Re: [Qemu-devel] [PATCH v4 1/3] coroutine: introduce coroutines

[Stefan Hajnoczi <stefanha@gmail.com>]

On Fri, May 20, 2011 at 1:09 PM, Paolo Bonzini <pbonzini@redhat.com> wrote:
> On 05/20/2011 12:59 PM, Stefan Hajnoczi wrote:
>>
>> This coroutines implementation is based on the gtk-vnc implementation
>> written by Anthony Liguori<anthony@codemonkey.ws>  but it has been
>> significantly rewritten by Kevin Wolf<kwolf@redhat.com>  to use
>> setjmp()/longjmp() instead of the more expensive swapcontext() and by
>> Paolo Bonzini<pbonzini@redhat.com>  for Windows Fibers support.
>>
>
> Not a blocker at all, but why did you move the pooling to the ucontext
> implementation?  It's less expensive to create the fiber in Windows because
> there are no system calls (unlike swapcontext), but a future pthread-based
> implementation will also need the pooling.
>
> It can be left to whoever writes the pthread stuff, though.

There are two options for pooling:
1. Thread-local pools
2. One global pool with a lock

One of these choices must be selected because otherwise the pool could
be accessed simultaneously from multiple threads.  I tried #2 first
because it was less code but it caused a noticable slow-down with
./check-coroutine --benchmark-lifecycle.  Option #1 had less impact
but requires using thread-local storage, which I've used pthread APIs
for.  Hence I moved it into coroutine-ucontext.c hoping that win32
would either be fast enough as-as or that we could find a better
solution if someone needs it.

Stefan