There are two things I see that reduce the viability of ref-count GCs with Clojure:
a) Clojure is insanely alloc heavy. Look at the source of the data structures, merging two hash-maps (for instance) requires about 2-3 allocations per assoc, per kv in the merged map: (merge map1 map2) allocs = ~((tree-depth of map1) * (count map2) b) Every time you transverse a persistent structure's tree you'll need to increment and decrement the refcounts. Consider multi-core, now the refcounts have to be thread safe (atomic inc & dec). So if you have two cores reading (not just writing) from the same hash-map, they'll be fighting over the cache lines that the refcount is stored in, and that'll kill performance. Timothy On Thu, May 30, 2013 at 8:47 AM, Gary Trakhman <gary.trakh...@gmail.com>wrote: > At first glance, those issues seem like they could be mitigated, so I > think I see room here for some real-time ref-counted clojure. I imagine > it'd still have a lot of allocations so it wouldn't be that great for > low-memory systems. > > > On Thu, May 30, 2013 at 9:44 AM, Jean Niklas L'orange < > jeann...@hypirion.com> wrote: > >> >> >> On Thursday, May 30, 2013 2:21:36 PM UTC+2, Gary Trakhman wrote: >>> >>> I just thought about this recently, but does the value-oriented nature >>> of clojure mostly void the need for a cycles-aware GC? It seems like you >>> won't ever have cycles without identities, or pointers (java references). >>> Maybe this would be a problem only when you need identities, ie deftype or >>> defprotocol implementing objects. >>> >> >> Sure thing, the value-oriented nature removes a lot of cycles in >> practice. However, you may for instance have an atom which contain itself, >> so they are not nonexistant. As such, the GC cannot be completely >> cycle-ignorant, but perhaps it doesn't have to be efficient at finding them >> either. >> >> Another place where cycles happen are in (mutually) recursive functions, >> they may be iffy if you define many recursive anonymous functions at >> runtime. >> >> -- Jean Niklas L'orange >> >> -- >> -- >> You received this message because you are subscribed to the Google >> Groups "Clojure" group. >> To post to this group, send email to clojure@googlegroups.com >> Note that posts from new members are moderated - please be patient with >> your first post. >> To unsubscribe from this group, send email to >> clojure+unsubscr...@googlegroups.com >> For more options, visit this group at >> http://groups.google.com/group/clojure?hl=en >> --- >> You received this message because you are subscribed to the Google Groups >> "Clojure" group. >> To unsubscribe from this group and stop receiving emails from it, send an >> email to clojure+unsubscr...@googlegroups.com. >> For more options, visit https://groups.google.com/groups/opt_out. >> >> >> > > -- > -- > You received this message because you are subscribed to the Google > Groups "Clojure" group. > To post to this group, send email to clojure@googlegroups.com > Note that posts from new members are moderated - please be patient with > your first post. > To unsubscribe from this group, send email to > clojure+unsubscr...@googlegroups.com > For more options, visit this group at > http://groups.google.com/group/clojure?hl=en > --- > You received this message because you are subscribed to the Google Groups > "Clojure" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to clojure+unsubscr...@googlegroups.com. > For more options, visit https://groups.google.com/groups/opt_out. > > > -- “One of the main causes of the fall of the Roman Empire was that–lacking zero–they had no way to indicate successful termination of their C programs.” (Robert Firth) -- -- You received this message because you are subscribed to the Google Groups "Clojure" group. To post to this group, send email to clojure@googlegroups.com Note that posts from new members are moderated - please be patient with your first post. To unsubscribe from this group, send email to clojure+unsubscr...@googlegroups.com For more options, visit this group at http://groups.google.com/group/clojure?hl=en --- You received this message because you are subscribed to the Google Groups "Clojure" group. To unsubscribe from this group and stop receiving emails from it, send an email to clojure+unsubscr...@googlegroups.com. For more options, visit https://groups.google.com/groups/opt_out.
