RAM question for everyone!

[Bryn M. Reeves]

Alan Evans wrote:
> On Fri, Jan 23, 2009 at 7:43 AM, Dan Track <dan.track at gmail.com> wrote:
>> I was recently asked a question about how much RAM should there be
>> within a server given that the APP uses 8GB of Memory, should I buy
>> 10Gig of memory and have a small harddrive and no swap space? Would
>> this configuration allow everything in my OS to run from RAM and not
>> from swap? If this is the case then there's no need to ever create
>> swap, is there?!?
>>
>> Your thoughts are appreciated.
> 
> This question, along with other recent discussion about swap, leads me
> to ask a question in response: Why is everyone so concerned about how
> to get away without swap?
> 
> Hard drives are cheap. Why does your server with potentially 10GB
> (!!!) of RAM have a hard drive so small that you can't sacrifice a few
> GB for swap?

I think many people aren't as concerned about sacrificing a bit of 
disk space as much as they are concerned about the performance impacts 
when the system begins to use the swap, especially for desktops.

Linux will attempt to move old data that has not been referenced for 
some time out to the swap device even when there is relatively little 
pressure to do so. This is generally a win since we are better 
utilising the physical memory of the system (storing more 
frequently/recently used data in it) but it may lead to nasty delays 
when the swapped-out data is needed again.

This is more of a problem today than 15 years ago because of the ever 
widening gulf between main memory speeds and (HD based) mass storage 
speeds (or at least, seek times).

As an example, try opening something in OpenOffice and then minimizing 
it for a week. Even if the box was fairly quiet for that period, 
chances are that much of OO's address space is now swapped out. 
Clicking the window in the task bar will cause the system to churn for 
a few seconds or more before the app returns to a usable state.

Regards,
Bryn.