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the fork exposition was weak. admittedly fork() was a mistake and constrains alot of implementations in strange ways. i still dont understand how local/global e

the fork exposition was weak. admittedly fork() was a mistake and constrains alot of implementations in strange ways. i still dont understand how local/global exactly matches the semantics of cow. transitive permissions are capabilities. while i'm sympathetic to the lack of market appeal to a capability based system, doesn't it seem like you could implement posix on top of one by compromising it? fd transfer over unix domain is already halfway there. seems like a better alternative.

deepnotderp 9y ago
> i still dont understand how local/global exactly matches the semantics of cow. It's not, it's "copy on reference".
- cwzwarich 9y ago
  They were light on the details, but what stops an OS from mapping the same physical page into two distinct local regions of the address space and implementing copy-on-write as usual?
  - deepnotderp 9y ago
    Presumably nothing, but it doesn't seem like that would break anything?
- igodard 9y ago
  No, it's aliasing, just as any fork() is. COW lowers the cost, for Mill as for any other. The Mill fork duplicates the address space, not the memory. The memory is duplicated page by page on demand, i.e. copy-on-write. Mill paging is quite conventional; it's the address space (SAS) that is different.
deleted 9y ago
[deleted]
igodard 9y ago
The biggest problem with caps and legacy apps is not the semantics - a caps systems can emulate POSIX with no problem. The problem is the data representation: you can't fit a capability into a pointer, so all the data layouts change. Sadly, there are tons of C programs that make rash assumptions about data layout, and they would all break. The guys are Cambridge have running caps systems that store the extra info in outboard data structures. We judge that the overhead is too great for commercial success. Customers buy benchmarks, and there are no security benchmarks. 'Tis true 'tis, 'tis pity. 'Tis pity 'tis, 'tis true.

deepnotderp 9y ago

> i still dont understand how local/global exactly matches the semantics of cow. It's not, it's "copy on reference".

cwzwarich 9y ago
They were light on the details, but what stops an OS from mapping the same physical page into two distinct local regions of the address space and implementing copy-on-write as usual?
- deepnotderp 9y ago
  Presumably nothing, but it doesn't seem like that would break anything?
igodard 9y ago
No, it's aliasing, just as any fork() is. COW lowers the cost, for Mill as for any other. The Mill fork duplicates the address space, not the memory. The memory is duplicated page by page on demand, i.e. copy-on-write. Mill paging is quite conventional; it's the address space (SAS) that is different.

deleted 9y ago

[deleted]

igodard 9y ago

The biggest problem with caps and legacy apps is not the semantics - a caps systems can emulate POSIX with no problem. The problem is the data representation: you can't fit a capability into a pointer, so all the data layouts change. Sadly, there are tons of C programs that make rash assumptions about data layout, and they would all break. The guys are Cambridge have running caps systems that store the extra info in outboard data structures. We judge that the overhead is too great for commercial success. Customers buy benchmarks, and there are no security benchmarks. 'Tis true 'tis, 'tis pity. 'Tis pity 'tis, 'tis true.