[systemd-devel] Is it possible to set a default for MemoryMax?

[McKay, Sean]

Thanks for the pointer, Lennart!

I've done some initial review of the commit you pointed me to, and it seems like it should be pretty straightforward to use that understanding to implement the other functions. Might take a bit depending on my local management's perspective on the priority of getting this done, but I'll put together a PR as soon as I have a chance.

In the meantime, can anyone tell me if there's any semblance of a 'getting started' page for systemd development? The main code changes should be easy enough, but I see that there are a number of tests included with the particular commit I was pointed to in addition to the code change itself. I'd like to make sure that I know what sort of tests are expected in a commit and how to run them (and any other things I might need to do to ensure things are in order before submitting the PR).

Thanks!
-Sean McKay

-----Original Message-----
From: Lennart Poettering <mznyfn at 0pointer.de> 
Sent: Monday, June 17, 2019 7:55 AM
To: McKay, Sean <sean.mckay at hpe.com>
Cc: systemd-devel at lists.freedesktop.org
Subject: Re: [systemd-devel] Is it possible to set a default for MemoryMax?

On Do, 13.06.19 22:06, McKay, Sean (sean.mckay at hpe.com) wrote:

> Hi there,
>
> First off, forgive me if this is documented or discussed somewhere 
> already. I couldn't find any reference to it in the man pages or in 
> the mailing list archives, and I'm relatively new to this community so 
> I could easily be missing something.
>
> Context that can be skipped if you only care about the technical part 
> of the question: We're building a customized linux environment using 
> systemd as the init system, and we've found that some of the daemons 
> that we're including in said environment aren't particularly good 
> citizens in regards to their resource consumption (admittedly most of 
> those are the ones that have been written in house). As a result, 
> while we try to track down and fix those bugs, we would like to limit 
> the memory that processes are allowed to consume by default (with 
> actual values in the .service files overriding that default). While we 
> understand that this is the sort of thing that's probably best set 
> individually per-daemon based on an understanding of how the daemon 
> should behave, it would at least give us a starting point and would 
> mean that a leaky process would be the one that crashed (when it hit 
> the memory limit), rather than taking down the whole system or the OOM 
> killer selecting a different process that's working correctly but 
> still happens to be using more memory at the time.
>
> We've been trying to encourage our developers to use memory accounting 
> and MemoryMax, since we're given to understand that's applied to the 
> entire cgroup's memory usage for a given daemon's slice. The 
> documentation, however, doesn't seem to indicate that a default value 
> for any of the memory variables can be set in the system.conf file, 
> and some quick experimental testing seems to indicate that's truly the 
> case. There does seem to be support for setting the memory resource 
> limit (limitas), but my understanding is that wouldn't apply to 
> subprocesses that are spawned off by the parent daemon - they'd 
> inherit the resource limit, but get their own instance of it (and I'm 
> actually still trying to figure out if it would apply to threads, 
> given the way I'm given to understand Linux treats threads. For 
> memory, it would probably still cap things, since they're all in the 
> same address space, but I'm unclear if that would be true of the other 
> limits that can be applied).
>
> Am I missing something and this is possible to set? Or is it something 
> that's come up and been rejected for some reason? Or has it come up 
> and not been important relative to other development priorities? Or 
> has it simply never come up because we're the first ones to want to do 
> something so foolish?
>
> Any insight that you could provide would be greatly appreciated! Thanks.

See https://github.com/systemd/systemd/pull/12211 

It adds this for MemoryLow=. I see no reason why we shouldn't add this for MemoryMax= too. Please consider pinging @cdown about this, and maybe prep a PR that adds this for you. It should be simple now, given that the MemoryLow= case is very very similar.

Or maybe MemoryLow= is actually the knob you want to use and thus what you are asking for already exists in current git?

Lennart

--
Lennart Poettering, Berlin