On Fri 25-02-11 22:44:12, Wu Fengguang wrote: > On Fri, Feb 25, 2011 at 02:56:32AM +0800, Jan Kara wrote: > > On Thu 24-02-11 23:25:09, Wu Fengguang wrote: > > > The bdi base throttle bandwidth is updated based on three class of > > > parameters. > > > > > > (1) level of dirty pages > > > > > > We try to avoid updating the base bandwidth whenever possible. The > > > main update criteria are based on the level of dirty pages, when > > > - the dirty pages are nearby the up or low control scope, or > > > - the dirty pages are departing from the global/bdi dirty goals > > > it's time to update the base bandwidth. > > > > > > Because the dirty pages are fluctuating steadily, we try to avoid > > > disturbing the base bandwidth when the smoothed number of dirty pages > > > is within (write bandwidth / 8) distance to the goal, based on the > > > fact that fluctuations are typically bounded by the write bandwidth. > > > > > > (2) the position bandwidth > > > > > > The position bandwidth is equal to the base bandwidth if the dirty > > > number is equal to the dirty goal, and will be scaled up/down when > > > the dirty pages grow larger than or drop below the goal. > > > > > > When it's decided to update the base bandwidth, the delta between > > > base bandwidth and position bandwidth will be calculated. The delta > > > value will be scaled down at least 8 times, and the smaller delta > > > value, the more it will be shrank. It's then added to the base > > > bandwidth. In this way, the base bandwidth will adapt to the position > > > bandwidth fast when there are large gaps, and remain stable when the > > > gap is small enough. > > > > > > The delta is scaled down considerably because the position bandwidth > > > is not very reliable. It fluctuates sharply when the dirty pages hit > > > the up/low limits. And it takes time for the dirty pages to return to > > > the goal even when the base bandwidth has be adjusted to the right > > > value. So if tracking the position bandwidth closely, the base > > > bandwidth could be overshot. > > > > > > (3) the reference bandwidth > > > > > > It's the theoretic base bandwidth! I take time to calculate it as a > > > reference value of base bandwidth to eliminate the fast-convergence > > > vs. steady-state-stability dilemma in pure position based control. > > > It would be optimal control if used directly, however the reference > > > bandwidth is not directly used as the base bandwidth because the > > > numbers for calculating it are all fluctuating, and it's not > > > acceptable for the base bandwidth to fluctuate in the plateau state. > > > So the roughly-accurate calculated value is now used as a very useful > > > double limit when updating the base bandwidth. > > > > > > Now you should be able to understand the information rich > > > balance_dirty_pages-pages.png graph. Here are two nice ones: > > > > > > http://www.kernel.org/pub/linux/kernel/people/wfg/writeback/dirty-throttling-v6/4G-60%25/btrfs-16dd-1M-8p-3927M-60%-2.6.38-rc6-dt6+-2011-02-24-23-14/balance_dirty_pages-pages.png > > > > > > http://www.kernel.org/pub/linux/kernel/people/wfg/writeback/dirty-throttling-v6/10HDD-JBOD-6G-6%25/xfs-1dd-1M-16p-5904M-6%25-2.6.38-rc5-dt6+-2011-02-21-20-00/balance_dirty_pages-pages.png > > Thanks for the update on your patch series :). As you probably noted, > > I've created patches which implement IO-less balance_dirty_pages() > > differently so we have two implementations to compare (which is a good > > thing I believe). The question is how to do the comparison... > > Yeah :) > > > I have implemented comments, Peter had to my patches and I have finished > > scripts for gathering mm statistics and processing trace output and > > plotting them. Looking at your test scripts I can probably use some > > of your workloads as mine are currently simpler. Currently I have some > > simple dd tests running, I'll run something over NFS, SATA+USB and > > hopefully several SATA drives next week. > The tests are pretty time consuming. It will help to reuse test > scripts for saving time and for ease of comparison. What my scripts gather is: global dirty pages & writeback pages (from /proc/meminfo) per-bdi dirty pages, writeback pages, threshold (from /proc/sys/kernel/debug/bdi/<bdi>/stats), read and write throughput (from /sys/block/<dev>/stat) Things specific to my patches are: estimated bdi throughput, time processes spend waiting in balance_dirty_pages(). Then I plot two graphs per test. The first graph is showing number of dirty and writeback pages, dirty threshold, real current IO throuput and estimated throughput for each BDI. The second graph is showing how much (in %) each process waited in last 2s window. Thinking about it, all these parameters would make sense for your patches as well. So if we just agreed on the format of tracepoints for process entering and leaving balance_dirty_pages(), we could use same scripts for tracking and plotting the behavior. My two relevant tracepoints are: TRACE_EVENT(writeback_balance_dirty_pages_waiting, TP_PROTO(struct backing_dev_info *bdi, unsigned long pages), TP_ARGS(bdi, pages), TP_STRUCT__entry( __array(char, name, 32) __field(unsigned long, pages) ), TP_fast_assign( strncpy(__entry->name, dev_name(bdi->dev), 32); __entry->pages = pages; ), TP_printk("bdi=%s pages=%lu", __entry->name, __entry->pages ) ); TRACE_EVENT(writeback_balance_dirty_pages_woken, TP_PROTO(struct backing_dev_info *bdi), TP_ARGS(bdi), TP_STRUCT__entry( __array(char, name, 32) ), TP_fast_assign( strncpy(__entry->name, dev_name(bdi->dev), 32); ), TP_printk("bdi=%s", __entry->name ) ); So I only have the 'pages' argument in writeback_balance_dirty_pages_waiting() which you maybe don't need (but you probably still have the number of pages dirtied before we got to balance_dirty_pages(), don't you?). In fact I don't currently use it for plotting (I just use it when inspecting the behavior in detail). > > The question is how to compare results? Any idea? Obvious metrics are > > overall throughput and fairness for IO bound tasks. But then there are > > I guess there will be little difference in throughput, as long as the > iostat output all have 100% disk util and full IO size. Hopefully yes, unless we delay processes far too much... > As for faireness, I have the "ls-files" output for comparing the > file size created by each dd task. For example, > > wfg ~/bee% cat xfs-4dd-1M-8p-970M-20%-2.6.38-rc6-dt6+-2011-02-25-21-55/ls-files > 131 -rw-r--r-- 1 root root 2783969280 Feb 25 21:58 /fs/sda7/zero-1 > 132 -rw-r--r-- 1 root root 2772434944 Feb 25 21:58 /fs/sda7/zero-2 > 133 -rw-r--r-- 1 root root 2733637632 Feb 25 21:58 /fs/sda7/zero-3 > 134 -rw-r--r-- 1 root root 2735734784 Feb 25 21:58 /fs/sda7/zero-4 This works nicely for dd threads doing the same thing but for less trivial load it isn't that easy. I thought we could compare fairness by measuring time the process is throttled in balance_dirty_pages() in each time slot (where the length of a time slot is for discussion, I use 2s but that's just an arbitrary number not too big and not too small) and then this should relate to the relative dirtying rate of a thread. What do you think? Another interesting parameter might be max time spent waiting in balance_dirty_pages() - that would tell something about the latency induced by the algorithm. > > more subtle things like how the algorithm behaves for tasks that are not IO > > bound for most of the time (or do less IO). Any good metrics here? More > > things we could compare? > > For non IO bound tasks, there are fio job files that do different > dirty rates. I have not run them though, as the bandwidth based > algorithm obviously assigns higher bandwidth to light dirtiers :) Yes :) But I'd be interested how our algorithms behave in such cases... Honza -- Jan Kara <jack@xxxxxxx> SUSE Labs, CR -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxxx For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
