Re: [gentoo-user] managing RAM usage

[Grant]

>> I'm trying to maximize the number of servers I can run on my machine
>> without running out of physical RAM.  I noticed something strange when
>> comparing top and free statistics:
>>
>> top says each PID associated with a particular server has a RES of
>> about 100M.  Does that mean each running instance of that server
>> reserves 100MB?  Since there are 20 of these servers running, this
>> seems contradictory to the output from free which indicates less than
>> 1GB used -/+ buffers/cache.
>
> RES does not mean reserved. It means resident.
>
> "man top" and searhc for RES for more info

OK but from what I can gather the concept is the same which is that
RES indicates how much RAM the process is using.

>> Is there any way to monitor RAM usage over time to see how close I'm
>> getting to using all of my physical RAM?
>
> Not really if instantaneous values is what you are looking for.
>
> All tools that purport to display "memory used" and "cpu load" values on
> an instantaneous basis basically lie through their teeth, and it's a
> fallacy to try and interpret such results to gain anything meaningful.
>
> It can't be any other way actually - memory usage can change
> dramatically 10,000 times a second, and that is precisely what the
> computer is designed to do, and to do it fast, and to do it invisibly.

Good point.  I suppose we would need the Linux memory system to do
some kind of built-in self-monitoring.

> "using all of my physical RAM" is also a concept that really makes no
> sense in any meaningful way. You can't treat it like eg the amount of
> water left in a bottle. All of memory is always in use all of the time,
> it's always doing *something*. Most of it is under the kernel's control
> and is used for caching, and the kernel is free to move things around as
> it sees fit. Memory is also shared, you can easily have a situation
> where 10 apps each have access to Y megs of RAM and total memory in use
> is still only a little bit more than Y

Does this explain why I can see 20 processes in top with 100MB
resident RAM each and free only indicates 1GB used -/+ buffers/cache?
If so, how can I go about setting apache2 MaxClients and other server
process limits so as to maximize performance and prevent an OOM
condition?

> The best you can really do is keep an eye on what free says, and to
> graph the trend.

Can you recommend an easy way to do that?

- Grant