On Fri, Jun 18, 2010 at 11:11:00AM +0200, Peter Zijlstra wrote: > On Fri, 2010-06-18 at 16:09 +1000, Dave Chinner wrote: > > > + bdi->wb_written_head = bdi_stat(bdi, BDI_WRITTEN) + wc->written; > > > > The resolution of the percpu counters is an issue here, I think. > > percpu counters update in batches of 32 counts per CPU. wc->written > > is going to have a value of roughly 8 or 32 depending on whether > > bdi->dirty_exceeded is set or not. I note that you take this into > > account when checking dirty threshold limits, but it doesn't appear > > to be taken in to here. > > The BDI stuff uses a custom batch-size, see bdi_stat_error() and > related. The total error is in the order of O(n log n) where n is the > number of CPUs. #define BDI_STAT_BATCH (8*(1+ilog2(nr_cpu_ids))) A dual socket server I have here: [ 0.000000] NR_CPUS:512 nr_cpumask_bits:512 nr_cpu_ids:32 nr_node_ids:2 Which means that the bdi per-cpu counter batch size on it would be 48 and the inaccuracy would be even bigger than I described. ;) > But yeah, the whole dirty_exceeded thing makes life more > interesting. I suspect the 8 vs 32 pages could go away without too much impact with the Jan's mechanism... Another thing to consider is the impact of mulipage writes on the incoming value to balance_dirty_pages - if we get 1024 page writes running, then that may impact on the way throttling behaves, too. Cheers, Dave. -- Dave Chinner david@xxxxxxxxxxxxx -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
