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From: shekita@provolone.cs.wisc.edu (E Shekita)
Newsgroups: comp.os.mach
Subject: Re: simple question on memory objects and copy-on-write
Message-ID: <10870@spool.cs.wisc.edu>
Date: 20 Jul 90 19:01:45 GMT
Sender: news@spool.cs.wisc.edu
Lines: 31

>Let's suppose I have a memory object server that is in charge of 
>paging object X. Further suppose that when a memory_object_data_request() 
>comes in for a page in X, my memory object server does the following:
>
>    (1) vm_allocate()'s a page P in its virtual address space
>    (2) reads the requested page from disk into P
>    (3) provides P to Mach via memory_object_data_provided()
>    (4) vm_deallocate()'s P
>
>The question is: will P be mapped COW in step (3), or will Mach copy 
>P immediately in step (3)? Obviously it would be nice if P was mapped 
>COW in step (3), because then an unnecessary copy is avoided.

Since nobody replied immediately, I ran a few tests, and using 
vm_statistics(), it looks like P is copied immediately in step (3)
and not mapped COW... Oh well. In this case, it probably doesn't matter
since the vm_allocate() in step (1) zeros P anyway, and this is
as expensive as a copy. Given what takes place, the above steps are
more efficiently written:

     (1) vm_allocate() page P once for all time when the program 
	 starts up.

Then on memory_object_datarequest():

     (2) reads the requested page from disk into P.
     (3) provide P to Mach via memory_object_data_provided().
	 Mach will copy the data it needs out of P.
	 

Gene