On Thu, Mar 08, 2012 at 04:12:21PM -0500, Ted Ts'o wrote: > On Thu, Mar 08, 2012 at 03:42:52PM -0500, Jeff Moyer wrote: > > > > So now we're back to figuring out how to tell how long I/O will take? > > If writeback is issuing random access I/Os to spinning media, you can > > bet it might be a while. Today, you could lower nr_requests to some > > obscenely small number to improve worst-case latency. I thought there > > was some talk about improving the intelligence of writeback in this > > regard, but it's a tough problem, especially given that writeback isn't > > the only cook in the kitchen. > > ... and it gets worse if there is any kind of I/O prioritization going > on via ionice(), or (as was the case in our example) I/O cgroups were > being used to provide proportional I/O rate controls. I don't think > it's realistic to assume the writeback code can predict how long I/O > will take when it does a submission. cgroups do make it much harder because it could be a simple IO priority inversion. The latencies are just going to be a fact of life for now and the best choice is to skip the stable pages. -chris -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
