stsievert / pytorch_ps_mpi

Why?

Features

It fits for our problem constraints:

• models fit on one device
• communication is reliable
• only one PS required
• have MPI available.

Additionally, this implementation of a PS

• uses MPI, a declarative standard common in clusters.
• can allow compression if concerned about bandwidth.

Limitations

or, why use other PS systems? You may...

• want multiple PSs.
• have models that may not fit on one machine.
• not want to use MPI.

Why not torch.distributed?

• torch.distributed only sends tensors. We may want to send generic Python objects.
• torch.distributed does not have any Igatherv, which is what we want for a PS.

Notes

• MPI_Gather requires the recv count of any particular message.
  1. Send sizes before, allocate largest size (and then trim down per size)
  2. (when recieinv from any source)
• Tensorflow does... (see tensorflow's docs for ConditionalAccumulator)
  1. make sure gradients are current
  2. waits until $n$ gradients have been received from any number of workers

plan

Implement a parameter server with mpi4py

0. Call encode
1. Convert to pickle object
2. (optional) compress
3. Allocate largest size expected + some padding (to see if overflowed)
4. Convert to NumPy buffer, use Igather
5. Convert back to pickle object when needed
6. Send dictionary to decode

Notes

• An MPI enabled parameter server will not fit models that cannot fit on every machine.
• Though we can safely (?) assume that data for every machine is costant

Resources

• Good summary of parameter servers at http://hunch.net/?p=151364

Async PS psuedo-code

• This is algorithm AsySG-InCon (for inconsistent reads) in [1]
• [1]:Asynchronous parallel stochastic gradient for nonconvex optimization, https://arxiv.org/abs/1506.08272

# optimizaiton step function
irequest_params()
for p in params:
    if rank == 0:
        comms = []
        while True:
            comms += [recv(MPI.ANY_SOURCE)]
            if len(comms) == 32:
                break
        params = [receive(comm) for comm in comms]
        p = sum(params)
        step()
    else:
        send(param, 0)
    req = ibcast(p.data)

It's easy to impelment inconsistent reads with ibcast. It's harder to do consistent reads; we need to do a buffered broadcast to allow workers to continue to compute gradients.

Iallgatherv

I tried playing with Iallgatherv. Some problems:

• Does not support sending objects of unknown sizes. Every rank needs to know the size of every object it's receiving.
• Sending the objects of these types involves sending a tuple as shown in the implementation and the tests