Re: [Stellarium-pubdevel] mmap patch for handling of big star catalogs

[Johannes G. <joh...@gm...>]

On 2007.06.12 22:09:59 CEST, Johannes Gajdosik wrote:
> On 2007.06.12 13:54:55 CEST, Fabien Chéreau wrote:
> > Hi,
> > 
> > Well done for the mmap patch. If we want to include that in the SVN, we
> > should make sure that it is portable to other plateforms,  and use a
> > fallback if needed.
> > There should be code in the CMakeLists.txt which check for mmap and define
> > HAVE_MMAP in the config.h or something like that.
> 
> We don't need HAVE_MMAP, because mmap exists everywhere. Even in Windows95,
> although it is called differently.
> 
> There are other problems:
> 
> 1) smooth operation vs. having the data on disk.
> Some people will prefer preloading of all data because this guarantees smooth operation
> without disk access (provided you have enough RAM).
> 
> 2) binary compatibility.
> The current patch work only for little endian machines, and I am unsure about compilers
> other than gcc. Well, this is not such a restriction: only the old Motorola G4 Macs
> and Sun, Hp, SGI, Alpha machines are out of luck. Conversion to native binary format can be done
> 
> A) before starting of stellarium
> B) right after loading the data in stellarium
> C) before rendering of each single star.
> 
> Currently B is implemented, but this is not possible with mmap.
> Maybe we will have to do both A and C. In any case it would be good to see how big
> the performance drawback is of method C.
> 
> Johannes

I have committed a first version of the mmap patch. The restriction is that it currently works for
little endian machines only, so users of G4-Macs are out of luck for now.

Nigel, If you want something for your old Mac, I can do it for you.
You just have to compile and test.

In order to use the mmap feature you must write in stars.ini:

cat_file_name_05               = mmap:stars_5_1v0_0.cat
cat_file_name_06               = mmap:stars_6_2v0_0.cat
cat_file_name_07               = mmap:stars_7_2v0_0.cat
cat_file_name_08               = mmap:stars_8_2v0_01.cat

So mmapping can be enabled/disabled for single files.
The fallback is the current loading into main memory.
I have implemented a windows version too, but was not even able to
compile it, because I currently have no working crosscompiling environment.

Yours,
Johannes