server : export max observed n_past value (#15361)

Add tracking for high watermark cache usage and make it available in /metrics endpoint.

Use-case: Tracking largest needed cache usage under realistic workload
to better understand memory requirements and be able to adjust
cache size/quantization for model/cache accordingly.

Author: okuvshynov

tools/server/server.cpp

@@ -1201,6 +1201,8 @@ struct server_task_result_metrics : server_task_result {
     uint64_t n_tokens_predicted_total        = 0;
     uint64_t t_tokens_generation_total       = 0;
 
+    uint64_t n_past_max = 0;
+
     uint64_t n_prompt_tokens_processed = 0;
     uint64_t t_prompt_processing       = 0;
 
@@ -1226,6 +1228,8 @@ struct server_task_result_metrics : server_task_result {
             { "n_tokens_predicted_total",        n_tokens_predicted_total },
             { "t_prompt_processing_total",       t_prompt_processing_total },
 
+            { "n_past_max",                      n_past_max },
+
             { "n_prompt_tokens_processed",       n_prompt_tokens_processed },
             { "t_prompt_processing",             t_prompt_processing },
             { "n_tokens_predicted",              n_tokens_predicted },
@@ -1587,6 +1591,8 @@ struct server_metrics {
     uint64_t n_tokens_predicted_total        = 0;
     uint64_t t_tokens_generation_total       = 0;
 
+    uint64_t n_past_max = 0;
+
     uint64_t n_prompt_tokens_processed = 0;
     uint64_t t_prompt_processing       = 0;
 
@@ -1605,6 +1611,10 @@ struct server_metrics {
         n_prompt_tokens_processed       += slot.n_prompt_tokens_processed;
         t_prompt_processing             += slot.t_prompt_processing;
         t_prompt_processing_total       += slot.t_prompt_processing;
+
+        if (slot.n_past > 0) {
+            n_past_max = std::max(n_past_max, (uint64_t) slot.n_past);
+        }
     }
 
     void on_prediction(const server_slot & slot) {
@@ -1620,6 +1630,9 @@ struct server_metrics {
             if (slot.is_processing()) {
                 n_busy_slots_total++;
             }
+            if (slot.n_past > 0) {
+                n_past_max = std::max(n_past_max, (uint64_t) slot.n_past);
+            }
         }
     }
 
@@ -2875,6 +2888,8 @@ struct server_context {
                     res->n_tokens_predicted_total        = metrics.n_tokens_predicted_total;
                     res->t_tokens_generation_total       = metrics.t_tokens_generation_total;
 
+                    res->n_past_max = metrics.n_past_max;
+
                     res->n_prompt_tokens_processed = metrics.n_prompt_tokens_processed;
                     res->t_prompt_processing       = metrics.t_prompt_processing;
                     res->n_tokens_predicted        = metrics.n_tokens_predicted;
@@ -4077,6 +4092,10 @@ int main(int argc, char ** argv) {
                     {"name",  "n_decode_total"},
                     {"help",  "Total number of llama_decode() calls"},
                     {"value",  res_metrics->n_decode_total}
+            }, {
+                    {"name",  "n_past_max"},
+                    {"help",  "Largest observed n_past."},
+                    {"value",  res_metrics->n_past_max}
             }, {
                     {"name",  "n_busy_slots_per_decode"},
                     {"help",  "Average number of busy slots per llama_decode() call"},