Tuning compcache/ramzswap to allow (X)ubuntu LiveCD install with 128MB of RAM

Eero Tamminen oak at helsinkinet.fi
Sat Oct 10 12:18:47 UTC 2009


On Saturday 10 October 2009, John McCabe-Dansted wrote:
> Using the default compcache size of 25% of RAM I was not able to use
> the ubiquity installer on the last alpha of the Ubuntu Live CD in a
> 128MB machine. I haven't tried the beta, or Xubuntu, but the LiveCD
> install doesn't load Gnome so I wouldn't imagine there would be any
> difference. Increasing the compcache size to 100% by swapping on four
> ramzswap partions of ram fixed this. 100% of RAM is reasonable because
> assuming a 2:1 compression ratio this means that compcache will use up
> to 50% of RAM before triggering the OOM killer (and using 50% of ram
> is probably better than triggering an OOM kill during an install). I
> tried to get more precise figures on how much ramzswap was needed to
> get a reliable install but I came across various difficulty in
> generating figures and benchmarks such as: lack of realistic 128MB era
> hardware to test on*, installs that work despite the OOM killer being
> triggered (e.g taking out pulseaudio). For example I with 75% I
> appeared to be able to install successfully, but the OOM killer was
> triggered.

What prevents using real swap?

If the problem is really running out of memory, assigning part of memory for
compcache makes things only worse.  It helps only if you really cannot have
real swap, your problem is performance & you have a use-case where compcache
is faster or you want to avoid disk writes (e.g. because you have flash with
relatively low number of write cycles.)

	- Eero

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