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SubjectRe: large files unnecessary trashing filesystem cache?
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On Tue, 2005-10-18 at 23:58 +0200, Bodo Eggert wrote:
> Badari Pulavarty <pbadari@gmail.com> wrote:
> > On Tue, 2005-10-18 at 22:01 +0200, Guido Fiala wrote:
>
> [large files trash cache]
>
> > Is there a reason why those applications couldn't use O_DIRECT ?
>
> The cache trashing will affect all programs handling large files:
>
> mkisofs * > iso
> dd < /dev/hdx42 | gzip > imagefile
> perl -pe's/filenamea/filenameb/' < iso | cdrecord - # <- never tried
>

Are these examples which demonstrate the thrashing problem.
Few product (database) groups here are trying to get me to
work on a solution before demonstrating the problem. They
also claim exactly what you are saying. They want a control
on how many pages (per process or per file or per filesystem
or system wide) you can have in filesystem cache.

Thats why I am pressing to find out the real issue behind this.
If you have a demonstratable testcase, please let me know.
I will be happy to take a look.


> Changing a few programs will only partly cover the problems.
>
> I guess the solution would be using random cache eviction rather than
> a FIFO. I never took a look the cache mechanism, so I may very well be
> wrong here.

Read-only pages should be re-cycled really easily & quickly. I can't
belive read-only pages are causing you all the trouble.


Thanks,
Badari

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