> On Sep 2, 2017, at 14:15, Chris Lattner via swift-evolution
> <swift-evolution@swift.org> wrote:
>
> My understanding is that GCD doesn’t currently scale to 1M concurrent queues
> / tasks.
Hi Chris!
[As a preface, I’ve only read a few of these concurrency related emails on
swift-evolution, so please forgive me if I missed something.]
When it comes to GCD scalability, the short answer is that millions of of tiny
heap allocations are cheap, be they queues or closures. And GCD has fairly
linear performance so long as the millions of closures/queues are non-blocking.
The real world is far messier though. In practice, real world code blocks all
of the time. In the case of GCD tasks, this is often tolerable for most apps,
because their CPU usage is bursty and any accidental “thread explosion” that is
created is super temporary. That being said, programs that create thousands of
queues/closures that block on I/O will naturally get thousands of threads. GCD
is efficient but not magic.
As an aside, there are things that future versions of GCD could do to minimize
the “thread explosion” problem. For example, if GCD interposed the system call
layer, it would gain visibility into *why* threads are stalled and therefore
GCD could 1) be more conservative about when to fire up more worker threads and
2) defer resuming threads that are at “safe” stopping points if all of the CPUs
are busy.
That being done though, the complaining would just shift. Instead of an
“explosion of threads”, people would complain about an “explosion of stacks"
that consume memory and address space. While I and others have argued in the
past that solving this means that frameworks must embrace callback API design
patterns, I personally am no longer of this opinion. As I see it, I don’t think
the complexity (and bugs) of heavy async/callback/coroutine designs are worth
the memory savings. Said differently, the stack is simple and efficient. Why
fight it?
I think the real problem is that programmers cannot pretend that resources are
infinite. For example, if one implements a photo library browsing app, it would
be naive to try and load every image at launch (async or otherwise). That just
won’t scale and that isn’t the operating system's fault.
Dave
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