Re: [Xen-devel] [PATCH 4/4] xen-netback: coalesce slots before copying

[Wei Liu <wei.liu2@xxxxxxxxxx>]

On Fri, Mar 22, 2013 at 11:19:56AM +0000, James Harper wrote:
> > 
> > On Thu, Mar 21, 2013 at 10:14:17PM +0000, James Harper wrote:
> > > >
> > > >> Actually it turns out GPLPV just stops counting at 20. If I keep
> > > >> counting I can sometimes see over 1000 buffers per GSO packet under
> > > >> Windows using "iperf -
> > > >
> > > > Do you think it is necessary to increase MAX_SKB_SLOTS_DEFAULT to 21?
> > > >
> > >
> > > Doesn't really matter. Under windows you have to coalesce anyway and
> > the number of cases where the skb count is 20 or 21 is very small so there 
> > will
> > be negligible gain and it will break guests that can't handle more than 19.
> > 
> > It's not about performance, it's about usability. If frontend uses more
> > slots than backend allows it to, it gets disconnected. In case we don't
> > push the wrong value upstream, it is important to know whether 20 is
> > enough for Windows PV driver.
> > 
> 
> Windows will accept whatever you throw at it (there may be some upper limit, 
> but I suspect it's quite high). Whatever Linux will accept, it will be less 
> than the 1000+ buffers that Windows can generate, so some degree of 
> coalescing will be required for Windows->Linux.
> 
> In GPLPV I already coalesce anything with more than 19 buffers, because I 
> have no guarantee that Dom0 will accept anything more (and who knows what 
> Solaris or BSD will accept, if those are still valid backends...), so 
> whatever you increase Dom0 to won't matter because I would still need to 
> assume that Linux can't accept more than 19, until such time as Dom0 (or 
> driver domain) advertises the maximum buffer count it can support in 
> xenstore...
> 
> So do what you need to do to make Linux work, just don't put the erroneous 
> comment that "windows has a maximum of 20 buffers" or whatever it was in the 
> comments :)
> 

OK, problem solved. :-)


Wei.

> James
> 

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxx
http://lists.xen.org/xen-devel