Re: [Qemu-devel] [RFC][PATCH 01/12] coroutine: Add gtk-vnc coroutines library

[Anthony Liguori <anthony@codemonkey.ws>]

On 01/26/2011 10:13 AM, Avi Kivity wrote:
> On 01/26/2011 06:00 PM, Anthony Liguori wrote:
>> On 01/26/2011 09:25 AM, Avi Kivity wrote:
>>> On 01/22/2011 11:29 AM, Stefan Hajnoczi wrote:
>>>> Asynchronous image format code is becoming very complex.  Let's try
>>>> using coroutines to write sequential code without callbacks but use
>>>> coroutines to switch stacks under the hood.
>>>>
>>>>
>>>> +
>>>> +int cc_swap(struct continuation *from, struct continuation *to)
>>>> +{
>>>> +    to->exited = 0;
>>>> +    if (getcontext(&to->last) == -1)
>>>> +        return -1;
>>>> +    else if (to->exited == 0)
>>>> +        to->exited = 1;
>>>> +    else if (to->exited == 1)
>>>> +        return 1;
>>>> +
>>>> +    return swapcontext(&from->uc,&to->uc);
>>>> +}
>>>
>>> swapcontext() is very slow, involving the fpu and a syscall.
>>>
>>> A nice trick I've used in the past is to use getcontext/makecontext 
>>> for the initial setup and setjmp/longjmp for switching.  Of course 
>>> this can be done later, as an optimization.
>>
>> Yeah, there are further optimizations that can be had but really, 
>> it's not important here.  In fact, I'd rather start with the threaded 
>> version just because it's far more portable than ucontext.
>>
>
> What do you mean by threaded version?

Stefan didn't post it, but the original code also has a GThread based 
implementation when ucontext isn't available (like on Windows).  It uses 
a mutex to control the execution of the coroutines.

Regards,

Anthony Liguori