At 2:51 PM +0100 8/10/02, Nicholas Clark wrote:
>On Fri, Aug 09, 2002 at 04:10:50PM -0400, Dan Sugalski wrote:
>> This has an implication for bytecode generation--it means that at
>> least one pointer-sized word at the end of the bytecode will get
>> altered at load time. This is OK--we can use it as the bytecode fixup
>> section. (I expect we'll find other things we need to use as fixups
>> along the way)
>
>Are you *sure* you want to do it this way?
No. :)
>My experience of Linux mmap is that if you map a file truly read-only
>(PROT_READ and MAP_SHARED) then no swap is allocated for the file. I
>assume that the physical RAM used to map the file in is also shared common
>to all processes (or threads) independently mapping the file in the same
>read-only way, but I don't know for sure.
>[This swap behaviour based on experimentation, not reading the source code.
>The assumption is because to me it appears an obvious optimisation, and is
>consistent with my understanding that the read only parts of an executable
>is only in RAM once, however many processes are running it]
>
>However, if the bytecode format requires that part of the file is required
>to be modified *in place* (as you appear to be saying) then the file needs
>to be mapped PROT_READ|PROT_WRITE|MAP_PRIVATE, which will mean that swap
>will be allocated for the whole file, even though only a small part of the
>file will need to be read-write. (probably over-committed swap, but possibly
>not. And even if it is over-committed swap, over-committing will only happen
>up to some multiple of real swap, so large amounts of parrot bytecode could
>start to eat into actual swap availability even though they will never need
>that swap space)
Urk. I was mildly afraid of something like this. I was hoping that
we'd not eat swap, but I expect we will.
The alternative is to embed the constant/fixup table pointer in the
sub PMC and vector that way.
--
Dan
--------------------------------------"it's like this"-------------------
Dan Sugalski even samurai
[EMAIL PROTECTED] have teddy bears and even
teddy bears get drunk
