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

[jmerkey <jmerkey@soleranetworks.com>]

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.
>>
>>    
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>>>      
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>>>>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