max memory

max memory

[Paavo Leinonen]

Hi,

I have read [1] and set heap_chunk_in_mb to 2048, but
max_memory.exe gives me only 1920 MB. I need (almost)
2048 MB to be able to manipulate BIG (map) images, how
can get 2048 megabytes available for my program?

Where has the 128 MB (2048 - 1920) gone? I also tried
increasing heap_chunk_in_mb to 4095, but max_memory.exe
still gives 1920 MB.

I'm running cygwin with 64-bit Windows 7 Ultimate in a
machine that has 4 GB of physical RAM, and lots of swap.

[2] says native 64-bit cygwin is not being planned, is
this still the situation? Or /LARGEADDRESSAWARE support,
would it increase memory available for cygwin apps [3]?

-Paavo

[1] http://www.cygwin.com/cygwin-ug-net/setup-maxmem.html
[2] http://www.cygwin.com/faq/faq-nochunks.html
[3] http://blogs.msdn.com/b/oldnewthing/archive/2005/06/01/423817.aspx


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Re: max memory

[Christopher Faylor]

On Sun, Jan 16, 2011 at 04:32:17PM +0200, Paavo Leinonen wrote:
>Hi,
>
>I have read [1] and set heap_chunk_in_mb to 2048, but
>max_memory.exe gives me only 1920 MB. I need (almost)
>2048 MB to be able to manipulate BIG (map) images, how
>can get 2048 megabytes available for my program?
>
>Where has the 128 MB (2048 - 1920) gone? I also tried
>increasing heap_chunk_in_mb to 4095, but max_memory.exe
>still gives 1920 MB.

Cygwin only uses as much memory as the OS gives it.  It can't
use a full 2048MB for the heap.

cgf

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Re: max memory

[Linda Walsh]

Christopher Faylor wrote:
> Cygwin only uses as much memory as the OS gives it.  It can't
> use a full 2048MB for the heap.
----

Using the /LARGEADDRESSAWARE flag would allow cygwin to access
up to a full 4GB of memory under Win7-64.

Wouldn't that give enough to Cygwin to allow it to give more to the
heap?

There's several 32-bit progs that should have been able to use over 2GB, with even XP being configurable to run with 3GB of user/1GB system space, but this was not very usable due to XP being hobbled to ignore extended memory (available on XEON processors many years back that allowed up to 64G on 32-bit machines).  

With 64-bit OS's User programs can easily be given more than 2G of addr space, but to allow this programs have to be linked with the largeaddressaware flag.
Then 32-bit cygwin could theoretically use up to 4GB of memory.  If that was the case, cygwin should be able to allocate more memory for user space (search firefox's bugdb for that flag for notes on its progress toward getting that switch enabled -- apparently ready but not yet enabled in shipping versions).

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Re: max memory

[Christopher Faylor]

On Sun, Jan 23, 2011 at 08:11:52PM -0800, Linda Walsh wrote:
>Christopher Faylor wrote:
>> Cygwin only uses as much memory as the OS gives it.  It can't
>> use a full 2048MB for the heap.
>----
>
>Using the /LARGEADDRESSAWARE flag would allow cygwin to access
>up to a full 4GB of memory under Win7-64.
>
>Wouldn't that give enough to Cygwin to allow it to give more to the
>heap?

No.

cgf

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Re: max memory

[Brian Ford]

On Mon, 24 Jan 2011, Christopher Faylor wrote:

> On Sun, Jan 23, 2011 at 08:11:52PM -0800, Linda Walsh wrote:
> >Christopher Faylor wrote:
> >> Cygwin only uses as much memory as the OS gives it.  It can't
> >> use a full 2048MB for the heap.
> >----
> >
> >Using the /LARGEADDRESSAWARE flag would allow cygwin to access
> >up to a full 4GB of memory under Win7-64.
> >
> >Wouldn't that give enough to Cygwin to allow it to give more to the
> >heap?
>
> No.

It does give Cygwin more to work with, and it is Cygwin compatible.  But
obviously the heap is still limited by many other address space mappings
(especially DLLs), so getting a full 2 or 4 Gb is not usually possible.

-- 
Brian Ford
Staff Realtime Software Engineer
VITAL - Visual Simulation Systems
FlightSafety International
the best safety device in any aircraft is a well-trained crew...

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