putting fedora on a (memory) diet... some suggestions for the kernel config

[Arjan van de Ven]

Looking at kernel memory use (based on 2.6.19 with a fedora config),
there's some not that hard ways to gain quite a bit of memory back.

Most important is CONFIG_NR_CPUS, right now that is set to 255 for
everyone; this value is used for scaling a LOT of things in the kernel,
and the sad thing is it's not even possible today to get a 255
core/processor machine (you run out of apic ids well before you get that
far).

Setting CONFIG_NR_CPUS to 64 (which is still huge, even 16 would be more
than 99.99999% of the people who use fedora will ever use) will save


user 		255		64	saving
----------------------------------------------
irq_desc	2154496		294912	1859584
irq_domain	269312		18432	250880
irq_lists	134656		36864	97792
irq_2_pin	100992		27648	73344
irq_timer_state	67328		18432	48896
msi_desc	67328		18432	48896
per_cpu__kstat	33728		9280	24448
-----------------------------------------------
total					2403839

this alone saves 2.4Mb of memory in *static* buffers! Add a bunch more
in smaller buffers and all dynamic kernel allocations and it'll be
closer to 3Mb.. Easy gain right there.

Another one is __log_buf; this is currently 128Kb. Arguably it's
important for debugging to get 128Kb of dmesg info, but I wonder if 64Kb
would be enough as well, it's another easy save by setting
CONFIG_LOG_BUF_SHIFT to 16.

A third one is disabling CPU hotplug. CPU hotplug keeps quite a bit of
code in the kernel, that otherwise is boot-time only and gets evicted
right after boot..... You could argue the same for PCI hotplug but
cardbus is sort of the party spoiler there I suppose.


I don't know what Dave will pick for CONFIG_STACK_PROTECTOR_ALL; I would
suggest to turn this off (while enabling the normal
CONFIG_STACK_PROTECTOR); _ALL doesn't add much if any security while it
creates bigger and slower code everywhere. (The difference is that _ALL
forces the canary on every function, while the normal setting only
forces it for functions with buffers on the stack)