Pixbuf loading question

[nico <nbd free fr>]

Hello
I m developing an application using pygtk under ubuntu.

For creation of thumbnails on an image list ( inside a gtk.Treeview), I
create a pixbuf for each thumbnail.

After power up of my laptop  , I load the program and my 500 images (
image loading is made on  background) ,  for the creation of pixbuf, I
can see many disc access to load the pixbuf ( I use the command
pixbuf_new_from_file ).

Then if I quit the application, and reload the program, The load time is
very short, and I can no more notice disc access. It's like my pixbufs
stays in memory, and gtk application did not need to reload from file.
It's very useful for loading time but I don't understand why  and how
the buffers  "stays in memory"

How it is possible to force my application to start with the same
behavior as the first launch ( many disc accces), without rebooting my
laptop each time (;-() . I need that to optimize the  loading of the
pixbuf on background.

Hoping  that my question was understandable.

Thanks in advance

Nico.