kvstore.md

KVStore API

Topics:

• Basic Push and Pull
• List Key-Value Pairs

Basic Push and Pull

Provides basic operation over multiple devices (GPUs) on a single device.

Initialization

Let's consider a simple example. It initializes a (int, NDArray) pair into the store, and then pulls the value out.

pdl> $kv = mx->kv->create('local')
pdl> $shape = [2,3]
pdl> $kv->init(3, mx->nd->ones($shape)*2)
pdl> $a = mx->nd->zeros($shape)
pdl> $kv->pull(3, out => $a)
pdl> print $a->aspdl
[
 [2 2 2]
 [2 2 2]
]

Push, Aggregation, and Updater

For any key that's been initialized, you can push a new value with the same shape to the key, as follows:

pdl> $kv->push(3, mx->nd->ones($shape)*8)
pdl> $a = mx->nd->zeros($shape)
pdl> $kv->pull(3, out => $a)
pdl> print $a->aspdl
[
 [8 8 8]
 [8 8 8]
]

The data that you want to push can be stored on any device. Furthermore, you can push multiple values into the same key, where KVStore first sums all of these values, and then you pull the aggregated value, as follows:

pdl> $kv->push(3, [mx->nd->ones($shape, ctx=>mx->cpu(0)), mx->nd->ones($shape, ctx=>mx->cpu(1))])
pdl> $kv->pull(3, out => $a)
pdl> print $a->aspdl
[
 [2 2 2]
 [2 2 2]
]

For each push command, KVStore applies the pushed value to the value stored by an updater. The default updater is ASSIGN. You can replace the default to control how data is merged.

pdl> $updater = sub { my ($key, $input, $stored) = @_; print "update on key: $key\n"; $stored += $input * 3; }
pdl> $kv->_set_updater($updater)
pdl> $kv->push(3, [mx->nd->ones($shape, ctx=>mx->cpu(0)), mx->nd->ones($shape, ctx=>mx->cpu(1))])
update on key: 3
pdl> $kv->pull(3, out => $a)
pdl> print $a->aspdl
[
 [8 8 8]
 [8 8 8]
]

Pull

You've already seen how to pull a single key-value pair. Similar to the way that you use the push command, you can pull the value into several devices with a single call.

pdl> $b = [mx->nd->zeros($shape, ctx=>mx->cpu(0)), mx->nd->zeros($shape, ctx=>mx->cpu(1))]
pdl> $kv->pull(3, out => $b)
pdl> print $b->[1]->aspdl
[
 [8 8 8]
 [8 8 8]
]

List Key-Value Pairs

All of the operations that we've discussed so far are performed on a single key. KVStore also provides the interface for generating a list of key-value pairs. For a single device, use the following:

pdl> $keys = [5,7,9]
pdl> $kv->init($keys, [map { mx->nd->ones($shape) } 0..@$keys-1])
pdl> $kv->push($keys, [map { mx->nd->ones($shape) } 0..@$keys-1])
update on key: 5
update on key: 7
update on key: 9
pdl> $b = [map { mx->nd->ones($shape) } 0..@$keys-1]
pdl> $kv->pull($keys, out => $b)
pdl> print $b->[1]->aspdl
[
 [4 4 4]
 [4 4 4]
]