Re: 2.6.9 reporting 1 Gigabyte/second throughput on bio's, timer skew possible?

jmerkey
2005-11-13T21:27:06Z

Jens Axboe wrote:

>On Sat, Nov 12 2005, Jeff V. Merkey wrote:
>
>
>>Jens Axboe wrote:
>>
>>
>>
>>>On Fri, Nov 11 2005, Jeff V. Merkey wrote:
>>>
>>>
>>>
>>>
>>>>I have allocated 393,216 bio buffers I statically maintain in a chain
>>>>and am running the dsfs file system with 3 x gigabit links fully
>>>>saturated. meta-data
>>>>increases the write sizes to 720 MB/Second on dual 9500 controllers with
>>>>8 drives each (total of 16) 7200 RPM Drives. I am seeing some
>>>>congestion and bursting on the bio chains as they are submitted.
>>>>
>>>>
>>>>
>>
>>
>>>16 disks on 2 controllers, I'm 100% sure they are lots of people
>>>pushing 2.6 much further than that! I wouldn't evne call that a big
>>>setup.
>>>
>>>
>>>
>>>
>>Probably not for this type of application.
>>
>>
>>
>>>
>>>
>>>>DSFS dynamically generates html status files form within the file
>>>>system. When the system gets somewhat behind, I am seeing bursts > 1
>>>>GB/Second which exceeds the theoretical limit of the bus. I have a
>>>>timer function that runs every second and profiles the I/O throughput
>>>>created by DSFS with bio submissions and captured packets. I am asking
>>>>if there is clock skew at these data rates with use of the timer
>>>>functions. The system appears to be sustaining 1GB/Second throughput on
>>>>dual controllers. I have verified through data rates the system is
>>>>sustaining 800 megabytes/second with these 1GB/S bursts. I am curious
>>>>if there is potentially timer skew at these higher rates since I am
>>>>having a hard time accepting that I can push 1GB/S through a bus rated
>>>>at only 850 MB/S for DMA based transfers. The unit is accessible by
>>>>
>>>>
>>>>
>>>>
>>>Note that the linux io stats accounting in 2.6.9 accounts queued io, not
>>>io completions. So it's quite possible to have burst rates > bus speeds
>>>for async io. 2.6.15-rc1 change this.
>>>
>>>
>>>
>>>
>>>
>>So you are willing to log into the unit and validate these numbers? I
>>would like for an
>>someone other than me to validate I am seeing these rates.
>>
>>
>
>If you average the bandwidth over a time long enough to eliminate the
>bursty queueing rates, your average rage should drop to what the
>hardware can actually do. Or dig out the patch from 2.6.15-rc1 for
>ll_rw_blk.c and apply it to 2.6.9, find it here:
>
>http://kernel.org/git/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commitdiff;h=d72d904a5367ad4ca3f2c9a2ce8c3a68f0b28bf0;hp=d83c671fb7023f69a9582e622d01525054f23b66
>
>
>
Jens,

Thanks. I'll dig out the patch. I am measuring the rates on the back end
and they are running at 720-800 MB/S apart from what's being reported from
the bio submission. At any rate, I ave to say the bio performance is
stunning in comparison to Windows 2003.

Jeff