Support KSM

[ktsaou]

I don't know if this the right place for this request.

Given the limited memory of a raspberry Pi, I suggest to provide compiled kernels with CONFIG_KSM=y

KSM is a memory de-duplication feature, for applications willing to use it. More about it here.

netdata uses this to lower its memory requirements. Other applications may use it too.

[popcornmix]

Have you tried this?
An option like this has the possibility to harm performance:

KSM's scans may use a lot of processing power: some installations will disable KSM for that reason.

so we won't enable this without strong evidence that the benefits outweigh the costs.

[ktsaou]

When KSM is enabled at the kernel is just available for the user to enable it.

So, if you build a kernel with CONFIG_KSM=y you will just get a few files in /sys/kernel/mm/ksm. Nothing else happens. There is no performance penalty (apart I guess from the memory this code occupies into the kernel).

The files that CONFIG_KSM=y offers include:

• /sys/kernel/mm/ksm/run by default 0. You have to set this to 1 for the kernel to spawn ksmd.
• /sys/kernel/mm/ksm/sleep_millisecs, by default 20. The frequency ksmd should evaluate memory for deduplication.
• /sys/kernel/mm/ksm/pages_to_scan, by default 100. The amount of pages ksmd will evaluate on each run.

So, by default ksmd is just disabled. It will not harm performance and the user/admin can control the CPU resources he/she is willing ksmd to use.

For netdata for example, I use sleep_millisecs=1000, pages_to_scan=100. With these settings ksmd does not even appear in the running process list.

[popcornmix]

So what is the effect for you when enabled?
Are you seeing increased free memory with this enabled?

[ktsaou]

It depends on the application.

The applications opt-in / register memory for deduplication.

For netdata, you can have 50% to 60% memory savings.

Check this:

[ktsaou]

And these the CPU resources it uses:

As you can see, 1% CPU every a few seconds.

[ktsaou]

The effect on netdata is so high, because netdata registers its in-memory round robin database. This database stores the values collected from the system. As you understand there are many metrics that are constant, or just zero.

Deduplication happens at the page level (4KB), and netdata uses by default 1 hour of data filled with 32bit numbers, so each metric is about 3,5 pages. If a metric is constant, 3 of these pages will be merged to 1, saving 2. If more metrics have the same constant value (zeros for example), then even more pages will be merged.

In the chart I posted above, you can see that sharing = 116MB merged to shared = 1,65MB, while unshared = 87MB were unique and could not be merged with anything else.

[P33M]

Do these applications ever encounter hard memory constraints in realistic scenarios (i.e. OOM-kills) without KSM enabled?

[ktsaou]

Do these applications ever encounter hard memory constraints in realistic scenarios (i.e. OOM-kills) without KSM enabled?

netdata can be sized according to your needs. The user / admin can configure the size of the in-memory round robin database. By default it will use about 15-20 MB. Even at this size however, KSM will offer 25-30% savings.

I am sure you will find netdata worths the trouble.
Check it at http://netdata.firehol.org.
It is good!

[Ruffio]

@ktsaou has your issue been resolved? If so, please close this issue. Thanks.

[ktsaou]

This is not resolved. KSM is still missing in rpi kernels.

[abpostelnicu]

I agree on this one KSM should be available in the default raspbian kernel. It would be very helpfl for monitoring application such as netdata

[ktsaou]

Do these applications ever encounter hard memory constraints in realistic scenarios (i.e. OOM-kills) without KSM enabled?

and keep in mind netdata adjusts its OOM score to 1000, so it is the first to be killed in case the system starves for memory.

Enabling KSM for RPi will really help a lot of people out there.

netdata is booming, we have 45k installations in the last 7 months (http://my-netdata.io). netdata is included on home automation systems (https://home-assistant.io/blog/2016/12/17/text-to-speech-aquostv-flic-zamg/?platform=hootsuite) and all kinds of embedded applications, due to its efficiency.

So, please give us KSM on RPi.

Thanks!

[sfx2000]

No impact to enable the item - it's default is not to run, so the user has to explicitly turn it on.

Saves a lot of time not having to build a custom kernel..

[sfx2000]

I'll update here - should be fine on Pi2/Pi3 for the arm7 kernel - I don't have a Pi1/PiZero/CM so can't say much about ARM6, but again, this is low risk - as one does need to enable it first, and even then, only code that is KSM aware will use it. Most folks will never know it's there, but for those that are looking to develop around it, or teach others, it's a good thing to have.

More info here - https://www.kernel.org/doc/Documentation/vm/ksm.txt

Those who are exploring things like containers and VM's can benefit here - and even Android has this option to enable/disable KSM.

[ktsaou]

Just to add: it is useful on RPi 1 too.