On 09/27/2012 06:58 PM, Tejun Heo wrote: > Hello, Mel. > > On Thu, Sep 27, 2012 at 03:43:07PM +0100, Mel Gorman wrote: >>> 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. > > But that happens only when pages enter and leave slab and if it still > is significant, we can try to further optimize charging. Given that > this is only for cases where memcg is already in use and we provide a > switch to disable it globally, I really don't think this warrants > implementing fully hierarchy configuration. > Not totally true. We still have to match every allocation to the right cache, and that is actually our heaviest hit, responsible for the 2, 3 % we're seeing when this is enabled. It is the kind of path so hot that people frown upon branches being added, so I don't think we'll ever get this close to being free. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>