Re: large files unnecessary trashing filesystem cache?

[gfiala@s.netic.de]

Zitat von Bodo Eggert <7eggert@gmx.de>:
> You can alyo cat a big file into /dev/null. I made those examples in order 
> to demonstrate the problem with using O_DIRECT.

O_DIRECT has to much impact at the mentioned "vdr" due to unwanted side effects 
either.

> 
> OTOH, I don't realtime stuff on my computer, so I'm not really affected, 
> but I'll try to show it anyway.
> 
> > > 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.
> 
> Just a q&d test:
> 
> $ time ls -l $DIR > /dev/null
> real    0m0.442s
> user    0m0.008s
> sys     0m0.024s
> 
> $ time ls -l $DIR > /dev/null
> real    0m0.077s
> user    0m0.008s
> sys     0m0.008s
> 
> cat $BIGFILES_1.5GB > /Dev/null
> 
> $ time ls -l $DIR > /dev/null
> real    0m0.270s
> user    0m0.008s
> sys     0m0.008s
> 
> $ time ls -l $DIR > /dev/null
> real    0m0.078s
> user    0m0.004s
> sys     0m0.004s
> 
> 
Thanks for pointing this out - this clearly shows the effect.
Now consider a mildly loaded multitasking environment running X, some services, 
window-manager, email, maybe some databases and a streaming video-application 
at once (so does mine) - the video-file will have unwanted impact on all the 
other applications - leading to unnecessary reloads of lots of files, inodes 
etc.