Re: Caching/buffers become useless after some time

[Vlastimil Babka <vbabka@suse.cz>]

On 11/2/18 12:49 PM, Michal Hocko wrote:
> On Fri 02-11-18 12:31:09, Marinko Catovic wrote:
>> Am Fr., 2. Nov. 2018 um 09:05 Uhr schrieb Michal Hocko <mhocko@suse.com>:
>>>
>>> On Thu 01-11-18 23:46:27, Marinko Catovic wrote:
>>>> Am Do., 1. Nov. 2018 um 14:23 Uhr schrieb Michal Hocko <mhocko@suse.com>:
>>>>>
>>>>> On Wed 31-10-18 20:21:42, Marinko Catovic wrote:
>>>>>> Am Mi., 31. Okt. 2018 um 18:01 Uhr schrieb Michal Hocko <mhocko@suse.com>:
>>>>>>>
>>>>>>> On Wed 31-10-18 15:53:44, Marinko Catovic wrote:
>>>>>>> [...]
>>>>>>>> Well caching of any operations with find/du is not necessary imho
>>>>>>>> anyway, since walking over all these millions of files in that time
>>>>>>>> period is really not worth caching at all - if there is a way you
>>>>>>>> mentioned to limit the commands there, that would be great.
>>>>>>>
>>>>>>> One possible way would be to run this find/du workload inside a memory
>>>>>>> cgroup with high limit set to something reasonable (that will likely
>>>>>>> require some tuning). I am not 100% sure that will behave for metadata
>>>>>>> mostly workload without almost any pagecache to reclaim so it might turn
>>>>>>> out this will result in other issues. But it is definitely worth trying.
>>>>>>
>>>>>> hm, how would that be possible..? every user has its UID, the group
>>>>>> can also not be a factor, since this memory restriction would apply to
>>>>>> all users then, find/du are running as UID 0 to have access to
>>>>>> everyone's data.
>>>>>
>>>>> I thought you have a dedicated script(s) to do all the stats. All you
>>>>> need is to run that particular script(s) within a memory cgroup
>>>>
>>>> yes, that is the case - the scripts are running as root, since as
>>>> mentioned all users have own UIDs and specific groups, so to have
>>>> access one would need root privileges.
>>>> My question was how to limit this using cgroups, since afaik limits
>>>> there apply to given UIDs/GIDs
>>>
>>> No. Limits apply to a specific memory cgroup and all tasks which are
>>> associated with it. There are many tutorials on how to configure/use
>>> memory cgroups or cgroups in general. If I were you I would simply do
>>> this
>>>
>>> mount -t cgroup -o memory none $SOME_MOUNTPOINT
>>> mkdir $SOME_MOUNTPOINT/A
>>> echo 500M > $SOME_MOUNTPOINT/A/memory.limit_in_bytes
>>>
>>> Your script then just do
>>> echo $$ > $SOME_MOUNTPOINT/A/tasks
>>> # rest of your script
>>> echo 1 > $SOME_MOUNTPOINT/A/memory.force_empty
>>>
>>> That should drop the memory cached on behalf of the memcg A including the
>>> metadata.
>>
>> well, that's an interesting approach, I did not know that this was
>> possible to assign cgroups to PIDs, without additionally explicitly
>> defining UID/GID. This way memory.force_empty basically acts like echo
>> 3 > drop_caches, but only for the memory affected by the PIDs and its
>> children/forks from the A/tasks-list, true?
> 
> Yup
>  
>> I'll give it a try with the nightly du/find jobs, thank you!
> 
> I am still a bit curious how that will work out on metadata mostly
> workload because we usually have quite a lot of memory on normal LRUs to
> reclaim (page cache, anonymous memory) and slab reclaim is just to
> balance kmem. But let's see. Watch for memcg OOM killer invocations if
> the reclaim is not sufficient.
> 
>>> [...]
>>>>>> As I understand everyone would have this issue when extensive walking
>>>>>> over files is performed, basically any `cloud`, shared hosting or
>>>>>> storage systems should experience it, true?
>>>>>
>>>>> Not really. You need also a high demand for high order allocations to
>>>>> require contiguous physical memory. Maybe there is something in your
>>>>> workload triggering this particular pattern.
>>>>
>>>> I would not even know what triggers it, nor what it has to do with
>>>> high order, I'm just running find/du, nothing special I'd say.
>>>
>>> Please note that find/du is mostly a fragmentation generator. It
>>> seems there is other system activity which requires those high order
>>> allocations.
>>
>> any idea how to find out what that might be? I'd really have no idea,
>> I also wonder why this never was an issue with 3.x
>> find uses regex patterns, that's the only thing that may be unusual.
> 
> The allocation tracepoint has the stack trace so that might help. This

Well we already checked the mm_page_alloc traces and it seemed that only
THP allocations could be the culprit. But apparently defrag=defer made
no difference. I would still recommend it so we can see the effects on
the traces. And adding tracepoints
compaction/mm_compaction_try_to_compact_pages and
compaction/mm_compaction_suitable as I suggested should show which
high-order allocations actually invoke the compaction.

> is quite a lot of work to pin point and find a pattern though. This is
> way out the time scope I can devote to this unfortunately. This might be
> some driver asking for more, or even the core kernel being more high
> order memory hungry.
>