Add entry about varying the size of the kernel address space.

svn path=/head/; revision=4551

FAQ/hackers.sgml

@@ -1,4 +1,4 @@
-<!-- $Id: hackers.sgml,v 1.12 1999-02-17 03:08:21 jkoshy Exp $ -->
+<!-- $Id: hackers.sgml,v 1.13 1999-03-20 13:17:41 des Exp $ -->
 <!-- The FreeBSD Documentation Project -->
 
   <sect>
@@ -502,4 +502,80 @@ Cc: current@FreeBSD.ORG
       <htmlurl url="http://www.freebsd.org/cgi/man.cgi?ld" 
       name="ELF linker">.
 
+      
+    <sect1>
+      <heading>Increasing or reducing the kernel address space</heading>
+
+      <p>
+      By default, the kernel address space is 256 MB on FreeBSD 3.x
+      and 1 GB on FreeBSD 4.x. If you run a network-intensive server
+      (e.g. a large FTP or HTTP server), you might find that 256 MB is
+      not enough.
+
+      <p>
+      So how do you increase the address space? There are two aspects
+      to this. First, you need to tell the kernel to reserve a larger
+      portion of the address space for itself. Second, since the
+      kernel is loaded at the top of the address space, you need to
+      lower the load address so it doesn't bump its head against the
+      ceiling.
+
+      <p>
+      The first goal is achieved by increasing the value of
+      <tt/NKPDE/ in <tt>src/sys/i386/include/pmap.h</tt>. Here's what
+      it looks like for a 1 GB address space:
+
+      <verb>
+#ifndef NKPDE
+#ifdef SMP
+#define NKPDE                   254     /* addressable number of page tables/pde's */
+#else
+#define NKPDE                   255     /* addressable number of page tables/pde's */
+#endif  /* SMP */
+#endif
+      </verb>
+
+      <p>
+      To find the correct value of <tt/NKPDE/, divide the desired
+      address space size (in megabytes) by four, then subtract one for
+      UP and two for SMP.
+
+      <p>
+      To achieve the second goal, you need to compute the correct load
+      address: simply subtract the address space size (in bytes) from
+      0x100100000; the result is 0xc0100000 for a 1 GB address space.
+      Set <tt/LOAD_ADDRESS/ in <tt>src/sys/i386/conf/Makefile.i386</tt>
+      to that value; then set the location counter in the beginning of
+      the section listing to the same value, as follows:
+
+      <verb>
+OUTPUT_FORMAT("elf32-i386", "elf32-i386", "elf32-i386")
+OUTPUT_ARCH(i386)
+ENTRY(btext)
+SEARCH_DIR(/usr/lib); SEARCH_DIR(/usr/obj/elf/home/src/tmp/usr/i386-unknown-freebsdelf/lib);
+SECTIONS
+{
+  /* Read-only sections, merged into text segment: */
+  . = 0xc0100000 + SIZEOF_HEADERS;
+  .interp     : { *(.interp)    }
+      </verb>
+
+      <p>
+      Then reconfig and rebuild your kernel. You will probably have
+      problems with <tt/ps(1)/, <tt/top(1)/ and the like; <tt/make
+      world/ should take care of it (or a manual rebuild of
+      <tt/libkvm/, <tt/ps/ and <tt/top/ after copying the patched
+      <tt/pmap.h/ to <tt>/usr/include/vm/</tt>.
+      
+      <p>
+      NOTE: the size of the kernel address space must be a multiple of
+      four megabytes.
+
+      <p>
+      <em>[<url url="mailto:dg@freebsd.org" name="David Greenman">
+      adds: <em> I think the kernel address space needs to be a power
+      of two, but I'm not certain about that. The old(er) boot code
+      used to monkey with the high order address bits and I think
+      expected at least 256MB granularity.]</em>
+
   </sect>