Re: [Boost-users] [msm] Why does it take so many bytes to represent a state machine?

Subject: Re: [Boost-users] [msm] Why does it take so many bytes to represent a state machine?
From: Christophe Henry (christophe.j.henry_at_[hidden])
Date: 2012-11-13 15:52:33

>Hi, 

>
>I'm using msm to store the state of many small objects. Each one is around 24 bytes on its own. However, I just found out 

>from experimentation that a MSM state machine takes many bytes to store. For the following program, the print out is: 

>entering: State1 

>104 

>48 

>This indicates that the very simple one-state, no-transition, machine is 104 bytes, and takes 48 bytes even to serialized. 

>The front::state_machine_def, however, is only 1 byte, though I suspect thet real state is stored only 

>in the 104-byte back::state_machine. 

Hi Josh,

It depends on several factors:

- MSM creates all the states at state machine creation and the states live until the state machine is destroyed, so even if the states have no data, it'll be a byte per state.

- you need to keep the id of the current state in each region (int)

- 2 bool for internal implementation details

- a queue for deferred events if you have some in your fsm (if none, MSM will deactivate this, but you'll still pay 1 byte).

- an event queue (enabled by default but you can deactivate it if not needed (see http://svn.boost.org/svn/boost/trunk/libs/msm/doc/HTML/ch03s02.html#d0e1139) but it will still cost 1 byte.

Both queues are per default a std::deque, which can be expensive on some systems. You can choose another container if you wish (see http://svn.boost.org/svn/boost/trunk/libs/msm/doc/HTML/ch03s05.html#d0e2680).
As you see, if you remove the queues, there is not much more to take away.

Also note that a text archive brings its own costs.

HTH,

Christophe