Allowing an activity to be launched multiple times in parallel

[Benjamin M. Schwartz]

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Deepak Saxena wrote:
> On the XO, if we try to edit two documents concurrently on Write, we should
> in my opinion only have on instance of write running which can switch quickly 
> between document objects so the user percieves it as two separate instances.

I disagree strongly, for several reasons.

1. Bitfrost requires that each instance be isolated from every other.
Each instance only has access to the Journal items to which the user has
explicitly granted it access.  Allowing multiple "apparent instances" to
share data behind the scenes represents a privilege-combining attack.
This is especially apparent if one instance has been launched with
P_NETWORK but not P_CAMERA, and the other has been launched with the
reverse privileges.

2. A key feature of the Sugar Activity system is that writing Activities
is _easy_.  The goal is to minimize the amount of work required to write
an Activity.  Asking Activity authors to juggle multiple virtual instances
creates tremendous complexity that is likely to produce bugs even when
performed by experts (e.g. Browse), for no user-visible gain.

3.  Two separate Activity instances already share a great deal, because
the Linux kernel automatically uses CoW to keep only one copy of read-only
memory needed by multiple processes.  Each Write instance uses no CPU when
idle, so RAM is the only overhead.

As a matter of efficiency, there is certainly more we can do to decrease
the memory overhead of running multiple instances.  People are working to
improve the efficiency of our X icon caching system and python launcher.
Even better would be to work on the python interpreter upstream to improve
memory usage and reduce the dirtying of pages that could be shared.

- --Ben
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