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docs/MemSavers/mem.html
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docs/MemSavers/mem.html
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<html>
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<head>
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<title>Memory Savers</title>
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<h1>Memory Savers</h1>
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</head>
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<body>
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<h3>Updated 30/08/95</h3><P>
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Paul Gortmaker<br>
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<a href="mailto:Paul.Gortmaker@anu.edu.au">Paul.Gortmaker@anu.edu.au</a>
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<P>
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<p><hr><h3>Linux Memory Savers
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<p></p>
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This page has various Linux memory saver patches that I have put together.
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</h3><hr></p>
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There are still lots of people out there running Linux on 386 boxes with
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4MB ram and older/smaller hard disks. These patches implement options
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that will allow these users to trim some excess (unused) functionality
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from their kernels and reclaim some of that valuable memory for running
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applications. Note that memory taken up by the kernel is hardwired to
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RAM and can't be swapped to disk, thus reducing the amount of real
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memory available to run user programs and the like.
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<P>
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Most of these patches are against the current 1.2.x kernel, as it is
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a bit smaller than the 1.3.x kernels to begin with. If you are tight
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on memory, you are best to run the latest 1.2.x kernel as opposed to
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the latest 1.3.x development series.
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<hr>
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Here are some of the areas where you can "trim the fat".
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<P>
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<a href="#ram">The RAM Disk</a><br>
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<a href="#hd">The "old" Disk Driver</a><br>
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<a href="#mod">The Modules Code</a><br>
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<a href="#ini">The Old init Code</a><br>
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<a href="#ser">The Serial Driver</a><br>
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<a href="#flo">The Floppy Driver</a><br>
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<a href="#ne">The ne2000 Driver</a><br>
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<a href="#3c">The 3c503 Driver</a><br>
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<a href="#scsi">The SCSI Blacklist</a><br>
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<a href="#pri">The printk() Ring Buffer</a><br>
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<a href="#ksw">The Kswap Patches</a><br>
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<a href="#thi">Removing IBM ThinkPad Support</a><br>
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<a href="#m386">The `-m386' GCC option</a><br>
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<P>
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As a more drastic alternative, you can `downgrade' to a 1.0.9 kernel.
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For those that can't recall, this was the last (i.e. stable) release
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of the 1.0 series before starting the 1.1 development series. As a quick
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test, I compiled a 1.0.9 kernel `out of the box' to see how it compared
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memorywise against my trimmed 1.2.13 kernel. It beat it hands down!
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For my config, the trimmed 1.2.13 kernel weighed in at 512k code and
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148kB data. The stock 1.0.9 kernel with the same config weighed in
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at a mere 388kB code and 168kB data. I then quickly applied by hand
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some of the above list (ram / modules / init / serial / ne2k / printk)
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and had it down to a lean-n-mean 384kB code and 152kB data!! That is
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a 104kB saving over the <em>trimmed</em> 1.2.13 kernel. This is an
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excellent kernel to run on 4MB machines. Here is a
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<a href="109.diff">patch</a>
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against the 1.0.9 tree that incorporates the above five from the list.
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Note that some (such as the Thinkpad hack or the reduced hd driver) don't
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apply to the 1.0.9 kernels, as they weren't in there yet to begin with,
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so there is nothing to `fix' so to speak.
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<hr>
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<p><h4><a name="ram">The RAM disk</h4></p>
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Patch to make the <em>ramdisk</em> a config option. Face it, you never
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use it except for boot/recovery disks and the like. Available for
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<a href="ramdisk.1212">v1.2.12</a>.
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With this in place, the file <code>ramdisk.c</code> is not even
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compiled unless you select ramdisk support.
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This should be another small memory saving that will help low memory
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machines.
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<p><h4><a name="hd">The "old" disk driver</h4></p>
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A patch to remove all IDE support from the "old" hard disk
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driver. Seeing as <code>ide.c</code> now uses
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all the IDE specific features in addition to the standard ST-506 hard
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disk specification, the old
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disk driver may as well return to its original featureless ST-506 form.
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This will be a good choice for users with MFM/RLL or ESDI drives that
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can use the memory savings a lot more than the IDE features. Users that
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don't wish to use the extended features of their IDE drive(s) can also
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use this patch as well. There should be no risk in using this patch,
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as all it mainly does is remove code that is IDE specific. I have
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tested it in a two drive IDE system and a one drive MFM system.
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Just compare the compiled sizes of hd.c with my patch, hd.c without my
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patch, and ide.c as seen below, for 1.3.14. (This also doesn't account
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for the extra 1/2kB per IDE drive that was dynamically allocated to
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to store the IDE drive information.)
