runtime: tight loops should be preemptible

[aclements]

Currently goroutines are only preemptible at function call points. Hence, it's possible to write a tight loop (e.g., a numerical kernel or a spin on an atomic) with no calls or allocation that arbitrarily delays preemption. This can result in arbitrarily long pause times as the GC waits for all goroutines to stop.

In unusual situations, this can even lead to deadlock when trying to stop the world. For example, the runtime's TestGoroutineParallelism tries to prevent GC during the test to avoid deadlock. It runs several goroutines in tight loops that communicate through a shared atomic variable. If the coordinator that starts these is paused part way through, it will deadlock.

One possible fix is to insert preemption points on control flow loops that otherwise have no preemption points. Depending on the cost of this check, it may also require unrolling such loops to amortize the overhead of the check.

This has been a longstanding issue, so I don't think it's necessary to fix it for 1.5. It can cause the 1.5 GC to miss its 10ms STW goal, but code like numerical kernels is probably not latency-sensitive. And as far as I can tell, causing a deadlock like the runtime test can do requires looking for trouble.

@RLH

[minux]

Another solution is to send a signal to the thread to force it to be preempted. i.e. once a thread is stopped, it masks a certain signal, and if there are some threads running user code that refuse to stop, we send that signal to the current process. In runtime.sigtramp, it can force the switch of the goroutine and then mask the signal. This solution introduces no overhead to the code, at the expense of complicating the runtime design.

[randall77]

The problem with interrupts is that the goroutine can be at an arbitrary address. The interrupted address is probably at a place where we have no data about where the pointers are. We'd either have to record a lot more frame layout/type/liveness information (including registers), or we'd have to simulate the goroutine forward to a safepoint. Both are challenging.

[minux]

The problem of inserting artificial preemption points is reduced numerical performance. Yet another solution is to just ignore the problem. User is supposed to insert runtime.Gosched() calls at appropriate points as needed. btw, isn't the long term GC plan is to never stop the world?

[ianlancetaylor]

It seems to me that we can preempt at a write barrier. So the only loops we are talking about are those that make no writes to the heap and make no function calls. If we think in terms of adding

    uint8 b
  loop:
    ++b
    if b == 0 {
        preemptCheck()
    }

then the normal path through the loop will have two extra instructions (add/beq) where the add may be to either a register or a memory location, depending on overall register pressure. This will be measurable in tight loops but for most cases shouldn't be too bad.

[randall77]

No pointer writes to the heap.

But maybe this is enough? If we can preempt at the first write barrier, then even if we let the mutator run it can't modify anything that the GC cares about. So maybe we just set the write barrier enabled bit and let goroutines run. Once a goroutine sees the write barrier bit (how do we force that?) it can't modify any memory the GC cares about. So it is safe to start the GC without waiting for every goroutine to stop.

[aclements]

The problem of inserting artificial preemption points is reduced
numerical performance.

True. That's why I suggested unrolling loops, which AFAIK is a standard solution to this problem.

However, I think the check can be done in just two instructions, even without adding Ian's loop counter. Just CMP the preempt flag and branch if it's set. That branch will almost never be hit, so it will be highly predictable, and the preempt flag should be in the L1, so the check may in fact be extremely cheap.

No pointer writes to the heap. But maybe this is enough?

This certainly reduces the set of programs that have this problem, but I don't think it actually helps with either of the examples I gave, since numerical kernels probably won't have heap pointer writes, and the runtime test that can deadlock the GC certainly has no pointer writes.

[RLH]

This is the reference for doing a GC safepoint at every instruction. It is
not something we want to do for x86 much less the various other
architectures we need to support.
http://dl.acm.org/citation.cfm?id=301652

Most systems I know of do what Austin suggests, unroll the loop and insert
a check. No numbers but 8 unrolls seems to be what I recall. Not only is
the branch highly predictable but the check does not introduce any
dependencies making it close to free on an out of order machine. There have
been other approaches such as code plugging and predicated branches but HW
has moved on. I had not seen Ian's suggestion.

