[Techtalk] Mystery cache

[David Sumbler]

I have been trying to run Linux on a Psion 7 computer.  This is a kind
of large hand-held with 16Mb RAM, and no hard drive, so it is using a
CF card as main storage, and another (64Mb) CF card as swap.
Obviously the swap is very slow.

Trying to run apt-get I discovered that "Building Dependency Tree"
takes well over 3 hours!  Having asked on an e-mail group if other
people have this problem, one person running Linux on a Psion 5mx -
basically a similar machine, also with 16Mb RAM - said that it takes
less than a minute.

Further investigation, reveals that, even immediately after booting,
over 11Mb are cached.  I assume, therefore, that apt-get is using swap
space because of this.  Running 'top' shows:

Mem: 14780k total, 14400k used, 380k free, 436k buffers
Swap: 62952k total, 260k used, 62692k free, 11612 cached

COMMAND         TTY     VIRT RES SHR %MEM
init            ?        268 244 220  1.7 
keventd         ?          0   0   0  0.0
ksoftirqd_CPU0  ?          0   0   0  0.0
kswapd          ?          0   0   0  0.0
bdflush         ?          0   0   0  0.0
kupdated        ?          0   0   0  0.0
kjournald       ?          0   0   0  0.0
bash            tty1    1096 940 784  6.4
bash            tty2     504 504 200  3.4
getty           tty3     264 244 216  1.7
getty           ttySA0   348 300 300  2.0
top             tty2     792 792 692  5.4

That is the full list of processes.

Does anyone have any idea what is going on here?  What is that 11612Kb
cached, and how can I free up real memory so that processes can run at
a reasonable speed?  Someone suggested something about
/proc/sys/vm/bdflush, but I have no idea what to do with the values in
there, nor where to find out.

The kernel is version 2.4.27.

David

-- 

david at aeolia.co.uk