[Techtalk] Mystery cache
david at aeolia.co.uk
Tue Apr 11 19:41:53 EST 2006
"Val Henson" <val.henson at gmail.com> writes:
> On 4/9/06, David Sumbler <david at aeolia.co.uk> wrote:
>> 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:
> Swap cache is actually not your problem (at least not directly) - swap
> cache reduces swap activity. Swap cache is a note that a page that is
> currently in memory matches a page already written to swap. If that
> page is not altered, then you don't have to write it out to swap again
> if it is evicted from main memory.
>> Mem: 14780k total, 14400k used, 380k free, 436k buffers
>> Swap: 62952k total, 260k used, 62692k free, 11612 cached
> This shows that you are only actually actively using 260k of your swap
> - the "cached" part just means that if you were to have to swap out a
> page from main memory, you wouldn't have to write it to swap. Think
> of it as a cache of pages that are already in main memory, on your
> swap device.
> Several things could be going on here. One way to narrow down the
> possibilities is to try booting with swap disabled - edit /etc/fstab -
> and see if it's still really slow. It looks like you might have just
> barely enough memory to do this, especially if you only run one getty
> and one shell.
I'll try that, once the current operation ('apt-get install less') has
finished. I just want to see how long it takes - so far it has got as
far as "Unpacking less" in 15 and a half hours!!!
>> The kernel is version 2.4.27.
> Oh. 2.4 is notoriously nondeterministic with regard to swap. I
> don't suppose a 2.6 kernel is available?
Unfortunately not - this version is based on Debian 3.1. Although
I've messed about quite a bit modifying an initrd to work on the Psion
7, I don't think I'm knowledgeable enough to attempt doing things from
scratch with a newer distro.
david at aeolia.co.uk
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