Preempt vs lowlatency kernel flavours summary

[Alessio Igor Bogani]

Hi All,

I want summarize the differences between -preempt and -lowlatency
(http://kernel.ubuntu.com/~abogani/) kernel flavour.
Both use the same -generic source code and differs only for kernel
configuration.

Preempt kernel flavour set CONFIG_HZ=1000 (CONFIG_HZ=100 on amd64
-generic) and CONFIG_PREEMPT=y (CONFIG_PREEMPT_VOLUNTARY=y on amd64
-generic).

Lowlatency kernel flavour contain preempt config changes (1000HZ and
PREEMPT=y) but also add these:

*) Is Built against i386 too: why we don't support 32bit(*) systems too?

*) CONFIG_NO_HZ turned off
Tickless is very important feature for reduce power consumption but is
also increase latency a lot. We can incur in high latencies every
times an event happens after a sufficient long time (it is a really
short) with cpu in idle state. So if you configure your system to be
very "ready" you could obtain the worst results.
We, -rt kernel users, turn it off by default with a simple patch since
Hardy (which could improve -generic kernel as well).

*) CONFIG_FTRACE turned off
Once some months ago Ftrace caused performance regression (one time at
least) Moreover, as far as I know, no one user have ever requested
ftrace available in the Ubuntu kernel so I should prefer turn it off
(-1 of things that could break).

*) CONFIG_SLAB turned on (instead of CONFIG_SLUB)
SLUB was born for achieve better results in memory management in
multi-core environment with a huge quantity of memory.
It work better than SLAB but is slower and require more cpu. So if you
want achieve better latency you should use SLUB for high-end systems
and SLAB for normal systems (desktop/laptop). I suspect that the last
ones are our target. :-)

*) CONFIG_TREE_PREEMPT_RCU=y
On -rt it is heavily used without sort of problem.

Personally I would want see these changes applied because, in some
cases, we could achieve enough good results for hope to replace -rt
kernel for most common uses.

(*) I don't checked PAE because I never had a i386 machine with more than 4GB.

Ciao,
Alessio

P.s.
Some background:
https://lists.ubuntu.com/archives/kernel-team/2010-January/008385.html
https://lists.ubuntu.com/archives/kernel-team/2010-January/008570.html