Re: multipe matches

[Ole Tange]

On Sun, Jul 6, 2014 at 11:22 AM, p sena <senapati2001@yahoo.com> wrote:

> I have a large file of some patterns and need to grep & find other
> associated things for every pattern in another large file.
:
> But at anytime when I do a ps aux |grep parallel |grep bigfile I see max 4/5
> & min 1 programs running only.Why is this so ? And also it take long long
> time to complete.

This can be due to disk I/O.

> What is the best way to solve this problem ? Thanks in advance.

I am considering adding this to the man page:

"""
EXAMPLE: Grepping n lines for m regular expressions.

The simplest solution to grep a big file for a lot of regexps is:

    grep -f regexps.txt bigfile

Or if the regexps are fixed strings:

    grep -F -f regexps.txt bigfile

There are 2 limiting factors: CPU and disk I/O. CPU is easy to
measure: If the grep takes >90% CPU (e.g. when running top), then the
CPU is a limiting factor, and parallelization will speed this up. If
not, then disk I/O is the limiting factor, and depending on the disk
system it may be faster or slower to parallelize. The only way to know
for certain is to measure.

If the CPU is the limiting factor parallelization should be done on the regexps:

    cat regexp.txt | parallel --pipe -L1000 --round-robin grep -f - bigfile

This will start one grep per CPU and read bigfile one time per CPU,
but as that is done in parallel, all reads except the first will be
cached in RAM. Depending on the size of regexp.txt it may be faster to
use --block 10m instead of -L1000. If regexp.txt is too big to fit in
RAM, remove --round-robin and adjust -L1000. This will cause bigfile
to be read more times.

Some storage systems perform better when reading multiple chunks in
parallel. This is true for some RAID systems and for some network file
systems. To parallelize the reading of bigfile:

    parallel --pipepart --block 100M -a bigfile grep -f regexp.txt

This will split bigfile into 100MB chunks and run grep on each of
these chunks. To parallelize both reading of bigfile and regexp.txt
combine the two using --fifo:

    parallel --pipepart --block 100M -a bigfile --fifo cat regexp.txt
\| parallel --pipe -L1000 --round-robin grep -f - {}

"""

How can this be expressed better?


/Ole