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Re: [libvirt] mingw: test-poll pipe part fails



On Thu, Jun 30, 2011 at 11:56:15AM -0600, Eric Blake wrote:
> [adding libvirt]
> 
> On 06/04/2011 12:24 AM, Paolo Bonzini wrote:
> > On Sat, Jun 4, 2011 at 00:37, Matthias Bolte
> > <matthias bolte googlemail com> wrote:
> >> After testing a while and reading MSDN docs the problem seems to be
> >> that MsgWaitForMultipleObjects doesn't work on pipes. It doesn't
> >> actually wait but just returns immediately. Digging MSDN and googling
> >> about this suggest that there is no simple solution to this.
> > 
> > Yes, Windows pipes are that broken. :(
> > 
> > Using socketpair is a possibly good idea, but I would do it on
> > libvirtd only.  I don't know exactly how libvirtd uses this pipe, but
> > perhaps it can be changed to an eventfd-like abstraction that can be
> > used with both Windows and Unix.  This is similar to Eric's
> > suggestion, but without the pipe at all.  It would also be a
> > libvirtd-specific suggestion.
> 
> I'm wondering if the problem here is that libvirt is trying to use the
> pipe-to-self mechanism as a fundamental event loop idiom.  That is, the
> reason libvirt is calling poll is in order to minimize CPU until
> something interesting happens, where interesting includes needing to
> wake up a helper thread to do an action inside locks in response to the
> receipt of a signal.
> 
> Maybe you are on to something, and replacing all uses of pipe() with
> virPipeToSelf() (which uses pipe() for efficiency on Linux, but
> socketpair() on mingw), would allow libvirt to continue to use the
> pipe-to-self idiom while also using fds that can actually be poll'd on
> mingw.

IIRC, we never resolved this for the last release. I think we should
do as you suggest and just use socketpair() on Win32. Given the way
in which libvirt uses these capabilities, I don't think the overheads
of socketpair() vs pipe() are so onerous that we need worry about an
even more fancy Win32 impl or eventfd for linux. Of course if someone
wants todo a full job for gnulib meawhile, we won't complain...

Regards,
Daniel
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