2013-05-02 01:30:25 +02:00
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<?xml version="1.0" encoding='UTF-8'?>
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<!DOCTYPE sect1 PUBLIC "-//OASIS//DTD DocBook V4.5//EN"
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"http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd">
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<sect1 id="setup-maxmem"><title>Changing Cygwin's Maximum Memory</title>
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<para>
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Cygwin's heap is extensible. However, it does start out at a fixed size
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and attempts to extend it may run into memory which has been previously
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allocated by Windows. In some cases, this problem can be solved by
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changing a field in the file header which is utilized by Cygwin since
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2016-03-18 22:52:04 +01:00
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to keep the initial size of the application heap. If the field contains 0,
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which is the default, the application heap defaults to a size of 384 Megabyte
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on 32 bit Cygwin, 512 Megabyte on 64 bit Cygwin. If the field is set to any
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other value between 4 and 2048, Cygwin tries to reserve as much Megabytes
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for the application heap. The field used for this is the "LoaderFlags" field
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in the NT-specific PE header structure
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(<literal>(IMAGE_NT_HEADER)->OptionalHeader.LoaderFlags</literal>).</para>
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2013-05-02 01:30:25 +02:00
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<para>
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This value can be changed for any executable by using a more recent version
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of the <command>peflags</command> tool from the <literal>rebase</literal>
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Cygwin package. Example:
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<screen>
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$ peflags --cygwin-heap foo.exe
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foo.exe: initial Cygwin heap size: 0 (0x0) MB
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$ peflags --cygwin-heap=500 foo.exe
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foo.exe: initial Cygwin heap size: 500 (0x1f4) MB
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</screen>
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</para>
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<para>
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Heap memory can be allocated up to the size of the biggest available free
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block in the processes virtual memory (VM). By default, the VM per process
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is 2 GB for 32 processes. To get more VM for a process, the executable
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must have the "large address aware" flag set in the file header. You can
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use the aforementioned <command>peflags</command> tool to set this flag.
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On 64 bit systems this results in a 4 GB VM for a process started from that
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executable. On 32 bit systems you also have to prepare the system to allow
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up to 3 GB per process. See the Microsoft article
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<ulink url="http://msdn.microsoft.com/en-us/library/bb613473%28VS.85%29.aspx">4-Gigabyte Tuning</ulink>
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for more information.
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</para>
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</sect1>
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