[Vm-dev] Unix heartbeat thread vs itimer

Denis Kudriashov dionisiydk at gmail.com
Tue Jan 10 21:01:43 UTC 2017

2017-01-10 21:54 GMT+01:00 Eliot Miranda <eliot.miranda at gmail.com>:

> Will event-driven VM fix this problem completely? Or heartbeat will be
>> needed anyway?
> As I understand it, the heartbeat us always needed.  In an event-driven VM
> it may be that the Smalltalk executive is called form the event loop, but
> the Smalltalk executive still has to break out of executing Smalltalk to
> return to the event loop to receive new events.
> The thing to understand about the JIT VM is that Smalltalk is executing in
> machine code just like simple code in some low level language.  That code
> can respond to Smalltalk events such as attempting to wait on a Semaphore
> with no outstanding signals, which may cause it to switch processes.  But
> it cannot respond to external asynchronous events unless it is informed of
> those events.  And it is very difficult to construct a VM that can accept
> interrupts at arbitrary times that activate Smalltalk code (imagine
> receiving an interrupt in the middle of a GC, or mid-way through looking up
> a send not found in the cache, etc, etc; essentially interrupts can only be
> accepted at limited times).  So the VM needs to check for events (and
> indeed interrupts) at safe points.  The efficient implementation of safe
> points is checking on the next frame-building send.  But if every
> frame-building send checked for interrupts and/or events, frame build would
> be very slow and the entire VM would crawl.  The function of the heart beat
> is to cause frame building sends to check for interrupts and/or events at
> regular, but (relative to frame-builling send frequency) infrequent
> occasions.
> Compare that to interrupts in a real processor.  The processor /also/ only
> tests e.g. the interrupt request pin at a safe point (perhaps at the end of
> each interaction decode cycle), and also provides means to disable
> interrupts for critical sections of code.  It's just that the quanta are
> much smaller than in a Smalltalk vm.

Thank's Eliot. It's very informative explanation
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