Processing a file using multithreads

[Roy Smith]

In article 
<c6cbd486-7e5e-4d26-93b9-088d48a25dea at g9g2000yqb.googlegroups.com>,
 aspineux <aspineux at gmail.com> wrote:

> On Sep 9, 12:49 am, Abhishek Pratap <abhishek.... at gmail.com> wrote:
> > 1. My input file is 10 GB.
> > 2. I want to open 10 file handles each handling 1 GB of the file
> > 3. Each file handle is processed in by an individual thread using the
> > same function ( so total 10 cores are assumed to be available on the
> > machine)
> > 4. There will be 10 different output files
> > 5. once the 10 jobs are complete a reduce kind of function will
> > combine the output.
> >
> > Could you give some ideas ?
> 
> You can use "multiprocessing" module instead of thread to bypass the
> GIL limitation.

I agree with this.

> First cut your file in 10 "equal" parts. If it is line based search 
> for the first line close to the cut. Be sure to have "start" and 
> "end" for each parts, start is the address of the first character of 
> the first line and end is one line too much (== start of the next 
> block)

How much of the total time will be I/O and how much actual processing?  
Unless your processing is trivial, the I/O time will be relatively 
small.  In that case, you might do well to just use the unix 
command-line "split" utility to split the file into pieces first, then 
process the pieces in parallel.  Why waste effort getting the 
file-splitting-at-line-boundaries logic correct when somebody has done 
it for you?