[discuss] memrlimit - potential applications that can use

[Balbir Singh <balbir@linux.vnet.ibm.com>]

After having discussed memrlimit at the container mini-summit, I've been
investigating potential users of memrlimits. Here are the use cases that I have
so far.

1. To provide a soft landing mechanism for applications that exceed their memory
limit. Currently in the memory resource controller, we swap and on failure OOM.
2. To provide a mechanism similar to memory overcommit for control groups.
Overcommit has finer accounting, we just account for virtual address space usage.
3. Vserver will directly be able to port over on top of memrlimit (their address
space limitation feature)

The case against 1 has been that applications, do not tolerate malloc failure,
does not imply that applications should not have the capability or will never be
allowed the flexibility of doing so

Other users of memory limits I found are

1. php - through php.ini allows setting of maximum memory limit
2. Apache - supports setting of memory limits for child processes (RLimitMEM
Directive)
3. Java/KVM all take hints about the maximum memory to be used by the application
4. google.com/codesearch for RLIMIT_AS will show up a big list of applications
that use memory limits.

With this background, I propose that we need a mechanism of providing a memory
overcommit feature for cgroups, the options are

1. We keep memrlimit and use it. It's very flexible, but on the down side it
does simple total_vm based accounting and provides functionality similar to
RLIMIT_AS for control groups.
2. We port the overcommit feature (Andrea did post patches for this), it's
harder to implement, but provides functionality similar to what exists for
overcommit.


Comments?

-- 
	Warm Regards,
	Balbir

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