Re: [PATCH 2/3] mm/vmscan: Enable kswapd to reclaim low-protected memory

Xunlei Pang
2018-12-04T02:40:41Z

On 2018/12/4 AM 1:22, Michal Hocko wrote:
> On Mon 03-12-18 23:20:31, Xunlei Pang wrote:
>> On 2018/12/3 下午7:56, Michal Hocko wrote:
>>> On Mon 03-12-18 16:01:18, Xunlei Pang wrote:
>>>> There may be cgroup memory overcommitment, it will become
>>>> even common in the future.
>>>>
>>>> Let's enable kswapd to reclaim low-protected memory in case
>>>> of memory pressure, to mitigate the global direct reclaim
>>>> pressures which could cause jitters to the response time of
>>>> lantency-sensitive groups.
>>>
>>> Please be more descriptive about the problem you are trying to handle
>>> here. I haven't actually read the patch but let me emphasise that the
>>> low limit protection is important isolation tool. And allowing kswapd to
>>> reclaim protected memcgs is going to break the semantic as it has been
>>> introduced and designed.
>>
>> We have two types of memcgs: online groups(important business)
>> and offline groups(unimportant business). Online groups are
>> all configured with MAX low protection, while offline groups
>> are not at all protected(with default 0 low).
>>
>> When offline groups are overcommitted, the global memory pressure
>> suffers. This will cause the memory allocations from online groups
>> constantly go to the slow global direct reclaim in order to reclaim
>> online's page caches, as kswap is not able to reclaim low-protection
>> memory. low is not hard limit, it's reasonable to be reclaimed by
>> kswapd if there's no other reclaimable memory.
>
> I am sorry I still do not follow. What role do offline cgroups play.
> Those are certainly not low mem protected because mem_cgroup_css_offline
> will reset them to 0.
>

Oh, I meant "offline groups" to be "offline-business groups", memcgs
refered to here are all "online state" from kernel's perspective.