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<P>
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<code>
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# size drivers/block/hd.o.patched<br>
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text data bss dec hex<br>
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5440 2228 4 7672 1df8<br>
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# size drivers/block/hd.o.orig<br>
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text data bss dec hex<br>
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8992 2324 48 11364 2c64<br>
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# size drivers/block/ide.o<br>
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text data bss dec hex<br>
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17992 4488 96 22576 5830<br>
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</code>
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<P>
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The "low-fat-diet" hard disk driver is available for
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<a href="hd_diff-1212">v1.2.12</a> kernels.
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<p><h4><a name="mod">The Modules Code</h4></p>
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A patch to
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<a href="no_modules_patch">remove all module support</a> for those that are on
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low memory machines, or those that never use modules.
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This is done
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by using <code>``#ifdef CONFIG_MODULES''</code> and adding it as
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a <em>make config</em> option. Note that this patch is for 1.2.x kernels.
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I will eventually update it for the 1.3.x kernels.
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<P>
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For those who were complaining about the modules stuff adding bloat,
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it can be pulled out quite easily.
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A big win is dumping modules.c, ksyms.c and a good chunk of scsi.c
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for loading and unloading host adaptors.
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Also gone are (un)register_netdev, unregister_filesystem and
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(un)register_binfmt. Note that you *can't* dump register_fiesystem,
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(un)register_blkdev and (un)register_chrdev because these are also
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used by non-modules. (However, this could be easily changed...)
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Of course /proc/ksyms and /proc/modules disappear as well.
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<P>
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The only files that didn't include <code><linux/config.h></code>
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already were scsi.c
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and modules.c (ironic huh?) Of course the module syscalls are still
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listed in unistd.h and entry.S, but they are just dummies that
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return -ENOSYS like those in kernel/sys.c, so that running insmod on
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a non-module kernel doesn't do anything silly. Hopefully this can
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go into 1.3.x someday.
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<p><h4><a name="ini">The Old init Code</h4></p>
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Many moons ago, when more people had heard of <em>Minix</em> than
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<em>Linux</em>, Linux did not have Poe's or MvS's init packages.
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The kernel would look for "/etc/rc" and/or "/bin/sh" to start as
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process number one. Even now that "/etc/init" and lately "/sbin/init"
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have been in widespread use for a long time, this old code still
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remains in the kernel. This
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<a href="init.patch">patch</a> against 1.2.x kernels
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removes that old code, along with a 1kB static buffer that it used.
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Ideally this old code should be removed from distribution kernels
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as well. (Thanks to Michael A. Griffith for reminding me of this one.)
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<p><h4><a name="ser">The Serial driver</h4></p>
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Almost all people have only two (or four) serial ports. Yet in
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<code>drivers/char/serial.c</code> there is a large table of
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possible serial devices called <code>rs_table</code>. This table
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has 37 entries. You can safely delete all but the ones you are
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using, that being the top two, or four if you have four serial
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ports installed. (Thanks to Alan Cox for mentioning this one.)
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<p><h4><a name="flo">The Floppy driver</h4></p>
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The average person has at either a 1.44MB 3.5" drive, and/or a 1.2MB
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5.25" drive. And they usually never use any strange floppy formats.
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The usual formats are 360kB, 720kB, 1.2MB and 1.44MB, and <em>none</em>
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of the others are used. This
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<a href="floppy.diff">patch</a>
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removes the support for 2.88MB drives, and for all the 30 or so other
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formats that 99% of us never use.
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<p><h4><a name="ne">The ne2000 driver</h4></p>
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For ne2000 users, you can make a one line change to
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<code>drivers/net/ne.c</code> that will get rid of a large table of
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non-conforming cards, and its associated code. If you don't have
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a "bad clone" you can just delete or comment out the line that says
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<P>
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<code>
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#define CONFIG_NE_BAD_CLONES
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</code>
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<P>
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<p><h4><a name="3c">The 3c503 driver</h4></p>
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As you may
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or may not know, the 3c503 has the ability to support PIO transfers
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to the card (akin to a ne2000) instead of the standard (read faster/better)
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shared memory implementation.
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However, since it is slower, you shouldn't use a 3c503 in PIO mode.
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If you do, the linux driver will print a message like
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"REJUMPER FOR SHARED MEMORY" at boot, and then still
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try and use it in the crippled PIO mode.
