On 6/27/19 11:15 AM, Michal Hocko wrote: > On Mon 24-06-19 13:42:19, Waiman Long wrote: >> With the slub memory allocator, the numbers of active slab objects >> reported in /proc/slabinfo are not real because they include objects >> that are held by the per-cpu slab structures whether they are actually >> used or not. The problem gets worse the more CPUs a system have. For >> instance, looking at the reported number of active task_struct objects, >> one will wonder where all the missing tasks gone. >> >> I know it is hard and costly to get a real count of active objects. > What exactly is expensive? Why cannot slabinfo reduce the number of > active objects by per-cpu cached objects? > The number of cachelines that needs to be accessed in order to get an accurate count will be much higher if we need to iterate through all the per-cpu structures. In addition, accessing the per-cpu partial list will be racy. >> So >> I am not advocating for that. Instead, this patch extends the >> /proc/sys/vm/drop_caches sysctl parameter by using a new bit (bit 3) >> to shrink all the kmem slabs which will flush out all the slabs in the >> per-cpu structures and give a more accurate view of how much memory are >> really used up by the active slab objects. This is a costly operation, >> of course, but it gives a way to have a clearer picture of the actual >> number of slab objects used, if the need arises. > drop_caches is a terrible interface. It destroys all the caching and > people are just too easy in using it to solve any kind of problem they > think they might have and cause others they might not see immediately. > I am strongly discouraging anybody - except for some tests which really > do want to see reproducible results without cache effects - from using > this interface and therefore I am not really happy to paper over > something that might be a real problem with yet another mode. If SLUB > indeed caches too aggressively on large machines then this should be > fixed. > OK, as explained in another thread, the main reason for doing this patch is to be able to do more accurate measurement of changes in kmem cache memory consumption. Yes, I do agree that drop_caches is not a general purpose interface that should be used lightly. Cheers, Longman
