[Devel] Re: [PATCH v3 04/13] kmem accounting basic infrastructure

[Mel Gorman]

On Thu, Sep 27, 2012 at 07:33:00AM -0700, Tejun Heo wrote:
> Hello, Michal.
> 
> On Thu, Sep 27, 2012 at 02:08:06PM +0200, Michal Hocko wrote:
> > Yes, because we have many users (basically almost all) who care only
> > about the user memory because that's what occupies the vast majority of
> > the memory. They usually want to isolate workload which would disrupt
> > the global memory otherwise (e.g. backup process vs. database). You
> > really do not want to pay an additional overhead for kmem accounting
> > here.
> 
> I'm not too convinced.  First of all, the overhead added by kmemcg
> isn't big. 

Really?

If kmemcg was globally accounted then every __GFP_KMEMCG allocation in
the page allocator potentially ends up down in
__memcg_kmem_newpage_charge which

1. takes RCU read lock
2. looks up cgroup from task
3. takes a reference count
4. memcg_charge_kmem -> __mem_cgroup_try_charge
5. release reference count

That's a *LOT* of work to incur for cgroups that do not care about kernel
accounting. This is why I thought it was reasonable that the kmem accounting
not be global.

-- 
Mel Gorman
SUSE Labs