md mirror / ext3 / dual core performance strange phenomenon?

Hi,

This may be a very stupid question or equally misplaced but I really
don't know where to ask it elsewhere.

I have this configuration (imho significant parts): dual core P915, 2x
sata harddisk configured as (root) md mirror on intel ICH7R (ahci mode),
1G ram.

The harddisks can sustain both around 55 Mb/s, according to hdparm -tT
(okay, I now that's only straight reads). Also they can deliver this
amount of data independent of each other, if both hdparms are run in
parallel, the same performance is reported. I also did similar tests
using ddrescue which shows the same.

Now, what puzzles me, is that compiling the kernel (2.6.17.7) using
either "make -j1 ..." or "make -j2 ..." or "make -j3 ..." makes the
building take about 6.5 minutes, which is really dissatisfying for this
cpu/harddisks combination. Also, top shows that most of the time both
core are between 10-40% idle.

BUT... starting from -j4 (and upwards) the compile time suddenly goes to
3.5 minutes!

I am really blown away here. It looks like disk access is the bottleneck
here, but I can't imagine my disks being so slow (at seeking, I guess)
it should matter that much.

So, I tried changing the readahead value of all relevant devices and...
there is no difference at all!

I changed journalling parameters to data writeback and commit time =
30s. Still no difference.

I kicked one of the two disks out of the mirror. Still no joy.

Then, I tried all i/o schedulers on all relevant devices (except md0 of
course) and... still no difference at all!

Is there anybody who can shed some light on this or give me a clue or
pointer?

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