens.size(), lcp_cur, f_keep_cur, sim_cur);
+        SRV_TRC("   - prompt with length %7zu, lcp = %7d, f_keep = %.3f, f_sim = %.3f\n", it->prompt.tokens.size(), lcp_cur, f_keep_cur, f_sim_cur);
 
         // don't trash large prompts
         if (f_keep_cur < 0.25f) {
             continue;
         }
 
-        if (f_keep_best < f_keep_cur && sim_best < sim_cur) {
+        if (f_keep_best < f_keep_cur && f_sim_best < f_sim_cur) {
             f_keep_best = f_keep_cur;
-            sim_best    = sim_cur;
+            f_sim_best  = f_sim_cur;
 
             it_best = it;
         }
     }
 
     if (it_best != states.end()) {
-        SRV_TRC(" - found better prompt with f_keep = %.3f, sim = %.3f\n", f_keep_best, sim_best);
+        SRV_TRC(" - found better prompt with f_keep = %.3f, f_sim = %.3f\n", f_keep_best, f_sim_best);
 
         {
             auto & data = it_best->data.main;