New Cygwin family member

[Ryan Johnson]

On 12/02/2013 6:00 AM, Corinna Vinschen wrote:
> Hi guys,
>
> On Feb  8 14:55, Corinna Vinschen wrote:
>> Another problem is the memory layout (the "fork" problem).  Fortunately
>> we have a 43 bit address space now.  Here's is our current model, still
>> open for discussion:
>>
>> - Keep clear of memory from 0x0 up to 0x0:7fffffff, and from
>>    0x700:00000000 up to 0x7ff:ffffffff since these areas are used
>>    by the OS for ... everything.
>>
>> - Cygwin thread stacks will be located between 0x0:80000000 and
>>    0x0:ffffffff.  With a default stack size of 1 Meg, we have room for
>>    2048 threads.
>>
>> - The Cygwin executables will be loaded to 0x1:00400000.
>>
>> - The Cygwin DLL will be loaded to 0x1:80040000.  That leaves 2 Gigs of
>>    space for the executable.  The space from 0x1:8000000 up to
>>    0x1:80040000 will be used for Cygwin's shared memory areas.
> A slight change to the  memory model occured to me, which should drop
> the potential DLL collisions even more:
>
>> - Other Cygwin distro DLLs are supposed to be rebased to the area
>>    between 0x2:00000000 up to 0x3:ffffffff.  This is an area of 8 Gigs
>>    for DLLs.  That should be enough for a while, I guess.
> The memory slot from 0x2:00000000 up to 0x4:00000000 == 8 Gigs will be
> used by rebase{all}...
>
> ...while ld's --auto-image-base will use the region from 0x4:00000000 up
> to 0x6:00000000.
>
> This means, rebased DLLs will never collide with auto-image-based DLLs,
> and both types of DLLs have a full 8 Gigs space each, which should be
> enough for the forseeable future.
>
>> - The heap will be located at 0x4:00000000, and it will be 512Megs by
>>    default.
> The heap will then be located at 0x6:00000000, of course.
>
>> - Then ... a big void ...
>>
>> - Eventually, mmap's will be allocated from 0x700:00000000 downwards.
> This means the VM space left for heap and mmaps is a mere 7152 Gigabyte,
> about 7 Terabytes.  I hope that's ok, even for our friends on the
> Fortran frontier.
>
> Comments?
If I count correctly:
- Total address space available is 8TB
- Windows no-man's land regions occupy 2GB and 1TB, respectively
- Cygwin thread stacks occupy 2GB
- Executable image gets 2GB
- distro and auto-image dlls together occupy 16GB (including some tens 
of MB for the cygwin dll and shared memory areas)
- heap/mmap area gets from 0x6: to 0x700:, which I compute as 7144GB

In other words, the proposed layout occupies only 20GB, or less than 
0.3% of the address space. If somebody wants significantly more heap 
space, they'd have to brave the 1TB no-man's land and risk fork 
failures. Or, more likely, just wait for Intel and MS to bump the 
address space limit by a few powers of two (which should happen roughly 
when the "normal" amount of memory to have in a Windows box approaches 
8TB). At that point, the heap/mmap area can easily take over the 
additional space.

What's not to like?
Ryan