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SubjectRe: 2.6.0 performance problems
> CONFIG_DEBUG_PAGEALLOC really does hurt on small machines.  Mainly because
> it rounds the size of all slab object which are >= 128 bytes up to a full
> 4k. So things like inodes and dentries take vastly more memory.
> The other debug options are less costly.

The patch below rationalizes the Kconfig documentation for the debugging
options a bit.

* removed one occurence of 'don't enable on production systems' as this
would imply that the other options are safe to enable on such systems.

* added a general warning that performance may suffer (but that you should
enable nonetheless in case of debugging), and two specific warnings, one
for slab poisoning, a big one for page alloc debugging.

* some spelling, added notice about /proc/sysrq-trigger to magic SysRQ

* Removed warning about SysRQ 'only if you know what it does' - I often ask
people to press alt-sysrq to get debugging information, only to find that
they have it turned off, even when I would be able to understand the

Against 2.6.0 (path is wrong), please consider applying:

--- linux-2.6.0-test11/arch/i386/Kconfig.orig Wed Dec 31 12:03:20 2003
+++ linux-2.6.0-test11/arch/i386/Kconfig Wed Dec 31 12:16:01 2003
@@ -1131,7 +1131,8 @@
bool "Kernel debugging"
Say Y here if you are developing drivers or trying to debug and
- identify kernel problems.
+ identify kernel problems. Enabling these features often incurs
+ a performance hit, but will help debug problems much faster.

bool "Check for stack overflows"
@@ -1143,7 +1144,7 @@
Say Y here to have the kernel do limited verification on memory
allocation as well as poisoning memory on free to catch use of freed
- memory.
+ memory. Hurts performance.

bool "Memory mapped I/O debugging"
@@ -1166,9 +1167,9 @@
immediately or dump some status information). This is accomplished
by pressing various keys while holding SysRq (Alt+PrintScreen). It
also works on a serial console (on PC hardware at least), if you
- send a BREAK and then within 5 seconds a command keypress. The
- keys are documented in <file:Documentation/sysrq.txt>. Don't say Y
- unless you really know what this hack does.
+ send a BREAK and then within 5 seconds a command keypress.
+ Additionally, /proc/sysrq-trigger can be used. More documentation
+ is in <file:Documentation/sysrq.txt>.

bool "Spinlock debugging"
@@ -1180,19 +1181,18 @@
deadlocks are also debuggable.

- bool "Page alloc debugging"
+ bool "Page alloc debugging (slow/resource intensive)"
depends on DEBUG_KERNEL
Unmap pages from the kernel linear mapping after free_pages().
- This results in a large slowdown, but helps to find certain types
- of memory corruptions.
+ This results in a large slowdown and requires a lot of memory,
+ but helps to find certain types of memory corruptions.

bool "Highmem debugging"
- This options enables addition error checking for high memory systems.
- Disable for production systems.
+ This options enables additional error checking for high memory systems.

bool "Compile the kernel with debug info"

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