Re: [boost] [context] Why does boost::ctx::minimum_stacksize() on Windows return 65536

Subject: Re: [boost] [context] Why does boost::ctx::minimum_stacksize() on Windows return 65536
From: Cory Nelson (phrosty_at_[hidden])
Date: 2012-09-01 22:25:20

On Sat, Sep 1, 2012 at 8:50 PM, Hartmut Kaiser <hartmut.kaiser_at_[hidden]> wrote:
>
>> > Why is the minimally possible stack size in Boost.Context on Windows
>> > set to be 64k? This seems to be way too high for applications where
>> > only a minimal amount of stack is required. I assume, that since
>> > Boost.Context allocates the stack using VirtualAlloc the minimum
>> > possible value should be equal to the page size (see
>> > http://msdn.microsoft.com/en-us/library/windows/desktop/aa366887(v=vs.
>> > 85).as
>> > px).
>>
>> A thread's minimum stack size is Windows' allocation granularity, which is
>> not the size of a single page. Currently this is 64KB. See
>> http://msdn.microsoft.com/en-us/library/windows/desktop/ms686774.aspx
>>
>> VirtualAlloc also rounds up allocations to this granularity. If you
>> request 4KB, you'll actually waste a lot of space and get 64KB.
>>
>> The allocation granularity can be determined using GetSystemInfo().
>
> This is definitely true for the stack allocated by CreateThread, CreateFiber
> et.al. However there is no limitation in actually using a smaller stack in
> Boost.Context as it allocates the stack outside of CreateThread or
> CreateFiber, directly using VirtalAlloc.
>
> The docs of VirtualAlloc specify that the minimal (enforced) allocation size
> there is 4k - i.e. the page size (see the link I provided above).

For VirtualAlloc, reserves are allocation granularity based and
commits are page based. For Context to use VirtualAlloc and give you
4KB without wasting 60KB, it'd have to build its own allocator and
might as well just use malloc/new.
-- 
Cory Nelson
http://int64.org