File opening/reading speed differs first time and after that

Not now, John! · 08-05-2004, 03:51 AM

Why is there a speed difference when opening/reading files first time and after that?
Here's an example.

Code:

$ time md5sum song.mp3
290abe89444add77b02815c55311e452  song.mp3

real    0m2.472s
user    0m0.940s
sys     0m0.360s
$ time md5sum song.mp3
290abe89444add77b02815c55311e452  song.mp3

real    0m0.065s
user    0m0.030s
sys     0m0.030s

Differences are larger with big files:

Code:

$ time md5sum file.tar.bz2
3c6cfd9fcd180481063b4058cf6faff2  file.tar.bz2

real    0m7.072s
user    0m1.230s
sys     0m0.960s
$ time md5sum file.tar.bz2
3c6cfd9fcd180481063b4058cf6faff2  file.tar.bz2

real    0m0.165s
user    0m0.110s
sys     0m0.040s

I thought it was Journal on ext3 file system, but same happens on ext2 systems too.
I think that on my previous system (RedHat 8.0, 2.4.20) this didn't happen.

Linux 2.4.26, ext3, HDD Seagate ST340014A.

Thanks in advance.

r0b0 · 08-05-2004, 09:08 AM

Everytime a file is read, its contents are stored in memory to speed-up possible future reads of the same file. This concept is known as caching, file-cache.
So the first time, the file is read from the disk (takes a long time), next times it is read from the cache, which is part of RAM and it is very fast.
R.

Not now, John! · 08-05-2004, 09:37 AM

Thanks for your reply.
On other forum I got same answer. It's nice feature.
I was confused with long time needed for first time accessing file.
Trying to solve this problem I activated VIA chipset support in my kernel. After that DMA was activated on boot, and all file accesses are 3 times faster then before.

Thanks again.