Go's GC story is evolving. And it's doing so at a fairly quick pace. Judging it based on a current implementation and current choices is premature, I think.
I think that the Go team's *implementation priority* focus on latency is a good one. And unlike others, I don't actually think there is a strong latency vs. throughput tradeoff in the long run. Achieving great latency in GC does not inherently come at the expense of GC throughput or application throughput. Both can one had at the same time. My view is that GC implementations in general (for all runtimes) will move towards good latency + good throughput at the same time at they mature. And Go is no different here. A key architectural choice that has not been completely finalized in Go (AFAICT) has to do with the ability to move objects. It is common to see early GC-based runtime implementations that do not move objects around succumb to an architectural "hole" that creates a reliance on that fact early in the maturity cycles. This usually happens by allowing native code extensions to assume object addresses are fixed (i.e. that objects will not move during their lifetime) and getting stuck with that "assumed quality" in the long run due to practical compatibility needs. From what I can tell, it is not too late for Go to avoid this problem. Go still has precise GC capability (it knows where all the heap references are), and technically can still allow the movement of objects in the heap, even if current GC implementations do not make use of that ability. My hope for Go is that it is not too late to forgo the undesirable architectural limitation of requiring objects to stay at fixed addresses, because much of what I think is ahead for GC in Go may rely on breaking that assumption. [to be clear, supporting various forms of temporary "pinning" of objects is fine, and does not represent a strong architectural limitation.] The ability to move objects is key for two main qualities that future Go GC's (and GCs in pretty much any runtime) will find desirable: 1. The ability to defragment the heap in order to support indefinite execution lengths that do not depend on friendly behavior patterns for object sizes over time. And defragmentation requires moving objects around before they die. Heaps that can compact are dramatically more resilient than ones that don't. Much like GC itself, self-defragmenting heaps allow the relatively easy construction of software that is free to deal with arbitrarily complex data shapes and scenarios. The value of this freedom is often disputed by people who focus on "things that have worked in C/C++ for a long time", but is easily seen in the multitude of application logic that breaks when restricted to using "malloc'ed heap" or "arena allocator" memory management mechanisms. 2. The ability of move objects is fundamental to many of the techniques that support high throughput GC and fast allocation logic. Generational GC is a proven, nearly-universally-used technique that tends to improve GC throughput by at least one order of magnitude [compared to non-generational uses of similar algorithms] across a wide spectrum of application behaviors in many different runtimes. It achieves that by fundamentally focusing some efficient-at-sparse-occupancy algorithm on a known-to-be-sparse logical subset of the heap. And it maintains the qualities needed (tracking the logical subset of the heap, and keeping it sparse) by using object relocation (e.g. for promotion). While there *may* exist alternative techniques that achieve similar throughput improvements without moving objects, I don't know of any that have been widely and successfully used so far, in any runtime. I would predict that as Go's use application cases expand it's GC implementations will evolve to include generational moving collectors. This *does not* necessarily mean a monolithic STW copying newgen. Given their [very correct] focus on latency, I'd expect even the newgen collectors in future generational GC collectors to prioritize the minimization or elimination of STW pauses, which will probably make them either fully concurrent, incremental-STW (with viable arbitrarily small incremental steps), or a combination of the two. And yes, doing that (concurrent generational GC) while supporting great latency, high throughput, and high efficiency all at the same time, is very possible. It is NOT the hard or inherent tradeoff many people seem to believe it to be. And practical proof points already exist. On Thursday, December 22, 2016 at 4:12:14 AM UTC-8, Vitaly Davidovich wrote: > > FWIW, I think the Go team is right in favoring lower latency over > throughput of their GC given the expected usage scenarios for Go. > > In fact, most of the (Hotspot based) Java GC horror stories involve very > long pauses (G1 and CMS not excluded) - I've yet to hear anyone complain > that their "Big Data" servers are too slow because the GC is chipping away > at throughput too much (most of those servers aren't CPU bound to begin > with or can't become CPU bound due to bottlenecks elsewhere). > > In the Hotspot world, there's also the unfortunate situation right now > where G1 isn't really solving any problem nor advancing the performance > (latency) boundaries. In fact, it taxes throughput quite heavily (e.g. > significantly more expensive write barriers) but isn't breaking any new > ground. > > > On Wed, Dec 21, 2016 at 11:38 PM Zellyn <zel...@gmail.com <javascript:>> > wrote: > >> You might be interested in this Golang-group follow-up thread too… >> https://groups.google.com/forum/#!topic/golang-nuts/DyxPz-cQDe4 >> (naturally a more pro-Go >> take on things…) >> >> On Wednesday, December 21, 2016 at 8:23:02 AM UTC-5, Greg Young wrote: >>> >>> Thought people on here would enjoy this >>> >>> >>> >>> https://medium.com/@octskyward/modern-garbage-collection-911ef4f8bd8e#.ptdzwcmq2 >>> >>> >>> >>> >>> >>> -- >>> >>> >>> Studying for the Turing test >>> >>> >>> >> >> >> >> >> >> >> >> -- >> >> >> You received this message because you are subscribed to the Google Groups >> "mechanical-sympathy" group. >> >> >> To unsubscribe from this group and stop receiving emails from it, send an >> email to mechanical-sympathy+unsubscr...@googlegroups.com <javascript:>. >> >> >> For more options, visit https://groups.google.com/d/optout. >> >> >> -- > Sent from my phone > -- You received this message because you are subscribed to the Google Groups "mechanical-sympathy" group. To unsubscribe from this group and stop receiving emails from it, send an email to mechanical-sympathy+unsubscr...@googlegroups.com. For more options, visit https://groups.google.com/d/optout.
