Asynchronous Simulation

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Asynchronous Simulation

Asynchronous simulation (ASIM) allows the game physics simulation to run alongside the 3D graphics rendering on a dual core CPU or better. This gives higher performance, and allows the engine to maintain a high rendering frame rate (FPS) at all times even in big battles.

Spring disables ASIM by default, unless a game has been specifically designed to support it. The reason is that ASIM gives rise to Lua related side effects that can be a bit hard to understand, especially for a game designer who has little experience with threaded programming.

Steps required to support ASIM

1. Choose a Lua Threading Model > 2 below

2. Rewrite the Lua code to conform with the rules imposed by that threading model

3. Learn about the side effects of the model chosen and tweak your code to work around it

Lua Threading Models

These modes can be set for a game using luaThreadingModel in Modrules.lua. The engine can override with the setting MultiThreadLua. A higher mode generally means that the game will perform better, with less side effects, but also requires more work. A game that specifies luaThreadingModel > 2 will be considered ASIM compatible, and will have ASIM enabled by default.

0. No MT, single threaded

For reference only, this mode should never be used in practice

1. Single state

This mode has a single Lua environment for everything. It will cause a significant slowdown because simulation and rendering threads block each other trying to access the shared Lua environment. Especially problematic is Lua rendering; being inherently time consuming (looping through large amounts of units etc.) it will force the simulation thread to wait. This gives rise to the classic phenomenon of having incredibly high FPS but still lagging behind (high ping) since simulation cannot keep up with the normal pace.

2. Single state, batching of unsynced events (default mode)

This mode is considerably better than mode(1) since it has a separate Lua environment for LuaUI.

All simulation events sent to LuaUI are also batched/delayed to reduce the need for the threads to block each other. For instance, if simulation triggers UnitMoved(), the event will not reach LuaUI right away, but is put in a batch and handled as soon as possible by the rendering thread. The batching system is managed, i.e. if the unit that triggered UnitMoved is about to be deleted (and thus invalidated) the batch is forcefully run before deletion occurs. Because the forced batch run is performed by the simulation thread, locking is required and this can degrade simulation performance for the same reasons as in mode(1).

The other means of unsynced communication, SendToUnsynced and Script.LuaUI.XXX() are also batched and managed to make sure any object ID or such sent are still valid once the message arrives. Note: Script.LuaUI.XXX() does because of the batching NOT have a return value.

This mode unfortunately suffers the same problems as mode(1) for rendering gadgets, since simulation events must be sent to the gadgets directly (without batching) in order not to desync the game.

With this mode, it is illegal to invoke LuaUI from another Lua environment (LuaGaia or LuaRules). This can happen depending on what call-ins the game and any user-enabled widgets implement. If such an attempt to invoke LuaUI is detected, it will be skipped to prevent a deadlock, and a warning will also be printed in the console. Unfortunately there is no other solution than to stop using the call-in that throws the error, or to use a higher threading model.

3. Dual states for synced, batching of unsynced events, synced/unsynced communication

via EXPORT table and SendToUnsynced

Same as mode(2), but the gadgets have separate Lua environments depending on whether the simulation or rendering thread is invoking it. This alleviates the negative performance impact for rendering gadgets, but makes it impossible to directly share synced gadget data to the unsynced part via _G --> SYNCED. To work around this limitation, this mode introduces _G.EXPORT that is automatically copied to SYNCED.EXPORT. Only small amounts of data should be stored in the EXPORT table, because the copying is performed very frequently. Cyclical and/or very deeply nested tables are not allowed. Larger data can be stored temporarily, e.g.

_G.EXPORT.myhugetable = huge_table

SendToUnsynced("huge_table_available")

_G.EXPORT.myhugetable = nil

4. Dual states for synced, batching of unsynced events, synced/unsynced communication

via SendToUnsynced only

Same as mode(3) but with the EXPORT table disabled to increase the performance further. You don't need EXPORT, SendToUnsynced alone is sufficient.

5. Dual states for all, all synced/unsynced communication (widgets included)

via SendToUnsynced only

Same as mode(4), but adds separate Lua environments for LuaUI depending on whether the simulation or rendering thread is invoking it. With dual LuaUI states, batching is no longer needed and therefore disabled. This makes it even faster, since batching overhead is eliminated and forced batch runs (see mode(2)) are not needed. However, this mode adds significant complexity for the programmer, since essentially two instances of the same widget are run, and only the calling thread determines which one is invoked. As a rule of thumb, all simulation events (GameFrame etc.) are invoked by the simulation thread. All UI and rendering events and Update() are invoked by the rendering thread. So, if a variable is modified in GameFrame(), that change cannot be seen in Update(). The widget must instead use SendToUnsynced to communicate any data from the simulation widget instance to the rendering widget instance. Design wise it may be suitable to write the widget similarly to a gadget, with clearly separated "synced" and "unsynced" parts, each with their own variables, functions and call-ins.

6. Dual states for all, all synced/unsynced communication (widgets included)

is unmanaged and via SendToUnsynced only

Same as mode(5) but the SendToUnsynced and Script.LuaUI.XXX() communication is unmanaged. The Lua coder must manually check if object references sent are still valid when the message arrives.

Common side effects

1. Out-of-order execution, applies to all threading models

All rendering call-ins (Draw*, Update, Mouse/Keyboard) must be designed so that they work correctly even if objects are deleted (and possibly replaced by another object with same ID) in between the rendering call-ins. This means rendering code that stores lists of e.g. units in global variables must update these according to UnitCreated/UnitDestroyed events or re-check the units for validity before use. Keep in mind that if an object has been replaced, the operation that you intended to perform may no longer be valid for the new object.

2. Performance hit for rendering call-ins in gadgets, applies to threading models < 3

Avoid writing any time-consuming rendering code in a gadget. The easy way out is to let a widget do the rendering instead. The gadget can send a message to the widget by calling Script.LuaUI.XXX(), but the value returned from this call is undefined if ASIM is enabled. Similarly, the result obtained when checking for the existence of a Script.LuaUI.XXX function can be unpredictable, instead the widget may send a LuaRules message to indicate that the function now exists or has ceased to exist. If the LUA-SYNC-CPU indicator appears in the upper right part of the window it usually indicates a performance problem

3. Batching glitches, applies to threading models 2 - 4

For example, a widget that processes an UnitEnteredLOS event cannot safely make any assumptions about the unit actually being in LOS. Because of batching delay, it may have left LOS already when the message is processed, and an attempt to get further info about the unit may therefore fail, since access to out-of-LOS units is restricted. Numerous events are more or less affected by this type of issue.