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

Burakov, Anatoly anatoly.burakov at intel.com
Fri Mar 29 11:33:28 CET 2019

On 29-Mar-19 9:50 AM, David Marchand wrote:
> On Fri, Feb 22, 2019 at 6:12 PM Anatoly Burakov 
> <anatoly.burakov at intel.com <mailto:anatoly.burakov at 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.

> Thanks.
> -- 
> David Marchand


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