Re: [PATCH 2/2] mm, slab: Extend vm/drop_caches to shrink kmem slabs

Waiman Long
2019-06-27T21:16:04Z

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