On Mon, Jul 12, 2010 at 1:53 PM, Edward Z. Yang
Excerpts from Evan Laforge's message of Mon Jul 12 16:43:45 -0400 2010:
Yeah, that's definitely the safest and simplest. But the copying defeats the purpose of passing a pointer in the first place, which was to not have to copy the giant array just to pass it.
Well, if your C code wasn't squirreling away the pointer, you could have avoided the copy. Memory copies in C are suprisingly cheap; I'm really surprised how often I see memcpy() in high performance code (though, it may be the case that the details of the algorithm are a much more important influence on performance.)
From some quick tests, copying 32 32mb chunks takes 0.5 seconds, which is too much lag for interactive use, but only one is only around 0.01 seconds, which is not a problem. At the moment, I expect 32mb will be an upper bound, so I think I can get away with copying for now.
However, I'm still curious about the funptr approach. Does a
"wrapper" funptr callback act as a GC root?
On Mon, Jul 12, 2010 at 4:26 PM, Felipe Lessa
... I would try to avoid having the MVar/IOVar as a CAF because of its unpredictability. You can always encapsulate it as somewhere else (e.g. Reader monad).
Yes, I would prefer a non-horrible hack too, but it's tricky and invasive to try to get a callback from C to haskell into the main haskell state monad. I would have to send a "delete this key" msg which the main loop will later pick up, which is certainly doable, but... invasive.