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Re: [LUG] Swap not swap


On 18 Nov 2013, at 13:05, Philip Hudson <phil.hudson@xxxxxxxxx> wrote:

> Performance has suddenly gone to hell on a x86_64 machine running debian testing. 
> I noticed in htop that nothing was ever showing in swap, so I checked with 'swapon 
> -s', expecting to find that swap was off for some reason, but instead got this:
> Filename                 Type            Size    Used    Priority
> /dev/sda5                partition       4805628 0       -1
> So the correct partition is allocated as swap as per /etc/fstab, but that "Used" 
> figure of zero is bugging me. The machine has 2GB RAM, which is usually 90%+ 
> allocated according to htop.
> Anyone recognize this?

Certainly sounds odd for box under memory pressure. I'd expect some pages to be 
paged out, although often not a lot these days. 

Have seen zero here and there before for some physical servers. Some of which I 
might have tweaked a bit.

Don't have Debian testing box to hand (well I could boot one from disk but it 
wouldn't be doing anything), but the Linux kernel use of swap is pretty much 
infinitely configurable, have you fiddled?

Really performance will go to hell if you start using swap actively anyway, swap is 
rarely the right solution in these days of cheap and abundant RAM.

Is the performance issue really memory limited, or did this just bring the odd swap 
usage to your attention?

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