Re: sharing memory map between processes (same parent)

On Mon, 28 Apr 2008 15:16:47 -0700 (PDT) David Schwartz <davids@xxxxxxxxxxxxx> wrote:
| On Apr 28, 2:44 pm, phil-news-nos...@xxxxxxxx wrote:
|
|> I do purposely try to avoid answering questions that go off track of what I
|> an seeking answers for. I used to not do that, and that would almost always
|> result in a pointless drawn out thread with someone else trying to suggest
|> ways to do things that would not really work because of many issues in the
|> program, that I didn't post in the first place because they were not relevant
|> to
|
| You can't use USENET that way. It's just not fair.
|
| If you ask a really unusual question without any discussion of why
| it's unusual, and you get (maybe even by luck) exactly the right
| answer, the exchange will help nobody but you. Worse, other people
| looking at the exchange may think the answer is the normal or
| appropriate way, though it's only right because of all your "many
| issues in the program", which they don't even know exists.
|
| If you want help from USENET, you have to explain your situation such
| that it's, at a minimum, not misleading to people in the future who
| hunt around for useful information.

If you want the whole can of worms, I'm writing a program to rapidly install
a system during the time the system is booting up. If you want to discuss
that whole thing, feel free to ... in another thread that I will not take
any part in. Until such time as I have completed the exploration of it, I
do don't plan to participate it what I suspect such a thread will become
(based on past Usenet experience). But if it interests you and others, go
right ahead on your own.

And you might wonder, if it is for Linux, why not ask in a Linux system
group. I'd rather find portable ways to do it in case I might ever want
to do the same thing with BSD.


|> | Any time you ask a question, expect the response "why don't you do it
|> | the usual way?" and give the answer in the question. Why not just have
|> | each process map it itself? That's more efficient, because it doesn't
|> | require extra coordination.
|
|> When I ask, expect that I have checked out the "usual" way, and that either
|> that way doesn't work for some reason, or that I am already doing it that way
|> and am exploring to find another way.
|
| Unfortunately, that doesn't work for two reasons. First, the time when
| you haven't checked out the usual way, it will lead you further
| astray. Second, people who need the more typical answer will be lead
| astray by the exchange.

If they misunderstood what I ask, and how narrow it is, I supposed so.
"How do I do X?" ... "To do X, there is method Y and also method Z".
Why would someone wanting to do something other than X try to apply
method Y or method Z?


| These are real, common, problems whose consequences have been learned
| by hard and painful experience. You are asking for things that just
| don't work on USENET.
|
|> If I describe some particular way to do something and ask if it is available
|> in some system, then I'm not interested in other ways, even though one of the
|> other ways might be the usual. If I describe what end result I want and ask
|> how to accomplish it, then any method to accomplish it is fair game.
|
| But you don't describe what end result you want. You describe how you
| think you could accomplish that end result.

That depends on the scope of what is being considered. It can be looked
at in a broad way or a narrow way. When you are considering just the
application, then what you are asking is what the application needs to
accomplish. What if no one else is doing that application? But if I am
asking the "how to" in a more narrow way ...


| For example, in this thread, was the end result the mapping at the
| same address in both processes or not?

No. Just that the mapped pages got transferred. They could have become
mapped into a different address. I did not confine the goal to include
keeping the same address. Clearly process B would need to know what the
addresses are. I can envision a number of ways to transfer mapped pages
between processes and by that, a number of ways for process B to find out
the addresses it got them at, or potentially even a way to specify where
to put them (options it has in the mmap call now).


| The only way to know what result you wanted was to understand what you
| were trying to accomplish. Otherwise, we'd have to tease through every
| tedious permutation of how two processes could wind up with the "same
| mapping".

If a mechanism had existed to do this, the specifics of that mechanism
would have been the answer. There would have been no pondering. If the
mechanism only supported inserting the mapping into process B at addresses
not yet mapped to anythingdetermined by the kernel, then whoever would
know about and suggest such a mechanism would not be pondering other ways
such as letting process B pick the addresses. If I has narrowed what I
was asking for to one where process B can pick the addresses, but the only
solution was one that did not, then that would not fit what I was asking.


|> If what I want to do cannot be done, then just say "no" or don't say. But do
|> not make assumptions about what other stuff I may or may not have explored.
|> If you are curious what I have explored, you can always ask. I may or may not
|> be interested in pursing that subthread.
|
| That's not a fair attitude on USENET. USENET is a give and take, and
| you cannot refuse to give. Spend the extra ten minutes explaining your
| problem. Not only will that help novices to better understand whether
| the answers you get apply to them or not, you will get better answers.
| Plus, you will spend less time explaining why the answers you get
| aren't helpful or saying "I already tried X, that was obvious" because
| you didn't make clear why X wasn't what you wanted.

It has been my experience that this approach has led to very long discourse,
often times rather heated; far more than this thread is even approaching.
I want to avoid that kind of thing.

OTOH, I do give in other ways. I do post answers to what other people ask.
Sometimes I do "guess" at what they are trying to do, and think maybe they
have overlooked a better path if they just back up one and go another way.
Maybe they didn't. If I am confident of my guess, I will make such a
suggestion. Most of the time my guesses have been wrong. Occaisionally
they are right. If I were to post such alternatives all the time, then I
would be more of a pest than I seem to be now.


| And this post is not just for you. It's more for anyone reading this
| thread who might think that your post serves as a good example of how
| to ask a question.
|
| This is probably the most frequently posted link on USENET:
| http://catb.org/~esr/faqs/smart-questions.html
| See especially the section no describing the ultimate goal and the
| section on being specific.

Now my goal is to find the words to get people to not drag me unto subthreads
of discussion that get off the subject of what I was asking. And these need
to be polite enough that I can stick them in every initiated question I ask.

--
|WARNING: Due to extreme spam, I no longer see any articles originating from |
| Google Groups. If you want your postings to be seen by more readers |
| you will need to find a different place to post on Usenet. |
| Phil Howard KA9WGN (email for humans: first name in lower case at ipal.net) |
.

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