Re: gc notes available

[Mikael Djurfeldt <mdj@nada.kth.se>]

hjstein@bfr.co.il (Harvey J. Stein) writes:

>  > > 4. I wrote a tetris game for STk.  It has a non generational
>  > >    conservative GC.  The pauses are unnoticable because I increased
>  > >    the heap allocation at the beginning & call gc at times when it
>  > >    won't be noticable to the user.
>  > 
>  > Well, what can I say.  I have a GUI for my simulator written in Guile
>  > and it doesn't feel nice to interact with due to the irregular short
>  > pauses.  But then I didn't change heap allocation or call gc.
> 
> How are you doing the GUI?  Using the guile interface to GTk?

I use gtcltk.  This means that there is a lot of scheme code executed,
which makes it a good test case.

> Is it basically an animation sort of issue - you want things to move
> smoothly around the screen & gc interrupts it?

No, not in that case.  It's a toolbar and a canvas where you can place
objects and connect them to eachother (MacDraw:ish).

> It relieves the user from memory management,

This is what I meant.

> How can a few (gc) calls in the code have a bad influence on program
> structure?  I think that's overblown.

It all depends on where you need to do them in order to avoid the
all-generation scans, doesn't it?

If you write a tetris game, placing the gc call directly after
generation of a new piece doesn't destroy code structure.

But where, for example, should you put the gc call in a GUI where the
flow of control is guided by the user's actions?  Should I then put it
at the top of every call-back?  No, that would be too heavy since
certain types of callbacks are invoked too often (like when dragging
an object in the canvas).  It's probably only certain call-backs which
need it, and then, maybe some of them need it conditionally upon what
the user does... etc etc.

>  > I haven't claimed that waiting for GC is a big deal in general.  I
>  > claim that it is a big deal when you're writing GUI code in Scheme.
> 
> I'd contend it's not GUI code, but animation.

It seems like both Russ and I have GUI:s which don't do animations but
which still have noticeable, irregular GC pauses.

>  > Agreed.  I want a behaviour "smooth" enough not to have to worry about
>  > GC when writing GUI:s.
> 
> The only way you'll get this is with a gc running in a separate thread.

No.  You can also get it in at least two more cases: 1. collecting
cells incrementally, either cell by cell, or in small groups.,
2. doing gengc but not collecting all heap blocks during one gc pause.

/mdj