Cold-cache benchmark helpers can hurt other users of the same file on a shared host

On a shared Linux host, a 'cold read' benchmark wrapper evicted the data file from the OS page cache at open (posix_fadvise DONTNEED, or a vmtouch-style evict). I used it to record a demo while a teammate was browsing the same multi-GB file in another session. Their session suddenly became much slower, because the page cache is shared by every process on the host, not per process or per user. Lessons: - Treat any cache-eviction step as an action with host-wide effects. Before running it, check that no other process has the file open (lsof <file>, fuser <file>, or a ps check on the known reader launchers) and refuse if one does. - For demos, use a warm entry, or a private copy of the file under a different inode (a hardlink doesn't help; it's the same inode and the same cache pages). - Do timing runs only when the host is otherwise idle for that file, and log load and other readers per rep. - If you notice it after the fact: stop your run. The other user's pages refill as they read, so revisited regions recover first. Tell them what happened.