Re: [dpdk-dev] [PATCH] eal: add option to not store segment fd's

["Burakov, Anatoly" <anatoly.burakov@intel.com>]

On 29-Mar-19 12:40 PM, Thomas Monjalon wrote:
> 29/03/2019 13:05, Burakov, Anatoly:
>> On 29-Mar-19 11:34 AM, Thomas Monjalon wrote:
>>> 29/03/2019 11:33, Burakov, Anatoly:
>>>> On 29-Mar-19 9:50 AM, David Marchand wrote:
>>>>> On Fri, Feb 22, 2019 at 6:12 PM Anatoly Burakov
>>>>> <anatoly.burakov@intel.com <mailto:anatoly.burakov@intel.com>> wrote:
>>>>>
>>>>>       Due to internal glibc limitations [1], DPDK may exhaust internal
>>>>>       file descriptor limits when using smaller page sizes, which results
>>>>>       in inability to use system calls such as select() by user
>>>>>       applications.
>>>>>
>>>>>       While the problem can be worked around using --single-file-segments
>>>>>       option, it does not work if --legacy-mem mode is also used. Add a
>>>>>       (yet another) EAL flag to disable storing fd's internally. This
>>>>>       will sacrifice compability with Virtio with vhost-backend, but
>>>>>       at least select() and friends will work.
>>>>>
>>>>>       [1] https://mails.dpdk.org/archives/dev/2019-February/124386.html
>>>>>
>>>>>
>>>>> Sorry, I am a bit lost and I never took the time to look in the new
>>>>> memory allocation system.
>>>>> This gives the impression that we are accumulating workarounds, between
>>>>> legacy-mem, single-file-segments, now no-seg-fds.
>>>>
>>>> Yep. I don't like this any more than you do, but i think there are users
>>>> of all of these, so we can't just drop them willy-nilly. My great hope
>>>> was that by now everyone would move on to use VFIO so legacy mem
>>>> wouldn't be needed (the only reason it exists is to provide
>>>> compatibility for use cases where lots of IOVA-contiguous memory is
>>>> required, and VFIO cannot be used), but apparently that is too much to
>>>> ask :/
>>>>
>>>>>
>>>>> Iiuc, everything revolves around the need for per page locks.
>>>>> Can you summarize why we need them?
>>>>
>>>> The short answer is multiprocess. We have to be able to map and unmap
>>>> pages individually, and for that we need to be sure that we can, in
>>>> fact, remove a page because no one else uses it. We also need to store
>>>> fd's because virtio with vhost-user backend needs them to work, because
>>>> it relies on sharing memory between processes using fd's.
>>>
>>> It's a pity adding an option to workaround a limitation of a corner case.
>>> It adds complexity that we will have to support forever,
>>> and it's even not perfect because of vhost.
>>>
>>> Might there be another solution?
>>>
>>
>> If there is one, i'm all ears. I don't see any solutions aside from
>> adding limitations.
>>
>> For example, we could drop the single/multi file segments mode and just
>> make single file segments a default and the only available mode, but
>> this has certain risks because older kernels do not support fallocate()
>> on hugetlbfs.
>>
>> We could further draw a line in the sand, and say that, for example,
>> 19.11 (or 20.11) will not have legacy mem mode, and everyone should use
>> VFIO by now and if you don't it's your own fault.
>>
>> We could also cut down on the number of fd's we use in single-file
>> segments mode by not using locks and simply deleting pages in the
>> primary, but yanking out hugepages from under secondaries' feet makes me
>> feel uneasy, even if technically by the time that happens, they're not
>> supposed to be used anyway. This could mean that the patch is no longer
>> necessary because we don't use that many fd's any more.
> 
> This last option is interesting. Is it realistic?
> 

I can do it in current release cycle, but i'm not sure if it's too late 
to do such changes. I guess it's OK since the validation cycle is just 
starting? I'll throw something together and see if it crashes and burns.

-- 
Thanks,
Anatoly