One of the less obvious things that has plagued Pcsx2's compatibility over the years is its event handling system. The system in place as of 0.9.6 is adequate for interpreter-based emulation but is not well-equipped to handle the methods that a recompiler uses to manage cpu cycle updates. This is something we aim to fix in the coming weeks.
Cycle-based Timing Explained
All cpus have a cycle rate, which is typically the Mhz/Ghz values you're most familiar with when talking about any cpu. An i7 clocked at 2.83ghz has a 2.83ghz cycle rate. Now, the actual throughput of instructions can vary greatly since each cycle of the cpu consists of several stages
and multiple piplines, each of which can have dependency stalls and has varying rules for when such stalls occur. The cycle rate, however, is always 2.83ghz. Because cycle rates are a known constant, they make a good barometer for synchronizing the activities of a multi-processor design like the Playstation 2.
Why do Recompilers Complicate Event Testing?
Recompilers work as a significant speedup over interpreters by doing two things:
- Recompile the machine code of a emulated CPU (in our case MIPS instructions) into code native to the host machine (ix86 instructions).
- Prefetch and pre-decode emulated instructions, and inline them into blocks.
The thing recompilers are most well-known for -- recompiling to native machine code -- is actually the less effective of the two things recompilers do for speeding up emulation. The primary speedup typically comes from the prefetching and inlining of instructions, which in addition to eliminating the instruction fetch/decode stage (by far the slowest part of any interpreter), also allows for cross-instruction optimizations such as constant propagation and register caching/mapping. In other words, a recompiler is effectively executing emulated instructions in pre-compiled bursts
. This is so important to performance that a recompiler without block-level execution would hardly be any faster than an interpreter.
As part of the design of block-level execution, the recompiled code only updates cpu cycle counts and tests for scheduled events at block boundaries. Blocks typically span 5 to 35 cycles, but in some cases can span a hundred cycles or more. When the subsequent Event Test is performed, several scheduled events may be pending execution. This is where problems can occur: The current event system implemented into Pcsx2 executes all pending events in no particular order, leading to events being executed out-of-order when multiple events time-out during a single block. Typically most events don't have dependencies on each other, or games don't use them in a way that execution order matters. But sometimes they do, and in those cases behavior can be unpredictable, or can cause the game to fail outright. To make matters worse, the pending events typically don't know how late they are, and will re-schedule subsequent events in increasingly belated fashion. The current implementation of EE and IOP counters have tons of complicated code meant to compensate for this limitation (both slow and were nearly impossible to get right).
The fix for this is to use an event system I'll call decremental delta time.
It has three advantages:
- Makes it easy to execute events in scheduled order regardless of the amount of time which has passed since the last Event Test.
- Maintains relative cycle scheduling at a high level so that none of the events being re-scheduled "lose time" due to belated block-boundary event testing.
- Simplifies event handling on all levels, and provides significant speedups for event testing and event dispatching.
It's hard to know beforehand just how beneficial in-order execution of events will be. I'm anticipating that it might actually fix a few emulation problems on the IOP recompiler in particular, since it has a slow cycle rate and also has a handful of events which can have potential inter-dependencies. For that reason I'll be implementing the system first into the IOP, and then when all the chinks in its armor are worked free we'll port the EE side of the emulator over to it.