[gentoo-server] CFQ low latency mode in 2.6.32

[gentoo-server] CFQ low latency mode in 2.6.32

[Kerin Millar]

Hello,

While browsing the list of kernel changes in 2.6.32 at
kernelnewbies.org, I noticed this:

http://kernelnewbies.org/Linux_2_6_32#head-e07f19bb79beab04cb9e68b942c9041b490b71db

I'm posting this as a heads up as the article alleges that it can
"cause noticeable performance issues". Moreover, it defaults to being
swtiched on but is not exposed an option in the kernel build system.
It can only be turned off via sysfs:

echo 0 > /sys/class/block/<device name>/queue/iosched/low_latency

Therefore, I would advise those who are using CONFIG_IOSCHED_CFQ - and
who to intend to upgrade to 2.6.32 - to test the impact of the option
upon their respective workloads.

Perhaps it's just my interpretation, but I have detected an upstream
trend of introducing changes which are either best suited to desktop
users or which have had a tendency to introduce performance
regressions that are of concern with respect to the server role
(gentoo bug #247453 to name but one example). Hopefully this turns out
to be a false alarm, but I thought that it would be worth mentioning
in any case.

Cheers,

--Kerin