ODP is slower than regular RDMA even with memory copy overhead
Example numbers:
- 3950000 random read iops without ODP vs 240000 iops with ODP
- 1447000 random write iops without ODP vs 101000 iops with ODP
Reference: https://tkygtr6.github.io/pub/ISPASS21_slides.pdf
Build problems fixed:
- void* pointer arithmetic which is a GNU extension (works as byte*)
- "variable size object may not be initialized" which is OK under GCC
- nullptr_t related error in json11 (it lacks 'operator <' in clang)
Warnings fixed:
- empty nested struct initializer { 0 } replaced by {}
- removed several unused lambda captures
This is the simplest and, as usual, the best implementation :)
100% zero-copy implementation is also possible (see rdma-zerocopy branch),
but it requires to create A LOT of queues (~128 per client) to use QPN as a 'tag'
because of the lack of receive tags and the server may simply run out of queues.
Hardware limit is 262144 on Mellanox ConnectX-4 which amounts to only 2048
'connections' per host. And even with that amount of queues it's still less optimal
than the non-zerocopy one.
In fact, newest hardware like Mellanox ConnectX-5 does have Tag Matching
support, but it's still unsuitable for us because it doesn't support scatter/gather
(tm_caps.max_sge=1).
Basic naive implementation works, but it's highly non-optimal as
RNR retransmissions occur all the time. RDMA expects the receiver
to always have place for incoming WRs...
Vitaliy Filippov