On Thu, Apr 18, 2002 at 08:44:16AM -0400, Smith, Brian N. wrote: > >John Hasler wrote > >I wrote: > >> Executables, being read-only, are mapped directly from disk and never > >> use any swap at all. Only data gets mapped to swap. > > >Karsten writes: > >> Is this a GNU/Linux thing or a more general Unix/POSIX thing? > > >Neither. It's a virtual-memory thing. It's the obvious thing to do as > >soon as you have vm. > > >> Recent development or long-time feature. > > >VMS had it from day one, I believe, and Berkeley introduced it into Unix > >when they ported it to the VAX. > > I am a little puzzled by the comment "executables do not use swap" and I > have to admit computer design is not my forte. However let me dive in :-)
Neither am I. Diving in too.. > I was under the impression the swap process did not care what the memory > was doing, in fact the user process or data did not know it was in ram or on > disk, but ran in the virtual memory. If there was a request from part of > the program running in this virtual memory (hence obviously in ram space > talking to the processor) and the instruction pointed to an area in virtual > memory that was not in ram space the MMU would generate an exception to > kernal space and the swap would occur. The trick is to guess what parts of > the virtual memory should be kept in ram space. (Hence all the design in the > kernal world) As well there are some means to request that parts of your > program remain in RAM space (this used to be a function of the sticky bit I > think) but I am not a programmer so I really cannot comment too much on > this. It looks like you're both right: executables *do* swap - there is no need for the loader to load in a complete executeable before starting to execute it. When new bits of the executable are needed, they will be paged in (or is it swapped? I managed to confuse myself again...) But when the memory manager decides to re-use a bit of memory currently holding bits of program, then there's *no need* to write it to the swap partition(s). It can't be dirty, and it can be re-read as required from the executable file. So: executables do get swapped, but they don't use swap *space*. On some unix's (e.g. HP if memory serves - it's been swapped out a long time), a side-effect of this is not being able to remove executable files that are currently running - you get "text file busy". The fact that currently running executables can be removed under Linux, I believe is due to the cooperation of the file system, which just "hides" removed files until all references to them are gone. But I may well be wrong on this, see sig. -- ______________________________________________________________ /\ \ \_| *DISCLAIMER* I do not know exactly what I'm talking about. | | Large grains of salt recommended to aid in digestion. | | _________________________________________________________|_ \_/___________________________________________________________/
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