Re: [PATCH] readahead:add blk_run_backing_dev

Hisashi Hifumi
2009-06-01T03:12:20Z

At 12:02 09/06/01, Wu Fengguang wrote:
>On Mon, Jun 01, 2009 at 10:51:56AM +0800, Hisashi Hifumi wrote:
>>
>> At 11:37 09/06/01, Wu Fengguang wrote:
>> >On Wed, May 27, 2009 at 11:06:37AM +0800, Hisashi Hifumi wrote:
>> >>
>> >> At 11:57 09/05/27, Wu Fengguang wrote:
>> >> >On Wed, May 27, 2009 at 10:47:47AM +0800, Hisashi Hifumi wrote:
>> >> >>
>> >> >> At 11:36 09/05/27, Wu Fengguang wrote:
>> >> >> >On Wed, May 27, 2009 at 10:21:53AM +0800, Hisashi Hifumi wrote:
>> >> >> >>
>> >> >> >> At 11:09 09/05/27, Wu Fengguang wrote:
>> >> >> >> >On Wed, May 27, 2009 at 08:25:04AM +0800, Hisashi Hifumi wrote:
>> >> >> >> >>
>> >> >> >> >> At 08:42 09/05/27, Andrew Morton wrote:
>> >> >> >> >> >On Fri, 22 May 2009 10:33:23 +0800
>> >> >> >> >> >Wu Fengguang <fengguang.wu@intel.com> wrote:
>> >> >> >> >> >
>> >> >> >> >> >> > I tested above patch, and I got same performance number.
>> >> >> >> >> >> > I wonder why if (PageUptodate(page)) check is there...
>> >> >> >> >> >>
>> >> >> >> >> >> Thanks! This is an interesting micro timing behavior that
>> >> >> >> >> >> demands some research work. The above check is to confirm
>if it's
>> >> >> >> >> >> the PageUptodate() case that makes the difference. So why
>that case
>> >> >> >> >> >> happens so frequently so as to impact the performance?
>Will it also
>> >> >> >> >> >> happen in NFS?
>> >> >> >> >> >>
>> >> >> >> >> >> The problem is readahead IO pipeline is not running smoothly,
>> >which is
>> >> >> >> >> >> undesirable and not well understood for now.
>> >> >> >> >> >
>> >> >> >> >> >The patch causes a remarkably large performance increase. A 9%
>> >> >> >> >> >reduction in time for a linear read? I'd be surprised if the
>workload
>> >> >> >> >>
>> >> >> >> >> Hi Andrew.
>> >> >> >> >> Yes, I tested this with dd.
>> >> >> >> >>
>> >> >> >> >> >even consumed 9% of a CPU, so where on earth has the kernel
>gone to?
>> >> >> >> >> >
>> >> >> >> >> >Have you been able to reproduce this in your testing?
>> >> >> >> >>
>> >> >> >> >> Yes, this test on my environment is reproducible.
>> >> >> >> >
>> >> >> >> >Hisashi, does your environment have some special configurations?
>> >> >> >>
>> >> >> >> Hi.
>> >> >> >> My testing environment is as follows:
>> >> >> >> Hardware: HP DL580
>> >> >> >> CPU:Xeon 3.2GHz *4 HT enabled
>> >> >> >> Memory:8GB
>> >> >> >> Storage: Dothill SANNet2 FC (7Disks RAID-0 Array)
>> >> >> >
>> >> >> >This is a big hardware RAID. What's the readahead size?
>> >> >> >
>> >> >> >The numbers look too small for a 7 disk RAID:
>> >> >> >
>> >> >> > > #dd if=testdir/testfile of=/dev/null bs=16384
>> >> >> > >
>> >> >> > > -2.6.30-rc6
>> >> >> > > 1048576+0 records in
>> >> >> > > 1048576+0 records out
>> >> >> > > 17179869184 bytes (17 GB) copied, 224.182 seconds, 76.6 MB/s
>> >> >> > >
>> >> >> > > -2.6.30-rc6-patched
>> >> >> > > 1048576+0 records in
>> >> >> > > 1048576+0 records out
>> >> >> > > 17179869184 bytes (17 GB) copied, 206.465 seconds, 83.2 MB/s
>> >> >> >
>> >> >> >I'd suggest you to configure the array properly before coming back to
>> >> >> >measuring the impact of this patch.
>> >> >>
>> >> >>
>> >> >> I created 16GB file to this disk array, and mounted to testdir, dd to
>> >> >this directory.
>> >> >
>> >> >I mean, you should get >300MB/s throughput with 7 disks, and you
>> >> >should seek ways to achieve that before testing out this patch :-)
>> >>
>> >> Throughput number of storage array is very from one product to another.
>> >> On my hardware environment I think this number is valid and
>> >> my patch is effective.
>> >
>> >What's your readahead size? Is it large enough to cover the stripe width?
>>
>> Do you mean strage's readahead size?
>
>What's strage? I mean if your RAID's block device file is /dev/sda, then
>
> blockdev --getra /dev/sda
>
>will tell its readahead size in unit of 512 bytes.

256 sectors.