Re: [MLUG] Linux time command -- "kernel mode" and "user mode"

[Stephen Montgomery-Smith <EMAIL:PROTECTED>]

Jonathan King wrote:
> On 11/3/06, Stephen Montgomery-Smith <EMAIL:PROTECTED> wrote:

> > Or another possibility.  The reformatting of the data in the C++ program
> > involves a lot of calling malloc, which in turn will probably call things
> > like mmap.  This would push a lot of the tasks from the user to the
> > kernel.  But this would not be a bad thing in of itself, if it actually
> > leads to slightly better performance.  (Whereas using the swap space a 
> > lot
> > strikes me as excessively ugly.)
>
>
> Something does seem pretty screwy here. He says he's keeping *300*
> files open in the Python version, while the C++ is doing things one at
> a time. I'm thinking the only thing that's keeping Python in the
> ballpark is that it's doing something better with caching writes and
> then doing all of them at once. Would it be possible to post the code,
> or at least a skeleton that shows the i/o? My guess is that as soon as
> they saw the code, half of the C++ dudes on the list could point to
> some crucial inefficiency you could fix.
>
> Or, maybe Python really can be pretty clever here.

My suspicion is that is the kind of problem where C++ isn't going to 
give you much of an edge.  So I think that to write it in python is a 
good way to go.

The caching you talk of does take place, but actually it takes place in 
just about any decent file-io implementation.  For example, 'printf' in 
C has caching.  It only writes the info to the files once it has 
collected about 128 bytes to write out.  The only function that has the 
possibility to write a byte at a time is the core function 'write' (or 
printf if you first apply the function 'setbuf' which allows you to tune 
the caching behavior).

Stephen

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