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Subject: Re: [boost] [context] Why does boost::ctx::minimum_stacksize() on Windows return 65536
From: Cory Nelson (phrosty_at_[hidden])
Date: 2012-09-01 23:09:56

On Sat, Sep 1, 2012 at 9:56 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
>> 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
>> >>
>> >>
>> >> 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 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.

Those with a 32-bit address space might have issue with something
wasting 90% of it.

> Besides, what would be so bad in building your own allocator or using
> malloc/new just as well?

A custom allocator could be good. malloc/new might be feasible if
you're not worried about possibly sharing pages with other contexts.

Cory Nelson

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