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runtime: replace GC coordinator with state machine #11970
As of 1.5, all GC phase changes are managed through straight-line code in func gc, which runs on a dedicated GC goroutine. However, much of the real work is done in other goroutines, and the coordination between these goroutines and the main GC goroutine delays phase changes and opens windows where, for example, the mutator can allocate uncontrolled, or nothing can be accomplished because everything is waiting on the coordinator to wake up. This has led to bugs like #11677 and #11911. We've tried to mitigate this by handing control directly to the coordinator goroutine when we wake it up, but the scheduler isn't designed for this sort of explicit co-routine scheduling, so this doesn't always work and it's more likely to fall apart under stress.
For 1.6, we should consider decentralizing the role of the coordinator so that the goroutine that notices it's time to make a transition takes care of performing the transition.
This could take the form of a state machine where whichever goroutine detects the termination condition of the current state is responsible for moving the system to the next state. Because these transitions could happen on user goroutines, we'll want to make sure that these state transitions are all short (or STW), which is currently not true of everything the coordinator does. The states would roughly correspond to the GC phases, but would further subdivide some of them (especially the concurrent ones).
I think we should further design this state machine such that if more than one goroutine detects the termination condition for the current state (including while another goroutine is working on transitioning out of it) it should block until the goroutine performing the transition finishes the transition (or, if possible, help with the transition), rather than simply continuing execution. We have various problems right now because we don't do this: for example, the first G to detect that the system is over the heap trigger will start the transition from "GC off" to sweep termination, but before we're actually in sweep termination, other Gs will also detect this condition as well but simply continue allocating, allowing them to over-allocate . If they instead blocked or helped until this transition was complete, this wouldn't happen.
 See issue #11911 and all of its linked CLs.
Currently there are two sensitive periods during which a mutator can allocate past the heap goal but mutator assists can't be enabled: 1) at the beginning of GC between when the heap first passes the heap trigger and sweep termination and 2) at the end of GC between mark termination and when the background GC goroutine parks. During these periods there's no back-pressure or safety net, so a rapidly allocating mutator can allocate past the heap goal. This is exacerbated if there are many goroutines because the GC coordinator is scheduled as any other goroutine, so if it gets preempted during one of these periods, it may stay preempted for a long period (10s or 100s of milliseconds). Normally the mutator does scan work to create back-pressure against allocation, but there is no scan work during these periods. Hence, as a fall back, if a mutator would assist but can't yet, simply yield the CPU. This delays the mutator somewhat, but more importantly gives more CPU time to the GC coordinator for it to complete the transition. This is obviously a workaround. Issue #11970 suggests a far better but far more invasive way to fix this. Updates #11911. (This very nearly fixes the issue, but about once every 15 minutes I get a GC cycle where the assists are enabled but don't do enough work.) Change-Id: I9768b79e3778abd3e06d306596c3bd77f65bf3f1 Reviewed-on: https://go-review.googlesource.com/13026 Reviewed-by: Russ Cox <email@example.com> Reviewed-by: Rick Hudson <firstname.lastname@example.org>
This begins the conversion of the centralized GC coordinator to a decentralized state machine by introducing the internal API that triggers the first state transition from _GCoff to _GCmark (or _GCmarktermination). This change introduces the transition lock, the off->mark transition condition (which is very similar to shouldtriggergc()), and the general structure of a state transition. Since we're doing this conversion in stages, it then falls back to the GC coordinator to actually execute the cycle. We'll start moving logic out of the GC coordinator and in to transition functions next. This fixes a minor bug in gcstoptheworld debug mode where passing the heap trigger once could trigger multiple STW GCs. Updates #11970. Change-Id: I964087dd190a639eb5766398f8e1bbf8b352902f Reviewed-on: https://go-review.googlesource.com/16355 Reviewed-by: Rick Hudson <email@example.com> Run-TryBot: Austin Clements <firstname.lastname@example.org>
This moves concurrent sweep termination from the coordinator to the off->mark transition. This allows it to be performed by all Gs attempting to start the GC. Updates #11970. Change-Id: I24428e8599a759398c2ef7ec996ba755a448f947 Reviewed-on: https://go-review.googlesource.com/16356 Reviewed-by: Rick Hudson <email@example.com> Run-TryBot: Austin Clements <firstname.lastname@example.org> TryBot-Result: Gobot Gobot <email@example.com>
This moves all of GC initialization, sweep termination, and the transition to concurrent marking in to the off->mark transition function. This means it's now handled on the goroutine that detected the state exit condition. As a result, malloc no longer needs to Gosched() at the beginning of the GC cycle to prevent over-allocation while the GC is starting up because it will now *help* the GC to start up. The Gosched hack is still necessary during GC shutdown (this is easy to test by enabling gctrace and hitting Ctrl-S to block the gctrace output). At this point, the GC coordinator still handles later phases. This requires a small tweak to how we start the GC coordinator. Currently, starting the GC coordinator is best-effort and may fail if the coordinator is about to park from the previous cycle but hasn't yet. We fix this by replacing the park/ready to wake up the coordinator with a semaphore. This is temporary since the coordinator will be going away in a few commits. Updates #11970. Change-Id: I2c6a11c91e72dfbc59c2d8e7c66146dee9a444fe Reviewed-on: https://go-review.googlesource.com/16357 Reviewed-by: Rick Hudson <firstname.lastname@example.org> Run-TryBot: Austin Clements <email@example.com> TryBot-Result: Gobot Gobot <firstname.lastname@example.org>
Currently mallocgc detects if the GC is in a state where it can't assist, but also can't allocate uncontrolled and yields to help out the GC. This was a workaround for periods when we were trying to schedule the GC coordinator. It is no longer necessary because there is no GC coordinator and malloc can always assist with any GC transitions that are necessary. Updates #11970. Change-Id: I4f7beb7013e85e50ae99a3a8b0bb708ba49cbcd4 Reviewed-on: https://go-review.googlesource.com/16392 Reviewed-by: Rick Hudson <email@example.com> Run-TryBot: Austin Clements <firstname.lastname@example.org> TryBot-Result: Gobot Gobot <email@example.com>