On Sun, Nov 22, 2015 at 09:15:02PM -0500, ira.weiny wrote: > On Thu, Nov 19, 2015 at 04:54:44PM -0800, Greg KH wrote: > > On Mon, Nov 09, 2015 at 06:34:44PM -0500, ira.weiny@xxxxxxxxx wrote: > > > From: Vennila Megavannan <vennila.megavannan@xxxxxxxxx> > > > > > > Add a module paramter to toggle prescan/Fast ECN Detection and remove the > > > Kconfig option which used to control this. > > > > Ick, no, not a module parameter, that's horrid (hint, it isn't a > > per-device option...) > > This is a good point. Previous to this patch we had a compile time option > which would have affected all devices and I think we just continued that. I do > like the idea of making this per port. I will respin the patch. > > However, I want to be clear on your hint. Are you saying that sysfs would be a > better place to put such a flag? Maybe, if you want it per-device, but really, you don't want to have any "knobs" a user has to tune. > > Why can't you do this dynamically? > > ECN is always on. The key is the reaction time of the individual port. > Attempting to turn this on and off would affect both the reaction time and the > processing time in a negative way. > > > Why would anyone ever want to make this "slow"? > > This is a tuning nob for over all fabric performance not individual node > performance. ECN controls congestion spreading through the network as is > explained in this paper. > > http://infocom2003.ieee-infocom.org/papers/28_01.PDF > > As is shown in figure 1 and 2 of that paper congestion at node Bc is affecting > traffic at node Bv. The default reaction time of ECN is likely to be > sufficient for most users based on our experience so far. > > However, should a particular network see system wide degradation, this option > can be turned on to increase the reaction time at node Bc. However, node Bc is > already overloaded so the trade off is likely acceptable. > > Unfortunately, it is hard to predict when a user may need this option as we > don't have the resources to build extreme scale fabrics for testing. Nor do we > know all users workloads or the fabric topologies those workloads may be > running on. > > We developed the code which this option enables based on lab experiments. So > we need this to be an option available to users. Ok, then make it able to be a per-device option, through sysfs, and choose the "best" option to set it to be as the default. thanks, greg k-h _______________________________________________ devel mailing list devel@xxxxxxxxxxxxxxxxxxxxxx http://driverdev.linuxdriverproject.org/mailman/listinfo/driverdev-devel
