[MLton-user] How to write performant network code

[Wesley W. Terpstra]

I have been working on a network oriented program in SML. Currently we
have two main bottlenecks in our code (according to mlprof and
confirmed via replacing the code with stubs).

The first problem has to do with copying data when assembling a
packet. We've already eliminated as many copies as possible through
careful use of slices, and only one copy happens when finally
construction the (contiguous) packet which is to be sent. This copy is
one of the slowest points in our code, using Word8ArraySlice.copy. The
problem is that Word8ArraySlice.copy does a byte-by-byte copy with a
not very optimized loop. It seems to me that all the
WordXArray[Slice].copy[Vec] functions could just call out to memcpy.
This should be a lot faster. Is there any objection to my preparing a
patch to apply this optimization in the basis?

The second bottleneck comes from hashing network addresses. We need to
identify who sent us a packet and we keep information for each peer in
a hash table. Unfortunately, it's practically impossible to hash the
address as obtained from Socket.recvArrFrom. The only method that
seems to be available to us is to first convert the address to a
string and then to hash that. Unbelievable as it may be, hashing the
address this way is one of the slowest parts to processing the packet!
The address type can't be passed through the FFI itself (to extract
the 32-bit IP+16-bit port) because it is wrapped inside a datatype. So
the only options that seem available to me at present are: 1)
completely replace all networking calls with direct FFI, by-passing
the basis or 2) add some extension in MLton.Socket that lets me get
the address out as a Word8VectorSlice.slice. Any better suggestions?