On Sunday 16 August 2009 20:56:59 Philip Webb wrote: > 090816 Raul Gonzales wrote: > > I have a 2G of physical RAM but even without any major activity > > output of free,vmstat and top reports only ~64M free. > > > > free -m > > total used free shared buffers cached > > Mem: 1953 1888 65 0 143 1557 > > -/+ buffers/cache: 187 1765 > > Swap: 2055 0 2055 > > The crucial figure is '187', which is what is usable. > You have a lot of something cached: any idea what it might be ? > -- the kernel should delete that stuff, if it needs the space.
top, free and every other tool that examines /proc lies through it's teeth. Well, not really, but it's necessary to know what exactly it is measuring and also how the kernel is written. But first: Raul, there is nothing wrong with your box. It is working as designed. The "used" column literally means memory that in use for something, somewhere. The "free" column literally means memory that is not in use anywhere. It's normally around 2 or 3 % of total physical RAM. In Raul's case, the kernel has decided to allocate 1557M of RAM to cache. A particular block might have cached data in it, or it might not. It might have been used then marked as up for not in use anymore, or it might never have been written to since last reboot. It doesn't matter. All that does matter is that the kernel has decided that those blocks of memory *may* be used for caching. The trick is that if more memory is needed by apps than what is listed as immediately available and free, the kernel will search out chunks that it can de-allocate, then it will do so. Why does the kernel do this? It's a programming technique called lazy- deallocation (similar to lazy-initialization) and it's used because it is efficient. It means the kernel only has to do work when it needs to do work. De-allocating memory as soon as it is not in use is expensive and usually a waste of time - it's very likely that that very chunk of memory will almost immediately be used once again. If so, why bother de-allocating it at all? Bottom line: You, the user, do not really know what the kernel is doing with memory. It's also changing about 100 million times a second and your eyes don't work that fast. The kernel is designed to do these things in an autonomous fashion because it knows what to do, and you the user do not. The most you can do about this is provide hints as to how the kernel should operate and then let it get on with it. top, free, et all, simply show snapshots of memory state at a point in time. You should ignore the absolute values they display and concentrate more on how the numbers are changing over time. A value that gradually increases while no real processes are running would indicate a problem for example. This is all very correct and follows good engineering principles. -- alan dot mckinnon at gmail dot com
