I have two related questions about the cpu and memory subsystems and at
most vs at least semantics.
## cpu
As I understand it, by default cpu.shares is used to give tasks at least
the specified cpu resources. Optional with cgroups_enable_cfs both
cpu.shares and cpu.cfs_period_us/cpu.cfs_quota_us to provide both at
most and at least guarantees [1]. This is a per slave/cluster(?)
setting as opposed to per task.
Since cgroups_enable_cfs potentially could leave cpu resources unused I
presume it has a corresponding advantage, but I'm not sure what it is.
When would one be preferred over the another?
## mem
Memory uses memory.limit_in_bytes to limit at most how much memory a
task can use. There does not seem to be a way to specify that a task
should have at least a certain amount of memory. This makes sense for
RSS (since 'bursting' to RSS > physical is not a desirable state and
part of what these limits should avoid). However, from the kernel docs
my understanding is that memory.limit_in_bytes also limits how much page
cache a task can use. The page cache is a shared resource and keeping
the kernel from per-fetching and caching as much as it can is a
surprising choice to me.
Is there a best practice for frameworks to choose memory limits that
include RSS + page cache? In particular, I'm unsure how to reason about
page cache use since a page is counted against whichever cgroup happened
to access it first.
[1]
http://mail-archives.apache.org/mod_mbox/mesos-user/201310.mbox/%3CCA%2B8RcoTbSY%3DKm2jKTVbCid-G76ytFA%2Bq_Zmzu5zRGC-wE206Xg%40mail.gmail.com%3E
[2] https://www.kernel.org/doc/Documentation/cgroups/memory.txt
