On Thu, Apr 30, 2020 at 04:29:50PM +0000, Christoph Lameter wrote: > On Mon, 27 Apr 2020, Roman Gushchin wrote: > > > > Why do you need this? Just slap a pointer to the cgroup as additional > > > metadata onto the slab object. Is that not much simpler, safer and faster? > > > > > > > So, the problem is that not all slab objects are accounted, and sometimes > > we don't know if advance if they are accounted or not (with the current semantics > > of __GFP_ACCOUNT and SLAB_ACCOUNT flags). So we either have to increase > > the size of ALL slab objects, either create a pair of slab caches for each size. > > > > > The first option is not that cheap in terms of the memory overhead. Especially > > for those who disable cgroups using a boot-time option. > > > If the cgroups are disabled on boot time then you can switch back to the > compact version. Otherwise just add a pointer to each object. It will make > it consistent and there is not much memory wastage. > > The problem comes about with the power of 2 caches in the kmalloc array. It's a very good point, and it's an argument to stick with the current design (an external vector of memcg pointers). > If one keeps the "natural alignment" instead of going for the normal > alignment of slab caches then the alignment will cause a lot of memory > wastage and thus the scheme of off slab metadata is likely going to be > unavoidable. > > But I think we are just stacking one bad idea onto another here making > things much more complex than they could be. Well at least this justifies > all our jobs .... (not mine I am out of work... hehehe) Sorry, but what exactly do you mean? I don't think reducing the kernel memory footprint by almost half is such a bad idea. Thanks!
