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

[Jens Axboe]

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.  I am 
> not aware of anyone pushing 2.6 to these limits at present with this 
> type of architecture.  I have split

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.

> 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.

-- 
Jens Axboe

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