On Tue, 2010-12-14 at 21:59 +0800, Wu Fengguang wrote: > Hi Richard, > > On Tue, Dec 14, 2010 at 09:37:34PM +0800, Richard Kennedy wrote: > > On Mon, 2010-12-13 at 22:46 +0800, Wu Fengguang wrote: > > > plain text document attachment > > > (writeback-speedup-per-bdi-threshold-ramp-up.patch) > > > Reduce the dampening for the control system, yielding faster > > > convergence. > > > > > > Currently it converges at a snail's pace for slow devices (in order of > > > minutes). For really fast storage, the convergence speed should be fine. > > > > > > It makes sense to make it reasonably fast for typical desktops. > > > > > > After patch, it converges in ~10 seconds for 60MB/s writes and 4GB mem. > > > So expect ~1s for a fast 600MB/s storage under 4GB mem, or ~4s under > > > 16GB mem, which seems reasonable. > > > > > > $ while true; do grep BdiDirtyThresh /debug/bdi/8:0/stats; sleep 1; done > > > BdiDirtyThresh: 0 kB > > > BdiDirtyThresh: 118748 kB > > > BdiDirtyThresh: 214280 kB > > > BdiDirtyThresh: 303868 kB > > > BdiDirtyThresh: 376528 kB > > > BdiDirtyThresh: 411180 kB > > > BdiDirtyThresh: 448636 kB > > > BdiDirtyThresh: 472260 kB > > > BdiDirtyThresh: 490924 kB > > > BdiDirtyThresh: 499596 kB > > > BdiDirtyThresh: 507068 kB > > > ... > > > DirtyThresh: 530392 kB > > > > > > CC: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx> > > > CC: Richard Kennedy <richard@xxxxxxxxxxxxxxx> > > > Signed-off-by: Wu Fengguang <fengguang.wu@xxxxxxxxx> > > > --- > > > mm/page-writeback.c | 2 +- > > > 1 file changed, 1 insertion(+), 1 deletion(-) > > > > > > --- linux-next.orig/mm/page-writeback.c 2010-12-13 21:46:11.000000000 +0800 > > > +++ linux-next/mm/page-writeback.c 2010-12-13 21:46:11.000000000 +0800 > > > @@ -145,7 +145,7 @@ static int calc_period_shift(void) > > > else > > > dirty_total = (vm_dirty_ratio * determine_dirtyable_memory()) / > > > 100; > > > - return 2 + ilog2(dirty_total - 1); > > > + return ilog2(dirty_total - 1) - 1; > > > } > > > > > > /* > > > > > > > > Hi Fengguang, > > > > I've been running my test set on your v3 series and generally it's > > giving good results in line with the mainline kernel, with much less > > variability and lower standard deviation of the results so it is much > > more repeatable. > > Glad to hear that, and thank you very much for trying it out! > > > However, it doesn't seem to be honouring the background_dirty_threshold. > > > The attached graph is from a simple fio write test of 400Mb on ext4. > > All dirty pages are completely written in 15 seconds, but I expect to > > see up to background_dirty_threshold pages staying dirty until the 30 > > second background task writes them out. So it is much too eager to write > > back dirty pages. > > This is interesting, and seems easy to root cause. When testing v4, > would you help collect the following trace events? > > echo 1 > /debug/tracing/events/writeback/balance_dirty_pages/enable > echo 1 > /debug/tracing/events/writeback/balance_dirty_state/enable > echo 1 > /debug/tracing/events/writeback/writeback_single_inode/enable > > They'll have good opportunity to disclose the bug. > > > As to the ramp up time, when writing to 2 disks at the same time I see > > the per_bdi_threshold taking up to 20 seconds to converge on a steady > > value after one of the write stops. So I think this could be speeded up > > even more, at least on my setup. > > I have the roughly same ramp up time on the 1-disk 3GB mem test: > > http://www.kernel.org/pub/linux/kernel/people/wfg/writeback/tests/3G/ext4-1dd-1M-8p-2952M-2.6.37-rc5+-2010-12-09-00-37/dirty-pages.png > > Given that it's the typical desktop, it does seem reasonable to speed > it up further. > > > I am just about to start testing v4 & will report anything interesting. > > Thanks! > > Fengguang I just mailed the trace log to Fengguang, it is a bit big to post to this list. If anyone wants it, let me know and I'll mail to them directly. I'm also seeing a write stall in some of my tests. When writing 400Mb after about 6 seconds I'm see a few seconds when there are no reported sectors written to sda & there are no pages under writeback although there are lots of dirty pages. ( the graph I sent previously shows this stall as well ) regards Richard -- 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
