Messages in this thread | | | Date | Mon, 29 Jun 2009 14:55:21 +0400 | From | Vladislav Bolkhovitin <> | Subject | Re: [RESEND] [PATCH] readahead:add blk_run_backing_dev |
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Wu Fengguang, on 06/29/2009 01:34 PM wrote: > On Sat, Jun 20, 2009 at 08:29:31PM +0800, Vladislav Bolkhovitin wrote: >> Wu Fengguang, on 06/20/2009 07:55 AM wrote: >>> On Fri, Jun 19, 2009 at 03:04:36AM +0800, Andrew Morton wrote: >>>> On Sun, 7 Jun 2009 06:45:38 +0800 >>>> Wu Fengguang <fengguang.wu@intel.com> wrote: >>>> >>>>>>> Do you have a place where the raw blktrace data can be retrieved for >>>>>>> more in-depth analysis? >>>>>> I think your comment is really adequate. In another thread, Wu Fengguang pointed >>>>>> out the same issue. >>>>>> I and Wu also wait his analysis. >>>>> And do it with a large readahead size :) >>>>> >>>>> Alan, this was my analysis: >>>>> >>>>> : Hifumi, can you help retest with some large readahead size? >>>>> : >>>>> : Your readahead size (128K) is smaller than your max_sectors_kb (256K), >>>>> : so two readahead IO requests get merged into one real IO, that means >>>>> : half of the readahead requests are delayed. >>>>> >>>>> ie. two readahead requests get merged and complete together, thus the effective >>>>> IO size is doubled but at the same time it becomes completely synchronous IO. >>>>> >>>>> : >>>>> : The IO completion size goes down from 512 to 256 sectors: >>>>> : >>>>> : before patch: >>>>> : 8,0 3 177955 50.050313976 0 C R 8724991 + 512 [0] >>>>> : 8,0 3 177966 50.053380250 0 C R 8725503 + 512 [0] >>>>> : 8,0 3 177977 50.056970395 0 C R 8726015 + 512 [0] >>>>> : 8,0 3 177988 50.060326743 0 C R 8726527 + 512 [0] >>>>> : 8,0 3 177999 50.063922341 0 C R 8727039 + 512 [0] >>>>> : >>>>> : after patch: >>>>> : 8,0 3 257297 50.000760847 0 C R 9480703 + 256 [0] >>>>> : 8,0 3 257306 50.003034240 0 C R 9480959 + 256 [0] >>>>> : 8,0 3 257307 50.003076338 0 C R 9481215 + 256 [0] >>>>> : 8,0 3 257323 50.004774693 0 C R 9481471 + 256 [0] >>>>> : 8,0 3 257332 50.006865854 0 C R 9481727 + 256 [0] >>>>> >>>> I haven't sent readahead-add-blk_run_backing_dev.patch in to Linus yet >>>> and it's looking like 2.6.32 material, if ever. >>>> >>>> If it turns out to be wonderful, we could always ask the -stable >>>> maintainers to put it in 2.6.x.y I guess. >>> Agreed. The expected (and interesting) test on a properly configured >>> HW RAID has not happened yet, hence the theory remains unsupported. >> Hmm, do you see anything improper in the Ronald's setup (see >> http://sourceforge.net/mailarchive/forum.php?thread_name=a0272b440906030714g67eabc5k8f847fb1e538cc62%40mail.gmail.com&forum_name=scst-devel)? >> It is HW RAID based. > > No. Ronald's HW RAID performance is reasonably good. I meant Hifumi's > RAID performance is too bad and may be improved by increasing the > readahead size, hehe. > >> As I already wrote, we can ask Ronald to perform any needed tests. > > Thanks! Ronald's test results are: > > 231 MB/s HW RAID > 69.6 MB/s HW RAID + SCST > 89.7 MB/s HW RAID + SCST + this patch > > So this patch seem to help SCST, but again it would be better to > improve the SCST throughput first - it is now quite sub-optimal.
No, SCST performance isn't an issue here. You simply can't get more than 110 MB/s from iSCSI over 1GbE, hence 231 MB/s fundamentally isn't possible. There is only room for 20% improvement, which should be achieved with better client-side-driven pipelining (see our other discussions, e.g. http://lkml.org/lkml/2009/5/12/370)
> (Sorry for the long delay: currently I have not got an idea on > how to measure such timing issues.) > > And if Ronald could provide the HW RAID performance with this patch, > then we can confirm if this patch really makes a difference for RAID. > > Thanks, > Fengguang
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