Re: --min-parallelism sets max parallelism too

[Brandon Enright <bmenrigh () ucsd edu>]

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On Sun, 2 Sep 2007 08:55:56 -0600 plus or minus some time David Fifield
<david () bamsoftware com> wrote:

> On Sun, Sep 02, 2007 at 09:45:23AM -0500, Kris Katterjohn wrote:
> > On 9/2/07, David Fifield <david () bamsoftware com> wrote:
> > > All this talk about parallelism made me look deeper into how its
> > > works. And I found something a little surprising. Due to a quirk in
> > > how options are processed, when you specify --min-parallelism without
> > > also specifying --max-parallelism, the maximum parallelism is also
> > > set to the minimum. In other words, the parallelism stays the same
> > > for the whole scan, never dropping below the minimum but never rising
> > > above it either.
> > >
> > > The relevant code is in NmapOps.cc:
> > >
> > >         max_parallelism = 0;
> > >
> > >         ...
> > >
> > >         /* Prevent performance values from getting out of whack */
> > >         if (min_parallelism > max_parallelism)
> > >                 max_parallelism = min_parallelism;
> > >
> > > This assignment should only take place if max_parallelism has been
> > > defined, I think. Or is this the intended behavior?
> >
> > Well, there is also this line above that:
> >
> >   if (max_parallelism && min_parallelism && (min_parallelism >
> > max_parallelism)) {
> >     fatal("--min-parallelism=%i must be less than or equal to
> > --max-parallelism=%i",min_parallelism,max_parallelism);
> >
> > So if they are both set, and min > max, then it bails.
> >
> > But otherwise, max = min because you need to have max >= min and (I
> > guess) you can't really gauge how much the user wants for a max.
>
> Right, but when I set --min-parallelism, I expect the parallelism to
> actually be able to increase above the minimum.
>
> We can just use the convention that 0 for min_parallelism and
> max_parallelism means "unset". The other parts of the code already work
> that way. ultra_scan, for example, uses 300 for the maximum congestion
> window if o.max_parallelism is 0.
>
> Or else we could just initialize the maximum to be something huge like
> 100000, like what is already done with o.max_host_group_sz.
>
> David Fifield

When I was doing all the previous test I did play with making maxP larger
than minP and letting Nmap try to find the optimal parallelism within the
range. The results I got were not good so I gave up on those tests.  Before
I start spouting numbers off the top of my head let me go do some tests and
actually report something useful.

I have a feeling though that because ultrascan() is tuned to be accurate,
and to get the speeds I want I need to lose accuracy, Nmap is going to
detect this and stay pinned at the minP value.

I'll go test this.

Brandon

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