PCACHE_MB
#define PCACHE_CACHE_SUBTREE_SIZE (4 * PCACHE_MB) /* 4MB total tree size */
#define PCACHE_CACHE_DEV_SIZE_MIN (512 * PCACHE_MB) /* 512 MB */
#define PCACHE_SEG_SIZE (16 * PCACHE_MB) /* Size of each PCACHE segment (16 MB) */