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Re: [patch] Give kjournald a IOPRIO_CLASS_RT io priority

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To: Andrew Morton <akpm@...>
Cc: Jens Axboe <jens.axboe@...>, Arjan van de Ven <arjan@...>, <linux-kernel@...>
Date: Monday, October 22, 2007 - 5:40 am

* Andrew Morton <akpm@linux-foundation.org> wrote:


reverting this oneliner is trivial. Finding bandwidth problems and 
tracking them down to this oneliner change is relatively easy too. 
Finding latency problems and fixing them is _not_ trivial.

Boot up a Linux desktop and start OOo or firefox, and measure the time 
it takes to start the app up. 10-20 seconds on a top-of-the-line 
quad-core 3.2 GHz system - which is a shame. Same box can do in excess 
of 1GB/sec block IO. Yes, one could blame the apps but in reality most 
of the blame is mostly on the kernel side. We do not make bloat and 
latency suckage apparent enough to user-space (due to lack of 
intelligent instrumentation), we make latencies hard to fix, we have an 
acceptance bias towards bandwidth fixes (because they are easier to 
measure and justify) - and that's all what it takes to let such a 
situation get out of control.

and i can bring up the scheduler as an example. CFS broke the bandwidth 
performance of one particular app and it took only a few days to get it 
back under control. But it was months to get good latency behavior out 
of the scheduler. And that is with the help of excellent scheduler 
instrumentation. In the IO space the latency situation is much, much 
worse. Really.

	Ingo
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Messages in current thread:
[patch] Give kjournald a IOPRIO_CLASS_RT io priority, Arjan van de Ven, (Mon Oct 15, 1:46 pm)
Re: [patch] Give kjournald a IOPRIO_CLASS_RT io priority, Andrew Morton, (Mon Oct 15, 2:47 pm)
Re: [patch] Give kjournald a IOPRIO_CLASS_RT io priority, Andrew Morton, (Mon Oct 15, 5:12 pm)
Re: [patch] Give kjournald a IOPRIO_CLASS_RT io priority, Andrew Morton, (Mon Oct 22, 5:23 am)
Re: [patch] Give kjournald a IOPRIO_CLASS_RT io priority, Ingo Molnar, (Mon Oct 22, 5:40 am)
Re: [patch] Give kjournald a IOPRIO_CLASS_RT io priority, Andrew Morton, (Mon Oct 22, 5:49 am)
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