Increase the default memory usage limit on "low-memory" systems.

Previously the default limit was always 40 % of RAM. The
new limit is a little bit more complex:

  - If 40 % of RAM is at least 80 MiB, 40 % of RAM is used
    as the limit.

  - If 80 % of RAM is over 80 MiB, 80 MiB is used as the limit.

  - Otherwise 80 % of RAM is used as the limit.

This should make it possible to decompress files created with
"xz -9" on more systems. Swapping is generally more expected
on systems with less RAM, so higher default limit on them
shouldn't cause too bad surprises in terms of heavy swapping.
Instead, the higher default limit should reduce the number of
bad surprises when it used to prevent decompression of files
created with "xz -9". The DoS prevention system shouldn't be
a DoS itself.

Note that even with the new default limit, a system with 64 MiB
RAM cannot decompress files created with "xz -9" without user
overriding the limit. This should be OK, because if xz is going
to need more memory than the system has RAM, it will run very
very slowly and thus it's good that user has to override the limit
in that case.

1 files changed, 15 insertions, 6 deletions

diff --git a/src/xz/xz.1 b/src/xz/xz.1
index 9dc354a0..aba0a693 100644
--- a/src/xz/xz.1
+++ b/src/xz/xz.1
@@ -5,7 +5,7 @@
 .\" This file has been put into the public domain.
 .\" You can do whatever you want with this file.
 .\"
-.TH XZ 1 "2010-01-15" "Tukaani" "XZ Utils"
+.TH XZ 1 "2010-03-07" "Tukaani" "XZ Utils"
 .SH NAME
 xz, unxz, xzcat, lzma, unlzma, lzcat \- Compress or decompress .xz and .lzma files
 .SH SYNOPSIS
@@ -195,9 +195,16 @@ is several gigabytes.
 .PP
 To prevent uncomfortable surprises caused by huge memory usage,
 .B xz
-has a built-in memory usage limiter. The default limit is 40 % of total
-physical RAM. While operating systems provide ways to limit the memory usage
-of processes, relying on it wasn't deemed to be flexible enough.
+has a built-in memory usage limiter. While some operating systems provide
+ways to limit the memory usage of processes, relying on it wasn't deemed
+to be flexible enough. The default limit depends on the total amount of
+physical RAM:
+.IP \(bu 3
+If 40\ % of RAM is at least 80 MiB, 40\ % of RAM is used as the limit.
+.IP \(bu 3
+If 80\ % of RAM is over 80 MiB, 80 MiB is used as the limit.
+.IP \(bu 3
+Otherwise 80\ % of RAM is used as the limit.
 .PP
 When compressing, if the selected compression settings exceed the memory
 usage limit, the settings are automatically adjusted downwards and a notice
@@ -588,9 +595,11 @@ can be specified as a percentage of physical RAM. Example:
 .IP \(bu 3
 The
 .I limit
-can be reset back to its default value (currently 40 % of physical RAM)
-by setting it to
+can be reset back to its default value by setting it to
 .BR 0 .
+See the section
+.B "Memory usage"
+for how the default limit is defined.
 .IP \(bu 3
 The memory usage limiting can be effectively disabled by setting
 .I limit