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: Hartmut Kaiser (hartmut.kaiser_at_[hidden])
Date: 2012-09-01 22:56:28

> >> > 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. 

The docs say:

<quote>
lpAddress:
The starting address of the region to allocate. If the memory is being 
reserved, the specified address is rounded down to the nearest multiple 
of the allocation granularity. 
</quote>

That means the starting address (only if specified) is aligned with the
allocation granularity, not the allocated size. 

Further:

<quote>
dwSize:
The size of the region, in bytes. If the lpAddress parameter is NULL, this 
value is rounded up to the next page boundary. Otherwise, the allocated 
pages include all pages containing one or more bytes in the range from 
lpAddress to lpAddress+dwSize.
</quote>

This implies that if lpAddress is NULL the function allocates minimally 4k.

> 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.

You're not 'wasting' 60k. Those are never committed in the first place.
Besides, what would be so bad in building your own allocator or using
malloc/new just as well? 

Regards Hartmut
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