Re: [LSF/MM/BPF TOPIC] Do not pin pages for various direct-io scheme

[Jens Axboe <axboe@kernel.dk>]

On 1/22/20 10:28 AM, Jerome Glisse wrote:
> On Wed, Jan 22, 2020 at 10:04:44AM -0700, Jens Axboe wrote:
>> On 1/22/20 9:54 AM, Jerome Glisse wrote:
>>> On Wed, Jan 22, 2020 at 08:12:51AM -0700, Jens Axboe wrote:
>>>> On 1/22/20 4:59 AM, Michal Hocko wrote:
>>>>> On Tue 21-01-20 20:57:23, Jerome Glisse wrote:
>>>>>> We can also discuss what kind of knobs we want to expose so that
>>>>>> people can decide to choose the tradeof themself (ie from i want low
>>>>>> latency io-uring and i don't care wether mm can not do its business; to
>>>>>> i want mm to never be impeded in its business and i accept the extra
>>>>>> latency burst i might face in io operations).
>>>>>
>>>>> I do not think it is a good idea to make this configurable. How can
>>>>> people sensibly choose between the two without deep understanding of
>>>>> internals?
>>>>
>>>> Fully agree, we can't just punt this to a knob and call it good, that's
>>>> a typical fallacy of core changes. And there is only one mode for
>>>> io_uring, and that's consistent low latency. If this change introduces
>>>> weird reclaim, compaction or migration latencies, then that's a
>>>> non-starter as far as I'm concerned.
>>>>
>>>> And what do those two settings even mean? I don't even know, and a user
>>>> sure as hell doesn't either.
>>>>
>>>> io_uring pins two types of pages - registered buffers, these are used
>>>> for actual IO, and the rings themselves. The rings are not used for IO,
>>>> just used to communicate between the application and the kernel.
>>>
>>> So, do we still want to solve file back pages write back if page in
>>> ubuffer are from a file ?
>>
>> That's not currently a concern for io_uring, as it disallows file backed
>> pages for the IO buffers that are being registered.
>>
>>> Also we can introduce a flag when registering buffer that allows to
>>> register buffer without pining and thus avoid the RLIMIT_MEMLOCK at
>>> the cost of possible latency spike. Then user registering the buffer
>>> knows what he gets.
>>
>> That may be fine for others users, but I don't think it'll apply
>> to io_uring. I can't see anyone selecting that flag, unless you're
>> doing something funky where you're registering a substantial amount
>> of the system memory for IO buffers. And I don't think that's going
>> to be a super valid use case...
> 
> Given dataset are getting bigger and bigger i would assume that we
> will have people who want to use io-uring with large buffer.
> 
>>
>>> Maybe it would be good to test, it might stay in the noise, then it
>>> might be a good thing to do. Also they are strategy to avoid latency
>>> spike for instance we can block/force skip mm invalidation if buffer
>>> has pending/running io in the ring ie only have buffer invalidation
>>> happens when there is no pending/running submission entry.
>>
>> Would that really work? The buffer could very well be idle right when
>> you check, but wanting to do IO the instant you decide you can do
>> background work on it. Additionally, that would require accounting
>> on when the buffers are inflight, which is exactly the kind of
>> overhead we're trying to avoid to begin with.
>>
>>> We can also pick what kind of invalidation we allow (compaction,
>>> migration, ...) and thus limit the scope and likelyhood of
>>> invalidation.
>>
>> I think it'd be useful to try and understand the use case first.
>> If we're pinning a small percentage of the system memory, do we
>> really care at all? Isn't it completely fine to just ignore?
> 
> My main motivation is migration in NUMA system, if the process that
> did register buffer get migrated to a different node then it might
> actualy end up with bad performance because its io buffer are still
> on hold node. I am not sure we want to tell application developer to
> constantly monitor which node they are on and to re-register buffer
> after process migration to allow for memory migration.

If the process truly cares, would it not have pinned itself to that
node?

-- 
Jens Axboe