What does VU Cycle Stealing do?

[flow.of.soul]

So I've been playing around with one of the latest PCSX2 builds (R1674) and I've noticed that while VU cycle stealing can really help in GPU intenisve games like Dragon Quest 8, it can also really slow down games such as Virtua Fighter 4. So I was wondering, what does it actually do? It is really only good in instances when the GPU in bottlenecking the game?

[Koji]

I find that it's most helpful in that situation but not only helpful in that situation. Basically what cycle steal does is it underclocks the PS2's video hardware. The more you "steal" the lower the clockrate, and thus the easier it is to emulate. But of course doing too much slows down games and can cause some serious graphic glitches.

[Shadow Lady]

It's not good only to GPU limited games, some games get a huge boost from it some others dont and sometimes it slows things (specially using frame limiter) or makes the games buggy. It is really game dependant.

[cottonvibes]

The ps2 has 2 VU (vector units) processors.

The VU cycle stealing hack adds more cycles to the EE processor whenever a VU program is run. This effectively tells the EE that more time has passed.
Its similar to EE cycle hacks, but done when VU programs are run.

[Koji]

Ahhh... thanks for the more in depth explanation. That explains why Cycle hacks + Cycle Steal = very interesting/slow results.