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Old 10-19-2006, 12:03 AM   #1
sixerjman
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Registered: Sep 2004
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Does the kernel cache directory info? ('find' command runs quicker after first use)


I've noticed during testing of bash scripts which use the 'find' command that after the first usage of all subsequent uses are much faster and I don't hear the disk arm moving about (I've got a noisy old
machine). This is a good thing. Is it due to filesystem and/or kernel
caching of the disk contents in RAM? If so, which file(s) might I see
the caching code in? (I've got the kernel source for 2.6.8).

Thanks in advance! :-D
 
Old 10-19-2006, 12:27 PM   #2
zidane_tribal
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whilst i'm not sure where the code itself is kept, yes, linux keeps *lots* cached.

use top to look at your ram usage stats, it shows how much of your ram is used as cache (i have a gig of ram, right now i'm using about 300 megs for the stuff i am running and using about 700megs as cache). your hard-drive will also have a few megabytes of cache built in. linux is awesome at using your free ram as cache, its a *very* cool thing, i tend to hammer my drives with various databases and the read cache makes things happen so much quicker. its also very good at releasing cache, so whilst my system is using 700megs of ram as cache, if i need it to run my software, it just releases it to whatever needs it.
 
Old 10-26-2006, 12:31 AM   #3
sixerjman
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Thanks for the reply! Good way to see the effects of the disk cache in action is to
do a recursive grep for 'cache' in the kernel source, starting in subdirectory 'fs' (filesystem): grep -r cache * | less. I hear the disk chugging for about 5 seconds then look at the results. Then I issue the same command and it takes about 1 second!

Likely suspects ( :-P ) for maintaining the disk cache are:

/fs/bio.c (block I/O?)
/fs/block_dev.c
/fs/buffer.c
/fs/dcache.c
/fs/mbcache.c

There are others, including the modules in /fs/ext3. Yes, good stuff indeed!
 
  


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