The Julia Klara package provides a generic engine for Markov Chain Monte Carlo (MCMC) inference.
Klara has undergone a major upgrade. Some of its recent changes include:
- Models are represented internally by graphs.
- Memory allocation and garbage collection have been reduced by using mutating functions associated with targets.
- It is possible to select storing output in memory or in file at runtime.
- Automatic differentiation is available allowing to choose between forward mode and reverse mode (the latter relying on source transformation).
Some of the old code has not been fully ported. The full porting of old functionality, as well as further developments, will be completed shortly. Progress is being tracked systematically via issues and milestones.
The documentation is out of date, but will be brought up-to-date fairly soon. In the meantime, this README file provides a few examples of the new interface, explaining how to get up to speed with the new face of Klara. More examples can be found in doc/examples.
using Klara
### Define the log-target as a function (generic or anonymous):
plogtarget(z::Vector{Float64}) = -dot(z, z)
### Define the parameter via BasicContMuvParameter (it is a continuous multivariate variable)
### The input arguments for BasicContMuvParameter are:
### 1) the variable key,
### 2) the log-target
p = BasicContMuvParameter(:p, logtarget=plogtarget)
### Define the model using the likelihood_model generator
### The second argument informs the likelihood_model generator that p.index has not been set
model = likelihood_model(p, false)
### Define a Metropolis-Hastings sampler with an identity covariance matrix
sampler = MH(ones(2))
### Set MCMC sampling range
mcrange = BasicMCRange(nsteps=10000, burnin=1000)
### Set initial values for simulation
v0 = Dict(:p=>[5.1, -0.9])
### Specify job to be run
job = BasicMCJob(model, sampler, mcrange, v0)
### Run the simulation
run(job)
### Get simulated values
chain = output(job)
chain.value
### Check that the simulated values are close to the zero-mean target
mean(chain)To reset the job, using a new initial value for the targeted parameter, run
reset(job, [3.2, 9.4])
run(job)
chain = output(job)To see how the acceptance rate changes during burnin, set the vanilla tuner in verbose mode
job = BasicMCJob(model, sampler, mcrange, v0, tuner=VanillaMCTuner(verbose=true))
run(job)
chain = output(job)If apart from the simulated chain you also want to store the log-target, then pass an additional dictionary to the job to
specify the output options. In particular, the :monitor key indicates which items will be monitored. In the example below,
both :value and :logtarget will be monitored, referring to the chain and log-target respectively. These can then be
accessed by the corresponding fields chain.value and chain.logtarget:
outopts = Dict{Symbol, Any}(:monitor=>[:value, :logtarget])
job = BasicMCJob(model, sampler, mcrange, v0, tuner=VanillaMCTuner(verbose=true), outopts=outopts)
run(job)
chain = output(job)
chain.logtargetThe acceptance ratio diagnostics can be stgored via the :diagnostics=>[:accept] entry of outopts:
outopts = Dict{Symbol, Any}(:monitor=>[:value, :logtarget], :diagnostics=>[:accept])
job = BasicMCJob(model, sampler, mcrange, v0, tuner=VanillaMCTuner(verbose=true), outopts=outopts)
run(job)
chain = output(job)Instead of saving the output in memory, it can be written in file via the output option :destination=>:iostream:
outopts = Dict{Symbol, Any}(
:monitor=>[:value, :logtarget],
:diagnostics=>[:accept],
:destination=>:iostream
)
job = BasicMCJob(model, sampler, mcrange, v0, tuner=VanillaMCTuner(verbose=true), outopts=outopts)
run(job)The chain, log-target and acceptance ratio diagnostics of the above example are stored in the respective CSV files
"value.csv", "logtarget.csv" and "diagnosticvalues.csv" of the current directory. To save the output in another directory,
use the :filepath=>"myfullpath.csv", where "myfullpath.csv" is substituted by the full path of your choice:
outopts = Dict{Symbol, Any}(
:monitor=>[:value, :logtarget],
:diagnostics=>[:accept],
:destination=>:iostream,
:filepath=>"/Users/theodore/workspace/julia"
