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SubjectRe: why swap at all?
Catalin BOIE wrote:
> Hello!
>
>> But swap behaviour kills performance even when memory is more than
>> adequate. Consider building a DVD image in a 4GB system. The i/o
>> forces all of the unused programs out, in spite of the fact that an
>> extra 100MB doesn't make a measurable difference in performance. But
>> when I click Mozilla paging most of it in from disk make a big
>> difference in performance to the user.
>
>
> I think that kernel cannot know that you need some data once or more.
> This is fadvise for.
> With my wrapper (http://kernel.umbrella.ro) for fadvise you can do this:
> NOCA_SIZE=128 NOCA_READ=1 NOCA_WRITE=1 NOCA_RA=1 \
> noca mkisofs -R -o /tmp/1.iso /tmp/data
>
> This means:
> NOCA_SIZE: Call fadvise only after 128KiB was read/wrote.
> NOCA_RA: call fadvise with POSIX_FADV_SEQUENTIAL
> NOCA_READ: use fadvise(POSIX_FADV_DONTNEED) for reads (because you don't
> need anymore the source files)
> NOCA_WRITE: use fadvise(POSIX_FADV_DONTNEED) for writes (because it's
> useless to cache the end of the ISO)
>
> Do this program resolve your problem?

It addresses one of the cases which trigger problems, certainly. Thank you.
>
>> The problems with small memory are different in kind, when not even
>> the programs will fit in memory at the same time, or will leave next
>> to nothing for i/o, swap is required for performance. But on a large
>> memory system I believe the gain to pain ratio is way too low with the
>> current VM. The solution at the moment is to turn off swap, which as
>> you note has other problems (can't move between zones without swap?)
>> which in theory could really hang a system.


--
-bill davidsen (davidsen@tmr.com)
"The secret to procrastination is to put things off until the
last possible moment - but no longer" -me
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