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<P>
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This
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<a href="3c503.mem">patch</a> for the 1.2.x series kernels
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just #ifdef's out all the PIO code based on whether you have
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defined CONFIG_EL2_PIO, for a 20% reduction in the driver size, as
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seen below.
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<P>
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With PIO support:<br>
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<code>
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text data bss dec hex<br>
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3576 44 0 3620 e24<br>
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-rw------- 1 root root 4964 Apr 13 19:50 3c503.o<br>
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</code>
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<P>
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Without PIO support:<br>
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<code>
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text data bss dec hex<br>
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2700 44 0 2744 ab8<br>
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-rw------- 1 root root 4005 Apr 15 15:33 3c503.o<br>
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</code>
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<p><h4><a name="scsi">The SCSI blacklist</h4></p>
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Some old (and even some new!) SCSI devices are poorly designed so that
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if they are "asked" by the SCSI host adapter as to whether they contain
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multiple devices such as a multiple disk CD-ROM, they will lock-up.
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The kernel keeps a <em>huge</em> list of (at present 34) devices that
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have this problem in <code>drivers/scsi/scsi.c</code>.
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You can cut this <code>device_list</code> down to just the
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NULL entry at the end if you don't have any of these devices, or if you
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configured your kernel to only ask about LUN #1.
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<p><h4><a name="pri">The printk() Ring Buffer</h4></p>
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The <code>printk()</code> routine uses a 4kB static ring buffer to store
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kernel messages in while waiting for <code>klogd()</code> to read them.
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The only time this many unread messages pile up is at boot before
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<code>klogd()</code> is started. But you can read those by using the
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Shift+Page_Up key sequence. You can go into <code>kernel/printk.c</code>
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and change the <code>#define LOG_BUF_LEN 4096</code> to a smaller value
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like 512, for a saving of 3.5kB. This will still leave room for over
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six full (80 character) lines of kernel messages to be buffered.
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<P>
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There is also a 1kB buffer ("buf") allocated to deal with the incoming
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data passed in with a printk() call. No single printk() calls in the kernel
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are so huge (or broken) as to dump a whole kB in a single call, and so you
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can cut this back to 256 bytes, which is still the equivalent of more
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than three full lines of text. (You could allocate this buffer
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dynamically with kmalloc(), but things will get ugly if you run out of
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memory, and then try to do allocate a buffer so you can do a
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<code>printk("Out of memory\n");</code>!!!)
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<p><h4><a name="ksw">The Kswap Patches</h4></p>
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These are patches by
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<a href="mailto:sct@dcs.ed.ac.uk">Stephen C. Tweedie</a> who has also
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done a lot of work on the ext2 filesystem. These patches don't actually
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save you any memory, but instead they improve the swapping performance
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on low memory machines. For more details on the gains/implementations
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and such, here are some of Stephen's mails that I saved. (The dates
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are encoded in the URLs)
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<P>
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Stephen's latest version of his kswap patches can be found
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<a href="ftp://ftp.presence.co.uk/pub/linux/kswap">here</a>.
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<p><h4><a name="thi">Removing IBM ThinkPad Support</h4></p>
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Once again IBM has made an incompatibility. They seem to have done
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something funny with the last 4kB of RAM at the end of the 640kB
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region. The Linux kernel won't touch this 4kB by default for fear
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of upsetting the IBM ThinkPad. If you don't have an IBM ThinkPad,
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you can re-claim this 4kB by simply changing the value <tt>0x9f000</tt>
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to an <tt>0xA0000</tt> in <tt>linux/arch/i386/mm/init.c</tt>.
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If you are real lazy, here is a <a href="thinkpad">patch</a>
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for the 1.2.x kernels to do that for you. (Thanks to Thomas Graichen
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for pointing this one out.)
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<p><h4><a name="m386">The `-m386' GCC option</h4></p>
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When you compile a kernel with the `-m486' option, GCC then alignes
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internal data structures on data boundaries that will be favourable
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for the cache structure on an i486 CPU. This can leave small `holes'
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between the end of one data structure, and the start of another, which
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accounts for the slightly larger output. So if you compile linux with
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`-m386' instead, then you will end up with a slightly smaller (actually
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more tightly packed) kernel. You won't be able to notice any significant
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performace decrease in everyday usage either.
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<P>
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<hr>
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If you have any other memory saver tips/patches,
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<a href="mailto:Paul.Gortmaker@anu.edu.au">e-mail</a>
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me, and I will add them to the list.
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<P>
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<a href="index.html">Back to the top...</a>
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</body>
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</html>
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