Re: [PATCH v5 14/14] Add documentation about the kmem controller

From: Aristeu Rozanski
Date: Tue Oct 16 2012 - 14:56:45 EST

On Tue, Oct 16, 2012 at 06:25:06PM +0000, Christoph Lameter wrote:
> On Tue, 16 Oct 2012, Glauber Costa wrote:
> >
> > + memory.kmem.limit_in_bytes # set/show hard limit for kernel memory
> > + memory.kmem.usage_in_bytes # show current kernel memory allocation
> > + memory.kmem.failcnt # show the number of kernel memory usage hits limits
> > + memory.kmem.max_usage_in_bytes # show max kernel memory usage recorded
> Does it actually make sense to limit kernel memory? The user generally has
> no idea how much kernel memory a process is using and kernel changes can
> change the memory footprint. Given the fuzzy accounting in the kernel a
> large cache refill (if someone configures the slab batch count to be
> really big f.e.) can account a lot of memory to the wrong cgroup. The
> allocation could fail.
> Limiting the total memory use of a process (U+K) would make more sense I
> guess. Only U is probably sufficient? In what way would a limitation on
> kernel memory in use be good?

It's about preventing abuses caused by bugs or malicious use and avoiding
groups stepping on each others' toes. You're saying that letting a group
to allocate 32GB of paged memory is the same as 32GB of kernel memory?

I don't belive sysadmins will keep a tight limit for kernel memory but rather
a safety limit in case something goes wrong. usage_in_bytes will provide
data to get the limits better adjusted.

The innacuracy of the kmem accounting is (AFAIK) a cost tradeoff.

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