I'm running kernbench (make -j 128 on a kernel source) back to back
multiple times on an SMP. Among every 10 runs, there's always at least one
run that has a run time around 40% longer than the other runs. (Before
kernbench starts timing, it does a sync.) 'vmstat 1' indicates that the
longer runs always have a couple of 1-sec intervals during which there are
10 times more block-outs (bo field) than the average traffic in the rest
of the run, and during these intervals, many cc1 processes are in the D
state. My file system is ext3 and all the things like journal commit
interval, pdflush interval, etc. have the default values.
I'm trying to understand why such variability occurs. I tested the same
thing with ext2 and did not see any variability. So I'm thinking about two
things: (1) for some reason, ext3/jbd occasionally issues a large volume
of bursty writes to the disk (but why does it occur just sometimes, not
always?), and (2) when there are bursty writes, the block device driver is
not able to handle them, causing I/O waits. But I don't really have a
clear understanding of the problem here...
Does anyone have any insight on this, or any suggestion on how to figure
it out?
Thanks,
tong
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