On Mon, 2010-06-14 at 14:58 +0100, Richard Kennedy wrote: > Hi all, > The fraction of vm cache allowed to each BDI as calculated by > get_dirty_limits (mm/page-writeback.c) respond very slowly to changes in > workload. > > Running a simple test that alternately writes 1Gb to sda then sdb, > twice, shows the bdi_threshold taking approximately 15 seconds to reach > a steady state value. This prevents a application from using all of the > available cache and forces it to write to the physical disk earlier than > strictly necessary. > As you can see from the attached graph, bdi_thresh_before.png, our > current control system responds to this kind of workload very slowly. > > The below patch speeds up the recalculation and lets it reach a steady > state value in a couple of seconds. see bdi_thresh_after.png. > > I get better throughput with this patch applied and have been running > some variation of this on and off for some months without any obvious > problems. > > (These tests were all run on 2.6.35-rc3, > where dm-2 is a sata drive lvm/ext4 and sdb is ide ext4. > I've got lots more results and graphs but won't bore you all with > them ;) ) > > I see this as a considerable improvement but I have found the magic > number of -4 empirically so it may just be tuned to my system. I'm not > sure how to decide on a value that is suitable for everyone. > > Does anyone have any suggestions or thoughts? > > Unfortunately I don't have any other hardware to try this on, so I would > be very interest to hear if anyone tries this on their favourite > workload. > > regards > Richard > > patch against 2.6.35-rc3 > > diff --git a/mm/page-writeback.c b/mm/page-writeback.c > index 2fdda90..315dd04 100644 > --- a/mm/page-writeback.c > +++ b/mm/page-writeback.c > @@ -144,7 +144,7 @@ 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) - 4; > } > > /* > Fixed Jens email address. I can send you the graphs privately if you haven't already got them. regards Richard -- 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/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>