On Tue, May 26, 2015 at 9:24 PM, Austin Clements notifications@github.com
wrote:

The problem of inserting artificial preemption points is reduced
numerical performance.

True. That's why I suggested unrolling loops, which AFAIK is a standard
solution to this problem.

However, I think the check can be done in just two instructions, even
without adding Ian's loop counter. Just CMP the preempt flag and branch if
it's set. That branch will almost never be hit, so it will be highly
predictable, and the preempt flag should be in the L1, so the check may in
fact be extremely cheap.

No pointer writes to the heap. But maybe this is enough?

This certainly reduces the set of programs that have this problem, but I
don't think it actually helps with either of the examples I gave, since
numerical kernels probably won't have heap pointer writes, and the runtime
test that can deadlock the GC certainly has no pointer writes.

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#10958 (comment).

[kingluo]

@aclements,

Currently goroutines are only preemptible at function call points.

To be precise, the preemption check seems only at newstack()?

For example,

package main

import (
    "fmt"
    "runtime"
    "time"
)

func test() {
    a := 100
    for i := 1; i < 1000; i++ {
        a = i*100/i + a
    }
}

func main() {
    runtime.GOMAXPROCS(1)
    go func() {
        for {
            test()
        }
    }()
    time.Sleep(100 * time.Millisecond)
    fmt.Println("hello world")
}

The test() is not inline, and the infinite loop calls test(), but it would not preempt at the calls, because the morestack() --> newstack() not involved, so the "hello world" would never be printed.

[randall77]

test() is not inlined, but since it is a leaf it is promoted to nosplit, so it doesn't have the preemption check any more. Fixing that sounds much easier than the rest of this bug. Maybe we could forbid nosplit promotion for functions called from loops?

[griesemer]

For the reference, the same problem appeared in the HotSpot JVM. I remember two approaches:

1. Around 1997/1998, Rene Schmidt (http://jaoo.dk/aarhus2007/speaker/Rene+W.+Schmidt) implemented the following mechanism: Threads running a tight loop w/o function calls would receive a signal to temporarily suspend them. The runtime then dynamically generated a partial "copy" of the loop instructions the thread was running, from the current PC to the (unconditional) backward branch, except that the backward branch was replaced with a call into the runtime (leading to proper suspension at a safe point). The thread was then restarted with the pc modified such that it would run the newly generated piece of code. That code would run to the end of the loop body and then suspend itself at which point the code copy was discarded and the pc (return address) adjusted to continue with the original code (after the safe point).

This mechanism was ingenious but also insanely complicated. It was abandoned eventually for:

2. A simple test and branch (2 additional instructions) at the end of a loop body which didn't contain any calls. As far as I recall, we didn't do any form of loop unrolling and the performance implications were not significant in the overall picture of a larger application.

My vote would be for 2) to start in loops where @ianlancetaylor's suggestion doesn't work. I suspect that for all but the smallest long-running inner loops (where unrolling might make sense independently), the performance penalty is acceptable.

If this is not good enough, and we don't want to pay the code size cost of unrolling a loop multiple times, here's another idea based on 1): Instead of the backward branch check as in 2) plus unrolling, keep the original loop, and generate (at compile time) a 2nd version of the loop body that ends in a runtime call to suspend itself instead of the backward branch. The code size cost is about the cost of having the loop unrolled twice. When the goroutine needs to run to a safe point, use a signal to temporarily suspend the goroutine, switch the pc to the corresponding pc in the copy of the loop body, continue with execution there and have the code suspend itself at a safe point. There's no dynamic code generation involved, generating the extra loop body is trivial at compile-time, the extra amount of code is less than with loop unrolling, and there's only a little bit of runtime work to modify the pc. The regular code would run at full speed if no garbage collection is needed.

[nightlyone]

Another idea, similiar to @ianlancetaylor's idea, is to estimate the cost (in ns) of the loop and only check for required suspension every N loops through that loop body.

Once the compiler can unroll/unpeel, that check-every-N logic can be either inlined after the unrolled body, if unrolling is beneficial, or kept when unrolling makes no sense for that loop body.

Such logic also reads better when using debuggers.

