Re: Android low memory killer vs. memory pressure notifications

[Anton Vorontsov <anton.vorontsov@linaro.org>]

On Tue, Dec 20, 2011 at 05:14:18PM -0800, Frank Rowand wrote:
[...]
> >>> Hm, assuming that metadata is no longer an issue, why do you think avoiding
> >>> cgroups would be a good idea?
> >>>
> >>
> >> It's helpful for certain end users, particularly those in the embedded 
> >> world, to be able to disable as many config options as possible to reduce 
> >> the size of kernel image as much as possible, so they'll want a minimal 
> >> amount of kernel functionality that allows such notifications.  Keep in 
> >> mind that CONFIG_CGROUP_MEM_RES_CTLR is not enabled by default because of 
> >> this (enabling it, CONFIG_RESOURCE_COUNTERS, and CONFIG_CGROUPS increases 
> >> the size of the kernel text by ~1%),
> > 
> > So for 2MB kernel that's about 20KB of an additional text... This seems
> > affordable, especially as a trade-off for the things that cgroups may
> > provide.
> 
> A comment from http://lkml.indiana.edu/hypermail/linux/kernel/1102.1/00412.html:
> 
> "I care about 5K. (But honestly, I don't actively hunt stuff less than
> 10K in size, because there's too many of them to chase, currently)."

I have just tried to turn off CGROUPS on my qemu test kernels:

$ diff -u cgroups no_cgroups 
    text           data     bss     dec     hex filename
-3869810         465976  565248 4901034  4ac8aa vmlinux
+3806374         460544  540672 4807590  495ba6 vmlinux

So, that's actually ~60KB. Which is serious. memcontrol.o text size
is about 23KB.

And my cgroups setup was just this:

$ cat .config | grep CGRO
CONFIG_CGROUPS=y
# CONFIG_CGROUP_DEBUG is not set
# CONFIG_CGROUP_FREEZER is not set
# CONFIG_CGROUP_DEVICE is not set
# CONFIG_CGROUP_CPUACCT is not set
CONFIG_CGROUP_MEM_RES_CTLR=y
# CONFIG_CGROUP_PERF is not set
# CONFIG_CGROUP_SCHED is not set
# CONFIG_BLK_CGROUP is not set

:-(

> > The fact is, for desktop and server Linux, cgroups slowly becomes a
> > mandatory thing. And the reason for this is that cgroups mechanism
> > provides some very useful features (in an extensible way, like plugins),
> > i.e. a way to manage and track processes and its resources -- which is the
> > main purpose of cgroups.
> 
> And for embedded and for real-time, some of us do not want cgroups to be
> a mandatory thing.  We want it to remain configurable.  My personal
> interest is in keeping the latency of certain critical paths (especially
> in the scheduler) short and consistent.

Much thanks for your input! That would be quite strong argument for going
with /dev/mem_notify approach. Do you have any specific numbers how cgroups
makes scheduler latencies worse?

Thanks!

-- 
Anton Vorontsov
Email: cbouatmailru@gmail.com

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