Re: Implementing poll(2) for Mini-OS?

[Wei Liu <wei.liu2@citrix.com>]

On Mon, 2013-02-18 at 17:12 +0000, Samuel Thibault wrote:
> Wei Liu, le Mon 18 Feb 2013 15:40:56 +0000, a écrit :
> > I sent a patch to switch cxenstored's event loop from using select to
> > using poll several weeks ago,
> 
> What is the rationale BTW?  Efficiency?
> 

Oh, the rationale is that select can only support 1024 fds in Linux. If
we run more than 1024 guest (in fact the real limit is like 1022),
xenstore hangs and makes the whole system unusable.

> > My main question is, is it possible to just wrap around select(2) in
> > Mini-OS to implement poll(2)? (as shown in the conceptual patch)
> 
> Yes, except that there are evil small differences between poll and
> select, which we need to take care of.
> 

Sure. :-)

> About the 32 fd limitation, it indeed comes from the days when we always
> had a bounded number of things to open.  We need to drop that limitation,
> but it should be easy, by turning the `files' array into a reallocable
> pointer.  There is just one important thing: the xenbus_event_queue
> events field has to be made a pointer to a dynamically-allocated queue,
> otherwise it will get moved by the reallocation and things will go wrong
> very badly.
> 

So apart from the modification needed, is this "32" really a bottleneck
for xenstore-stubdom? How many domains can a xenstore-stubdom serve? (I
think these two questions are for Daniel)


Wei.


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