Building an initramfs is unreasonably slow. On Fedora 16
dracut-011 takes almost a minute when installing a new kernel:
real 56s
user 20s \__ dracut is CPU bound, not I/O bound.
sys 31s /
The final "gzip -9" takes 12 seconds, and the cpio 1 second,
which leaves 43 seconds for the rest of dracut. That's a
factor of 3 or 4 too long. The output initramfs is 14.9MB
(41MB unzipped) and contains 1619 files, including 367 .ko
kernel modules.
Running
strace -o strace.out -f -e trace=execve dracut test.img
and applying some text processing to strace.out shows
12518 SIGCHLD (processes terminated)
So dracut fondles each file with an average of (12518 / 1619)
= 7.7 processes. No wonder building an initramfs is slow!
Again in strace.out:
8917 execve (address-space images instantiated)
and taking (#SIGCHLD - #execve) gives:
3591 fork-and-no-exec (shell builtins that need a process)
because there is almost no chaining of execve without a fork.
The sorted histogram of execve begins:
3803 execve("/bin/egrep"
1343 execve("/bin/cp"
858 execve("/lib64/ld-linux-x86-64.so.2"
760 execve("/usr/bin/ldd"
375 execve("/sbin/modinfo"
359 execve("/bin/chmod"
344 execve("/bin/rm"
341 execve("/sbin/modprobe"
256 execve("/bin/mkdir"
222 execve("/bin/readlink"
100 execve("/bin/cat"
This data, and a glance at the source of dracut, suggests
considering the bash shell regexp operator "[[ string =~ pattern ]]"
and the expansion substitution operator "${parameter/pattern/string}"
to replace most instances of egrep.
The uses of cp, ldd, chmod, and modinfo should be investigated for
the possibility of batching more than one file at a time. Operating
inside one directory at a time can effectively remove the threat of
exceeding the 32KB limit on the arglist to execve.
Using pipelines (possibly including bash's "while read fname ; do")
to filter streamed lists of filenames can reduce overhead significantly
in contrast to "for fname in ...; do <<execve>>". A pipeline may also
introduce effective parallelism.
"sort --uniq" handily removes duplicates.
In most cases "cat filename |" should be replaced with ordinary
redirection "< filename", and similarly "$(cat filename)" should
be "$(< filename)". If SELinux denies access by dracut (etc.)
but allows /bin/cat, then such a comment is REQUIRED.
Yes, I'm going to work on it.
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