[Bugfix][CPU] Fix CPU KV cache fallback memory allocation (#29604)

Signed-off-by: Gauri Sahnan <gauri.sahnan@arm.com>
Co-authored-by: Li, Jiang <jiang1.li@intel.com>

vllm/platforms/cpu.py

@@ -10,6 +10,7 @@ import sys
 from dataclasses import dataclass
 from typing import TYPE_CHECKING
 
+import psutil
 import regex as re
 import torch
 
@@ -147,11 +148,21 @@ class CpuPlatform(Platform):
         from vllm.utils.mem_constants import GiB_bytes
 
         kv_cache_space = envs.VLLM_CPU_KVCACHE_SPACE
+        node_dir = "/sys/devices/system/node"
         if kv_cache_space is None:
-            kv_cache_space = 4 * GiB_bytes  # type: ignore
+            nodes = (
+                [d for d in os.listdir(node_dir) if d.startswith("node")]
+                if os.path.exists(node_dir)
+                else []
+            )
+            num_numa_nodes = len(nodes) or 1
+            free_cpu_memory = psutil.virtual_memory().total // num_numa_nodes
+            DEFAULT_CPU_MEM_UTILIZATION = 0.5
+            kv_cache_space = int(free_cpu_memory * DEFAULT_CPU_MEM_UTILIZATION)
+            kv_cache_space_gib = kv_cache_space / GiB_bytes
             logger.warning_once(
-                "Environment variable VLLM_CPU_KVCACHE_SPACE (GiB) "
-                "for CPU backend is not set, using 4 by default."
+                "VLLM_CPU_KVCACHE_SPACE not set. Using "
+                f"{kv_cache_space_gib:.2f} GiB for KV cache."
             )
         else:
             kv_cache_space *= GiB_bytes