Shared memory between two primary DPDK processes

[Antonio Di Bacco]

Il giorno ven 8 apr 2022 alle ore 15:26 Dmitry Kozlyuk <
dmitry.kozliuk at gmail.com> ha scritto:

> 2022-04-08 14:31 (UTC+0200), Antonio Di Bacco:
> > I know that it is possible to share memory between a primary and
> secondary
> > process using rte_memzone_reserve_aligned to allocate memory in primary
> > that is "seen" also by the secondary. If we have two primary processes
> > (started with different file-prefix) the same approach is not feasible. I
> > wonder how to share a chunk of memory hosted on a hugepage between two
> > primaries.
> >
> > Regards.
>
> Hi Antonio,
>
> Correction: all hugepages allocated by DPDK are shared
> between primary and secondary processes, not only memzones.
>
> I assume we're talking about processes within one host,
> because your previous similar question was about sharing memory between
> hosts
> (as we have discussed offline), which is out of scope for DPDK.
>
> As for the question directly, you need to map the same part of the same
> file
> in the second primary as the hugepage is mapped from in the first primary.
> I don't recommend to work with file paths, because their management
> is not straightforward (--single-file-segments, for one) and is
> undocumented.
>
> There is a way to share DPDK memory segment file descriptors.
> Although public, this DPDK API is dangerous in the sense that you must
> clearly understand what you're doing and how DPDK works.
> Hence the question: what is the task you need this sharing for?
> Maybe there is a simpler way.
>
> 1. In the first primary:
>
>         mz = rte_memzone_reserve()
>         ms = rte_mem_virt2memseg(mz->addr)
>         fd = rte_memseg_get_fd(ms)
>         offset = rte_memseg_get_fd_offset(ms)
>
> 2. Use Unix domain sockets with SCM_RIGHTS
>    to send "fd" and "offset" to the second primary.
>
> 3. In the second primary, after receiving "fd" and "offset":
>
>         flags = MAP_SHARED | MAP_HUGE | (30 << MAP_HUGE_SHIFT)
>         addr = mmap(fd, offset, flags)
>
> Note that "mz" may consist of multiple "ms" depending on the sizes
> of the zone and hugepages, and on the zone alignment.
> Also "addr" may (and probably will) differ from "mz->addr".
> It is possible to pass "mz->addr" and try to force it,
> like DPDK does for primary/secondary.
>


Thank you Dmitry, it is really incredible how deep your knowledge is. I
will give it a try.
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