What is the mutator?

[Malcolm Wallace]

>  Say you are
>  implementing a network server, for example -- you don't want
>  to have big spikes in the request latency due to GC.
>
>    We think
>    concurrent GC is unlikely to be practical in the Haskell
>    setting, due to the extra synchronisation needed in the
>    mutator.
>     -- Simon Marlow

It is perfectly possible to do real-time concurrent garbage collection  
for Haskell, in an incremental fashion that guarantees a small maximum  
bound on each packet of GC work.  The tradeoff is that the percentage  
of time devoted to GC in total is much greater, and the mutator must  
do more work to co-operate with the GC.  In other words, the program  
runs slower.  This tradeoff is the same for all real-time garbage  
collection schemes as far as I am aware, in any language - either you  
can have responsiveness, or you can have better overall application  
speed, but you cannot have both.

>  So I wonder, to what degree is GC latency controllable in
>  Haskell? It seems that, pending further research, we can not
>  hope for concurrent GC.

There have been several papers on real-time GC in Haskell (including  
one of my own).  There is no technical problem, only performance  
worries.  This is what I think Simon means by "unlikely to be  
practical".

Regards,
     Malcolm