server: enforce prompt cache RAM limit (#25070)

Before this commit, --cache-ram was not a hard limit:

- The cache always kept at least one entry, even if that entry exceeded the
  RAM/token limits.
- Old entries were only evicted for the RAM/token limits after saving the new
  one, which could cause the cache to temporarily exceed the RAM/token limits
  even if individual entries were below the limit.

Now, ensure that the RAM limit is strict with these changes:

- Skip saving state to cache if by itself it exceeds the RAM limit.
- Evict old entries as necessary to make the new entry fit.

Additionally, token-limit cleanup may now evict the last remaining cache entry
instead of always preserving one.

tools/server/server-task.cpp

@@ -1631,7 +1631,22 @@ server_prompt * server_prompt_cache::alloc(const server_prompt & prompt, size_t
         }
     }
 
-    // next, remove any cached prompts that are fully contained in the current prompt
+    // calculate checkpoints size to see if it will fit with the prompt
+    size_t checkpoints_size = 0;
+    for (const auto & ckpt : prompt.checkpoints) {
+        checkpoints_size += ckpt.size();
+    }
+
+    const size_t state_size_new = state_size_tgt + state_size_dft + checkpoints_size;
+
+    // skip over-limit entries to avoid disturbing the cache
+    if (limit_size > 0 && state_size_new > limit_size) {
+        SRV_WRN(" - prompt state size %.3f MiB exceeds cache size limit %.3f MiB, skipping\n",
+                state_size_new / (1024.0 * 1024.0), limit_size / (1024.0 * 1024.0));
+        return nullptr;
+    }
+
+    // remove any cached prompts that are fully contained in the current prompt
     for (auto it = states.begin(); it != states.end();) {
         const int len = it->tokens.get_common_prefix(prompt.tokens);
 
@@ -1644,6 +1659,16 @@ server_prompt * server_prompt_cache::alloc(const server_prompt & prompt, size_t
         }
     }
 
+    if (limit_size > 0) {
+        // make room before allocating the new vectors to avoid breaching the limit
+        while (!states.empty() && size() + state_size_new > limit_size) {
+            SRV_WRN(" - making room for prompt cache entry, removing oldest entry (size = %.3f MiB)\n",
+                    states.front().size() / (1024.0 * 1024.0));
+
+            states.pop_front();
+        }
+    }
+
     std::vector<uint8_t> state_data_tgt;
     std::vector<uint8_t> state_data_dft;
 
@@ -1752,12 +1777,7 @@ bool server_prompt_cache::load(server_prompt & prompt, const server_tokens & tok
 
 void server_prompt_cache::update() {
     if (limit_size > 0) {
-        // always keep at least one state, regardless of the limits
-        while (states.size() > 1 && size() > limit_size) {
-            if (states.empty()) {
-                break;
-            }
-
+        while (!states.empty() && size() > limit_size) {
             SRV_WRN(" - cache size limit reached, removing oldest entry (size = %.3f MiB)\n", states.front().size() / (1024.0 * 1024.0));
 
             states.pop_front();
@@ -1771,11 +1791,7 @@ void server_prompt_cache::update() {
     const size_t limit_tokens_cur = limit_size > 0 ? std::max<size_t>(limit_tokens, limit_size/size_per_token) : limit_tokens;
 
     if (limit_tokens > 0) {
-        while (states.size() > 1 && n_tokens() > limit_tokens_cur) {
-            if (states.empty()) {
-                break;
-            }
-
+        while (!states.empty() && n_tokens() > limit_tokens_cur) {
             SRV_WRN(" - cache token limit (%zu, est: %zu) reached, removing oldest entry (size = %.3f MiB)\n",
                     limit_tokens, limit_tokens_cur, states.front().size() / (1024.0 * 1024.0));