> > >> bunzip2 -c $file.bz2 |gzip -9 >$file.gz
So here are some actual results from a dual P3-1Ghz machine (2.6.21.1,
CFSv9). First lets time each operation individually:
$ time bunzip2 -k linux-2.6.21.tar.bz2
real 1m5.626s
user 1m2.240s
sys 0m3.144s
$ time gzip -9 linux-2.6.21.tar
real 1m17.652s
user 1m15.609s
sys 0m1.912s
The compress was the most complex (no surprise there) but they are close
enough that efficient overlap will definitely affect the total wall time. If
we can both decompress and compress in 1:17, we are optimal. First, let's
try the normal way:
$ time (bunzip2 -c linux-2.6.21.tar.bz2 | gzip -9 > test1)
real 1m45.051s
user 2m16.945s
sys 0m2.752s
1:45, or 1/3 over optimal. Now, with a 32MB non-blocking cache between the
two processes ('accel' creates a 32MB cache and uses 'select' to fill from
stdin and empty to stdout without blocking either direction):
$ time (bunzip2 -c linux-2.6.21.tar.bz2 | ./accel | gzip -9 > test2)
real 1m18.361s
user 2m19.589s
sys 0m6.356s
Within testing accuracy of optimal.
So it's not the scheduler. It's the fact that bunzip2/gzip have inadequate
input/output buffering. I don't think it's unreasonable to consider this a
defect in those programs.
DS
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