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SubjectRe: [RFC 5/5] Memory controller soft limit reclaim on contention
Paul Menage wrote:
> On Fri, Jun 27, 2008 at 8:19 AM, Balbir Singh <balbir@linux.vnet.ibm.com> wrote:
>> +/*
>> + * Create a heap of memory controller structures. The heap is reverse
>> + * sorted by size. This heap is used for implementing soft limits. Our
>> + * current heap implementation does not allow dynamic heap updates, but
>> + * eventually, the costliest controller (over it's soft limit should
>
> it's -> its
>

Yes

>> + old_mem = heap_insert(&mem_cgroup_heap, mem,
>> + HEAP_REP_LEAF);
>> + mem->on_heap = 1;
>> + if (old_mem)
>> + old_mem->on_heap = 0;
>
> Maybe a comment here that mem might == old_mem?
>
>> + * When the soft limit is exceeded, look through the heap and start
>> + * reclaiming from all groups over thier soft limit
>
> thier -> their
>

Will fix

>> + if (!res_counter_check_under_soft_limit(&mem->res)) {
>> + /*
>> + * The current task might already be over it's soft
>> + * limit and trying to aggressively grow. We check to
>> + * see if it the memory group associated with the
>> + * current task is on the heap when the current group
>> + * is over it's soft limit. If not, we add it
>> + */
>> + if (!mem->on_heap) {
>> + struct mem_cgroup *old_mem;
>> +
>> + old_mem = heap_insert(&mem_cgroup_heap, mem,
>> + HEAP_REP_LEAF);
>> + mem->on_heap = 1;
>> + if (old_mem)
>> + old_mem->on_heap = 0;
>> + }
>> + }
>
> This and the other similar code for adding to the heap should be
> refactored into a separate function.
>

OK, I can look into that.

>> +static int mem_cgroup_compare_soft_limits(void *p1, void *p2)
>> +{
>> + struct mem_cgroup *mem1 = (struct mem_cgroup *)p1;
>> + struct mem_cgroup *mem2 = (struct mem_cgroup *)p2;
>> + unsigned long long delta1, delta2;
>> +
>> + delta1 = res_counter_soft_limit_delta(&mem1->res);
>> + delta2 = res_counter_soft_limit_delta(&mem2->res);
>> +
>> + return delta1 > delta2;
>> +}
>
> This isn't a valid comparator, since it isn't a constant function of
> its two input pointers - calling mem_cgroup_compare_soft_limits(m1,
> m2) can give different results at different times. So your heap
> invariant will become invalid over time.
>
> I think if you want to do this, you're going to need to periodically
> take a snapshot of each cgroup's excess and use that snapshot in the
> comparator; whenever you update the snapshots, you'll need to restore
> the heap invariant.
>

I'll fix it by taking snapshots only before inserting an element into the heap
(I think I responded to this one in another email, but missed out on the typos).

--
Warm Regards,
Balbir Singh
Linux Technology Center
IBM, ISTL


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