On Thu, 2 Jun 2011 20:27:25 -0400, Tony Harminc wrote:
>On 2 June 2011 19:56, Paul Gilmartin <[email protected]> wrote:
>> On Thu, 2 Jun 2011 16:29:44 -0700, Edward Jaffe wrote:
>>>
>>>Not pre-zeroed per se, but the pages are in first-reference state.
>>>
>> Is this a hardware feature? (I haven't a PoOp (PrOp) handy.)
>>
>> If not, I could envision doing it in software: point the page
>> table entry at a physical page containing zeroes; widely shared,
>> and entirely write-protected. Then a protection exception
>> could be handled as if it were a page fault.
>>
>> Is it worth the cost of implementation, whether done in hardware
>> or in software? It's merely coddling programmers who perform
>> a fetch long before they store to the same location.
>
>It's not *wrong* or even misleading or obscure, as long as you know
>that the storage contains zeroes. If you obtained the storage using a
>form of STORAGE OBTAIN or GETMAIN that guarantees to give you zeroed
>storage, then where's the problem?
>
>> They deserve to be treated according to the second definition of "coddle"
>> rather than the first.
>
>It's not merely a matter of coddling programmers (even ovoid ones);
>the system cannot hand out storage that contains someone else's data,
>even if you didn't ask for it to contain zeroes.
>
I understand that quite well, but:
o That's why I asked in an earlier ply whether the page is
zeroed even if it was previously used by the same address
space. I suppose:
- it's not worth keeping track.
- even within a job there may be security perimeters
that must be respected.
o I'm puzzled by the value of the first-reference state. Previous
plies suggested there is a mechanism such that fetches from a
(virtual) page in that state will succeed and return zeroes without
handling a page fault, dedicating a physical page to the address
space, and clearing that storage. This is of value only if the
first fetch from the page precedes the first store into that page
by a significant interval (or the page is fetched from and never
stored into). This seems to me to be a program structure that's
unlikely and poorly designed.
-- gil
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