[griesemer]

@nightlyone The check-every-N seems more complex than just having two extra instructions (compare and branch). And if unrolling is done, that compare-and-branch is already needed only every N loop iterations only (where N is the number of times the loop was unrolled).

[nightlyone]

@griesemer not sure how the test will avoid atomic loads, that's why I suggested the check-every-N.

So the pseudo-go-code before unrolling would look like this:

loop:   
        // loop-body        

        if counter > N {          
                counter = 0                       

                if need_stop = atomic.LoadBool(&runtime.getg().need_stop); need_stop {
                        runtime.Gosched()                                    
                }                                                               
        }                                                                                                                       
        counter++                                                                                                           
        goto loop

after unrolling it would look like

loop:   
        // loop-body0
        // loop-body2
        // ...
        // loop-bodyM        

        if counter > N/M {          
                counter = 0                       

                if need_stop = atomic.LoadBool(&runtime.getg().need_stop); need_stop {
                        runtime.Gosched()                                    
                }                                                               
        }                                                                                                                       
        counter++                                                                                                           
        goto loop

So the code inserted would be a constant overhead, but still run every N loops.

[aclements]

The load doesn't have to be atomic. The current preemption check in the function prologue isn't atomic.

One tricky bit with the compare and branch is that the actual preemption point needs to have no live registers. Presumably, for performance reasons, we want the loop to be able to keep things in registers, so the code we branch to on a preempt has to flush any live registers before the preempt and reload them after the preempt. I don't think this will be particularly hard, but it is something to consider, since it might affect which stage of the compiler is responsible for generating this code.

[griesemer]

@aclements Indeed. Which is why perhaps switching the pc to a 2nd version of the loop body that ends in a safe point might not be much more complex and permit the loop to run at full speed in the normal case.

[aclements]

@aclements Indeed. Which is why perhaps switching the pc to a 2nd version of the loop body that ends in a safe point might not be much more complex and permit the loop to run at full speed in the normal case.

From the runtime's perspective, I think this would be more complicated because stealing a signal is a logistic pain and we'd have to deal with tables mapping from fast loop PCs to slow loop PCs. The compiler would have to generate these tables. This seems like a very clever plan B, but I think first we should trying adding a no-op compare and branch and see if it's actually a problem for dense numerical kernels.

[randall77]

The new SSA register allocator handles situations like this well, keeping everything in registers on the common edges and spill/call/restore on the unlikely case.

[RLH]

The load is not dependent on anything in the loop. Assuming the loop is
tight, the value is almost certainly to a location already in the L1 cache,
the branch will be highly predictable. Just to make sure the branch
predictor doesn't even need to be warmed up, we can make it a backward
branch. I would be sort of surprised that if on an out of order machine the
cost would even be noticed. That said build and measure is the only way to
be sure.

On Fri, Sep 11, 2015 at 3:38 PM, Keith Randall notifications@github.com
wrote:

The new SSA register allocator handles situations like this well, keeping
everything in registers on the common edges and spill/call/restore on the
unlikely case.

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#10958 (comment).

[gopherbot]

CL https://golang.org/cl/19516 mentions this issue.

[gopherbot]

Change https://golang.org/cl/204340 mentions this issue: runtime: support preemption on windows/{386,amd64}

[gopherbot]

Change https://golang.org/cl/207779 mentions this issue: runtime: ensure thread handle is valid in profileloop1

[gopherbot]

Change https://golang.org/cl/207961 mentions this issue: runtime: support non-cooperative preemption on windows/arm

[gopherbot]

Change https://golang.org/cl/208218 mentions this issue: runtime: stress testing for non-cooperative preemption

[aclements]

Go 1.14 will handle loop preemption far better than previous versions, but there are still some loose ends to tie up in 1.15, so I'm bumping this issue to 1.15.

[aclements]

Actually, rather than bump this to 1.15, since tight loops are preemptible now, I've created a new issue to keep track of loose ends (#36365) and will go ahead and close this issue as fixed.

[gopherbot]

Change https://golang.org/cl/213837 mentions this issue: runtime: protect against external code calling ExitProcess