Thanks, Ian. On Fri, 2020-03-06 at 14:01 -0800, Ian Lance Taylor wrote: > On Fri, Mar 6, 2020 at 1:40 PM Dan Kortschak <d...@kortschak.io> > wrote: > > > > This sort of follows on from the EINTR discussion a while back, but > > was > > prompted by a claim in the gophers #general slack channel that "the > > Go > > scheduler will actively try to interrupt CGO calls that take too > > long"[1]. > > > > This doesn't seem right to me, but I wanted to confirm from people > > who > > know (also is there any documentation about the behaviour of the > > new > > pre-emptive scheduler?). > > There is a sense in which it is right. When a cgo call starts it has > a P attached to it (in the scheduler, a P is a virtual processor; > there are exactly GOMAXPROCS P's at all times). If the cgo call > completes quickly, it will simply carry on with the same P. When the > system monitoring thread wakes up, it will check each P to see if it > has been waiting for a cgo call to complete for more than a scheduler > tick (20 microseconds, more or less). If so, the P will be stolen > and > some other M (operating system thread) will be woken up to start > using > the P and running Go code. When the cgo call completes, the > goroutine > will see that it no longer has a P, and will go to sleep waiting for > a > P to become available (more or less as though the goroutine called > runtime.Gosched). > > So in that sense, cgo calls will be interrupted: the P will be > removed > and reassigned to do other work. > > However, the actual C code running on the M (operating system thread) > will not be affected. And the G (goroutine) will of course remain > asleep waiting for the cgo call to complete. > > (This is all a description of the current 1.14 scheduler, and it may > be different in other releases.) > > Ian
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