On Fri, Sep 30, 2011 at 10:35:25AM +0300, Pekka Enberg wrote: > Hi Johannes! > > On Fri, Sep 30, 2011 at 10:17 AM, Johannes Weiner <jweiner@xxxxxxxxxx> wrote: > > But there is a flaw in that we have a zoned page allocator which does > > not care about the global state but rather the state of individual > > memory zones. And right now there is nothing that prevents one zone > > from filling up with dirty pages while other zones are spared, which > > frequently leads to situations where kswapd, in order to restore the > > watermark of free pages, does indeed have to write pages from that > > zone's LRU list. This can interfere so badly with IO from the flusher > > threads that major filesystems (btrfs, xfs, ext4) mostly ignore write > > requests from reclaim already, taking away the VM's only possibility > > to keep such a zone balanced, aside from hoping the flushers will soon > > clean pages from that zone. > > The obvious question is: how did you test this? Can you share the results? Meh, sorry about that, they were in the series introduction the last time and I forgot to copy them over. I did single-threaded, linear writing to an USB stick as the effect is most pronounced with slow backing devices. [ The write deferring on ext4 because of delalloc is so extreme that I could trigger it even with simple linear writers on a mediocre rotating disk, though. I can not access the logfiles right now, but the nr_vmscan_writes went practically away here as well and runtime was unaffected with the patched kernel. ] Test results 15M DMA + 3246M DMA32 + 504M Normal = 3765M memory 40% dirty ratio, 10% background ratio 16G USB thumb drive 10 runs of dd if=/dev/zero of=disk/zeroes bs=32k count=$((10 << 15)) seconds nr_vmscan_write (stddev) min| median| max xfs vanilla: 549.747( 3.492) 0.000| 0.000| 0.000 patched: 550.996( 3.802) 0.000| 0.000| 0.000 fuse-ntfs vanilla: 1183.094(53.178) 54349.000| 59341.000| 65163.000 patched: 558.049(17.914) 0.000| 0.000| 43.000 btrfs vanilla: 573.679(14.015) 156657.000| 460178.000| 606926.000 patched: 563.365(11.368) 0.000| 0.000| 1362.000 ext4 vanilla: 561.197(15.782) 0.000|2725438.000|4143837.000 patched: 568.806(17.496) 0.000| 0.000| 0.000 -- 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
