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Date: Fri, 11 May 2007 13:50:48 -0400
From: "Matt McClure" <mlm AT aya DOT yale DOT edu>
To: cygwin AT cygwin DOT com
Subject: peculiar behavior of max_memory and heap_chunk_in_mb
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What explains this peculiar behavior of max_memory and heap_chunk_in_mb?

Following the instructions at:
http://www.cygwin.com/cygwin-ug-net/setup-maxmem.html, I increased
Cygwin's memory limit to 1024MB:

$ regtool -i set /HKLM/Software/Cygnus\ Solutions/Cygwin/heap_chunk_in_mb 1024

and after closing all Cygwin programs, and restarting, then I see:

$ $ ./max_memory.exe
3fffe000 bytes (1024.0Mb)

Makes sense so far.  Curious what the default limit on my system is, I
unset the limit and tried again:

$ regtool unset /HKLM/Software/Cygnus\ Solutions/Cygwin/heap_chunk_in_mb
$ regtool -v list /HKLM/Software/Cygnus\ Solutions/Cygwin
mounts v2\ (Cygwin)
Program Options\ (cygnus)

After closing and restarting:

$ ./max_memory.exe
5fffe000 bytes (1536.0Mb)

Huh?  The documentation says the default limit is 384MB.  Okay, maybe
the documentation is wrong, so I tried setting the limit to 384MB.

$ regtool -i set /HKLM/Software/Cygnus\ Solutions/Cygwin/heap_chunk_in_mb 384
$ regtool -v list /HKLM/Software/Cygnus\ Solutions/Cygwin
mounts v2\ (Cygwin)
Program Options\ (cygnus)
heap_chunk_in_mb = 0x00000180 (384)

And after quitting and restarting:

$ ./max_memory.exe
5fffe000 bytes (1536.0Mb)

Now something is fishy for sure.  The good news is that when I set the
limit to 2048, the program I'm really trying to run works.  I'd like
to understand why the max_memory test is reporting strange results,
though.

Matt

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