)
job = BasicMCJob(model, sampler, mcrange, v0, tuner=VanillaMCTuner(verbose=true), outopts=outopts)
run(job)To use Julia tasks for running the job, set plain=false:
outopts = Dict{Symbol, Any}(:monitor=>[:value, :logtarget], :diagnostics=>[:accept])
job = BasicMCJob(model, sampler, mcrange, v0, tuner=VanillaMCTuner(verbose=true), outopts=outopts, plain=false)
run(job)
chain = output(job)Task-based jobs can also be reset:
reset(job, [-2.8, 3.4])
run(job)
chain = output(job)To run a sampler which requires the gradient of the log-target, such as MALA, try
using Klara
plogtarget(z::Vector{Float64}) = -dot(z, z)
pgradlogtarget(z::Vector{Float64}) = -2*z
p = BasicContMuvParameter(:p, logtarget=plogtarget, gradlogtarget=pgradlogtarget)
model = likelihood_model(p, false)
### Set driftstep to 0.9
sampler = MALA(0.9)
mcrange = BasicMCRange(nsteps=10000, burnin=1000)
v0 = Dict(:p=>[5.1, -0.9])
### Save grad-log-target along with the chain (value and log-target)
outopts = Dict{Symbol, Any}(:monitor=>[:value, :logtarget, :gradlogtarget], :diagnostics=>[:accept])
job = BasicMCJob(model, sampler, mcrange, v0, tuner=VanillaMCTuner(verbose=true), outopts=outopts)
run(job)
chain = output(job)
chain.gradlogtarget
mean(chain)To adapt the MALA drift step empirically during burnin towards an intended acceptance rate of 60%, run
job = BasicMCJob(
model,
sampler,
mcrange,
v0,
tuner=AcceptanceRateMCTuner(0.6, verbose=true),
outopts=outopts
)
run(job)
chain = output(job)The examples below demonstrates how to run MCMC using automatic differentiation (AD).
To use forward mode AD, try the following:
using Klara
plogtarget(z::Vector) = -dot(z, z)
p = BasicContMuvParameter(:p, logtarget=plogtarget, autodiff=:forward)
model = likelihood_model(p, false)
sampler = MALA(0.9)
mcrange = BasicMCRange(nsteps=10000, burnin=1000)
v0 = Dict(:p=>[5.1, -0.9])
outopts = Dict{Symbol, Any}(:monitor=>[:value, :logtarget, :gradlogtarget], :diagnostics=>[:accept])
job = BasicMCJob(model, sampler, mcrange, v0, tuner=VanillaMCTuner(verbose=true), outopts=outopts)
run(job)
chain = output(job)Note that plogtarget takes an argument of type Vector instead of Vector{Float64}, as required by the ForwardDiff
package. Furthermore, notice that in the definition of parameter p, the gradient of its log-target is not provided
explicitly; instead, the optional argument autodiff=:forward enables computing the gradient via forward mode AD.
To employ reverse mode AD, try
using Klara
plogtarget(z::Vector) = -dot(z, z)
p = BasicContMuvParameter(:p, logtarget=plogtarget, autodiff=:reverse, init=[(:z, ones(2))])
model = likelihood_model(p, false)
sampler = MALA(0.9)
mcrange = BasicMCRange(nsteps=10000, burnin=1000)
v0 = Dict(:p=>[5.1, -0.9])
outopts = Dict{Symbol, Any}(:monitor=>[:value, :logtarget, :gradlogtarget], :diagnostics=>[:accept])
job = BasicMCJob(model, sampler, mcrange, v0, tuner=VanillaMCTuner(verbose=true), outopts=outopts)
run(job)
chain = output(job)In this case the optional argument autodiff=:reverse enables computing the gradient via reverse mode AD using source
transformation. Notice also the init=[(:z, ones(2))] optional argument, which allows passing the required input to the
init optional argument of rdiff() of the ReverseDiffSource package. The :z symbol in the init argument refers to the
symbol used as the input argument of plogtarget.
Finally, it is possible to run reverse mode AD by passing an expression for the log-target (or log-likelihood or log-prior) instead of a function. An example follows where the log-target is specified via an expression:
using Klara
p = BasicContMuvParameter(:p, logtarget=:(-dot(z, z)), autodiff=:reverse, init=[(:z, ones(2))])
model = likelihood_model(p, false)
sampler = MALA(0.9)
mcrange = BasicMCRange(nsteps=10000, burnin=1000)
v0 = Dict(:p=>[5.1, -0.9])
outopts = Dict{Symbol, Any}(:monitor=>[:value, :logtarget, :gradlogtarget], :diagnostics=>[:accept])
job = BasicMCJob(model, sampler, mcrange, v0, tuner=VanillaMCTuner(verbose=true), outopts=outopts)
run(job)
chain = output(job)The user guide is currently being written up.
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