On Fri, 18 Feb 2005 16:31:56 +0000, Paul Howarth <paul@xxxxxxxxxxxx> wrote: > Bob Hou wrote: > > Hi, > > I just found the inconsistency of used memory report from top/free and > > System monitor. The top/free reports ~254MB memory used while System > > Monitor shows > > only 120MB. > > I am using FC3 Understanding free memory versus used memory is commonly misunderstood. It's not just a Linux thing, I've seen many advanced Unix people also confuse it. You actually want all of your memory to be in use (except perhaps on an idle single-user desktop system where there's just not enough workload). If you've got unused memory just sitting around then you're wasting the fastest resource your system has. This is especially true on I/O-bound systems, such as webservers, fileservers, etc. In many cases you can improve performance on these servers just by adding more memory, even if your applications weren't using all of what they had before. What happens behind the scenes is that the kernel will *borrow* any physicial memory that your applications aren't using and temporarily use that to buffer disk I/O and so forth. The less memory your applications use, the more the kernel can borrow to use for caching. However, as soon as an application needs more memory, the kernel will automatically give up some of the cache it was using. Your disk I/O may in response be a little slower, but your application gets the memory it wants without knowing any different, and when your application doesn't want it the kernel makes sure it gets used for something useful rather than just sitting around storing useless bits. -- Deron Meranda
